MLEM/MLEM.Ui/Elements/Element.cs

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Diagnostics;
using System.Linq;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;
using MLEM.Graphics;
using MLEM.Input;
using MLEM.Maths;
using MLEM.Misc;
using MLEM.Sound;
using MLEM.Textures;
using MLEM.Ui.Style;

namespace MLEM.Ui.Elements {
    /// <summary>
    /// This class represents a generic base class for ui elements of a <see cref="UiSystem"/>.
    /// </summary>
    public abstract class Element : GenericDataHolder {

        /// <summary>
        /// This field holds an epsilon value used in element <see cref="Size"/>, position and resulting <see cref="Area"/> calculations to mitigate floating point rounding inaccuracies.
        /// If ui elements used are extremely small or extremely large, this value can be reduced or increased.
        /// </summary>
        public static float Epsilon = 0.01F;

        /// <summary>
        /// The ui system that this element is currently a part of
        /// </summary>
        public UiSystem System {
            get => this.system;
            private set {
                this.system = value;
                this.Controls = value?.Controls;
                this.AndChildren(e => e.Style = e.Style.OrStyle(value?.Style));
            }
        }
        /// <summary>
        /// The controls that this element's <see cref="System"/> uses
        /// </summary>
        public UiControls Controls;
        /// <summary>
        /// This element's parent element.
        /// If this element has no parent (it is the <see cref="RootElement"/> of a ui system), this value is <c>null</c>.
        /// </summary>
        public Element Parent { get; private set; }
        /// <summary>
        /// This element's <see cref="RootElement"/>.
        /// Note that this value is set even if this element has a <see cref="Parent"/>. To get the element that represents the root element, use <see cref="RootElement.Element"/>.
        /// </summary>
        public RootElement Root { get; private set; }
        /// <summary>
        /// The scale that this ui element renders with
        /// </summary>
        public float Scale => this.Root.ActualScale;
        /// <summary>
        /// The <see cref="Anchor"/> that this element uses for positioning within its parent
        /// </summary>
        public Anchor Anchor {
            get => this.anchor;
            set {
                if (this.anchor == value)
                    return;
                this.anchor = value;
                this.SetAreaDirty();
            }
        }
        /// <summary>
        /// The size of this element, where X represents the width and Y represents the height.
        /// If the x or y value of the size is between 0 and 1, the size will be seen as a percentage of its parent's size rather than as an absolute value.
        /// If the x (or y) value of the size is negative, the width (or height) is seen as a percentage of the element's resulting height (or width).
        /// Additionally, if auto-sizing is used, <see cref="AutoSizeAddedAbsolute"/> can be set to add or subtract an absolute value from the automatically calculated size.
        /// </summary>
        /// <example>
        /// The following example, ignoring <see cref="Scale"/>, combines both types of percentage-based sizing.
        /// If this element is inside of a <see cref="Parent"/> whose width is 20, this element's width will be set to <c>0.5 * 20 = 10</c>, and its height will be set to <c>2.5 * 10 = 25</c>.
        /// <code>
        /// element.Size = new Vector2(0.5F, -2.5F);
        /// </code>
        /// </example>
        public Vector2 Size {
            get => this.size;
            set {
                if (this.size == value)
                    return;
                this.size = value;
                this.SetAreaDirty();
            }
        }
        /// <summary>
        /// The <see cref="Size"/>, but with <see cref="Scale"/> applied.
        /// </summary>
        public Vector2 ScaledSize => this.size * this.Scale;
        /// <summary>
        /// If auto-sizing is used by setting <see cref="Size"/> less than or equal to 1, this property allows adding or subtracting an additional, absolute value from the automatically calculated size.
        /// If this element is not using auto-sizing, this property is ignored.
        /// </summary>
        /// <example>
        /// Ignoring <see cref="Scale"/>, if this element's <see cref="Size"/> is set to <c>0.5, 0.75</c> and its <see cref="Parent"/> has a size of <c>200, 100</c>, then an added absolute size of <c>-50, 25</c> will result in a final <see cref="Area"/> size of <c>0.5 * 200 - 50, 0.75 * 100 + 25</c>, or <c>50, 100</c>.
        /// </example>
        public Vector2 AutoSizeAddedAbsolute {
            get => this.autoSizeAddedAbsolute;
            set {
                if (this.autoSizeAddedAbsolute == value)
                    return;
                this.autoSizeAddedAbsolute = value;
                this.SetAreaDirty();
            }
        }
        /// <summary>
        /// The <see cref="AutoSizeAddedAbsolute"/>, but with <see cref="Scale"/> applied.
        /// </summary>
        public Vector2 ScaledAutoSizeAddedAbsolute => this.autoSizeAddedAbsolute * this.Scale;
        /// <summary>
        /// This element's offset from its default position, which is dictated by its <see cref="Anchor"/>.
        /// Note that, depending on the side that the element is anchored to, this offset moves it in a different direction.
        /// </summary>
        public Vector2 PositionOffset {
            get => this.offset;
            set {
                if (this.offset == value)
                    return;
                this.offset = value;
                this.SetAreaDirty();
            }
        }
        /// <summary>
        /// The <see cref="PositionOffset"/>, but with <see cref="Scale"/> applied.
        /// </summary>
        public Vector2 ScaledOffset => this.offset * this.Scale;
        /// <summary>
        /// The <see cref="Padding"/>, but with <see cref="Scale"/> applied.
        /// </summary>
        public Padding ScaledPadding => this.Padding.Value * this.Scale;
        /// <summary>
        /// The <see cref="ChildPadding"/>, but with <see cref="Scale"/> applied.
        /// </summary>
        public Padding ScaledChildPadding => this.ChildPadding.Value * this.Scale;
        /// <summary>
        /// This element's current <see cref="Area"/>, but with <see cref="ScaledChildPadding"/> applied.
        /// </summary>
        public RectangleF ChildPaddedArea => this.UnscrolledArea.Shrink(this.ScaledChildPadding);
        /// <summary>
        /// This element's area, without respecting its <see cref="ScrollOffset"/>.
        /// This area is updated automatically to fit this element's sizing and positioning properties.
        /// </summary>
        public RectangleF UnscrolledArea {
            get {
                this.UpdateAreaIfDirty();
                return this.area;
            }
        }
        /// <summary>
        /// The <see cref="UnscrolledArea"/> of this element, but with <see cref="ScaledScrollOffset"/> applied.
        /// </summary>
        public RectangleF Area => this.UnscrolledArea.OffsetCopy(this.ScaledScrollOffset);
        /// <summary>
        /// The area that this element is displayed in, which is <see cref="Area"/> shrunk by this element's <see cref="ScaledPadding"/>.
        /// This is the property that should be used for drawing this element, as well as mouse input handling and culling.
        /// </summary>
        public RectangleF DisplayArea => this.Area.Shrink(this.ScaledPadding);
        /// <summary>
        /// The offset that this element has as a result of <see cref="Panel"/> scrolling.
        /// </summary>
        public Vector2 ScrollOffset;
        /// <summary>
        /// The <see cref="ScrollOffset"/>, but with <see cref="Scale"/> applied.
        /// </summary>
        public Vector2 ScaledScrollOffset => this.ScrollOffset * this.Scale;
        /// <summary>
        /// Set this property to <c>true</c> to cause this element to be hidden.
        /// Hidden elements don't receive input events, aren't rendered and don't factor into auto-anchoring.
        /// </summary>
        public virtual bool IsHidden {
            get => this.isHidden;
            set {
                if (this.isHidden == value)
                    return;
                this.isHidden = value;
                this.SetAreaDirty();
            }
        }
        /// <summary>
        /// The priority of this element as part of its <see cref="Parent"/> element.
        /// A higher priority means the element will be drawn first, but not anchored higher up if auto-anchoring is used.
        /// </summary>
        public int Priority {
            get => this.priority;
            set {
                if (this.priority == value)
                    return;
                this.priority = value;
                if (this.Parent != null)
                    this.Parent.SetSortedChildrenDirty();
            }
        }
        /// <summary>
        /// This element's transform matrix.
        /// Can easily be scaled using <see cref="ScaleTransform"/>.
        /// Note that, when this is non-null, a new <c>SpriteBatch.Begin</c> call is used for this element.
        /// </summary>
        public Matrix Transform = Matrix.Identity;
        /// <summary>
        /// Set this field to false to disallow the element from being selected.
        /// An unselectable element is skipped by automatic navigation and its <see cref="OnSelected"/> callback will never be called.
        /// </summary>
        public virtual bool CanBeSelected {
            get => this.canBeSelected;
            set {
                this.canBeSelected = value;
                if (!this.canBeSelected && this.Root?.SelectedElement == this)
                    this.Root.SelectElement(null);
            }
        }
        /// <summary>
        /// Set this field to false to disallow the element from reacting to being moused over.
        /// </summary>
        public virtual bool CanBeMoused { get; set; } = true;
        /// <summary>
        /// Set this field to false to disallow this element's <see cref="OnPressed"/> and <see cref="OnSecondaryPressed"/> events to be called.
        /// </summary>
        public virtual bool CanBePressed { get; set; } = true;
        /// <summary>
        /// Set this field to false to cause auto-anchored siblings to ignore this element as a possible anchor point.
        /// </summary>
        public virtual bool CanAutoAnchorsAttach {
            get => this.canAutoAnchorsAttach;
            set {
                if (this.canAutoAnchorsAttach != value) {
                    this.canAutoAnchorsAttach = value;
                    this.SetAreaDirty();
                }
            }
        }
        /// <summary>
        /// Set this field to true to cause this element's width to be automatically calculated based on the area that its <see cref="Children"/> take up.
        /// To use this element's <see cref="Size"/>'s X coordinate as a minimum or maximum width rather than ignoring it, set <see cref="TreatSizeAsMinimum"/> or <see cref="TreatSizeAsMaximum"/> to true.
        /// </summary>
        public virtual bool SetWidthBasedOnChildren {
            get => this.setWidthBasedOnChildren;
            set {
                if (this.setWidthBasedOnChildren != value) {
                    this.setWidthBasedOnChildren = value;
                    this.SetAreaDirty();
                }
            }
        }
        /// <summary>
        /// Set this field to true to cause this element's height to be automatically calculated based on the area that its <see cref="Children"/> take up.
        /// To use this element's <see cref="Size"/>'s Y coordinate as a minimum or maximum height rather than ignoring it, set <see cref="TreatSizeAsMinimum"/> or <see cref="TreatSizeAsMaximum"/> to true.
        /// </summary>
        public virtual bool SetHeightBasedOnChildren {
            get => this.setHeightBasedOnChildren;
            set {
                if (this.setHeightBasedOnChildren != value) {
                    this.setHeightBasedOnChildren = value;
                    this.SetAreaDirty();
                }
            }
        }
        /// <summary>
        /// If this field is set to true, and <see cref="SetWidthBasedOnChildren"/> or <see cref="SetHeightBasedOnChildren"/> are enabled, the resulting width or height will always be greather than or equal to this element's <see cref="Size"/>.
        /// For example, if an element's <see cref="Size"/>'s Y coordinate is set to 20, but there is only one child with a height of 10 in it, the element's height would be shrunk to 10 if this value was false, but would remain at 20 if it was true.
        /// Note that this value only has an effect if <see cref="SetWidthBasedOnChildren"/> or <see cref="SetHeightBasedOnChildren"/> are enabled.
        /// </summary>
        public virtual bool TreatSizeAsMinimum {
            get => this.treatSizeAsMinimum;
            set {
                if (this.treatSizeAsMinimum != value) {
                    this.treatSizeAsMinimum = value;
                    this.SetAreaDirty();
                }
            }
        }
        /// <summary>
        /// If this field is set to true, and <see cref="SetWidthBasedOnChildren"/> or <see cref="SetHeightBasedOnChildren"/>are enabled, the resulting width or height weill always be less than or equal to this element's <see cref="Size"/>.
        /// Note that this value only has an effect if <see cref="SetWidthBasedOnChildren"/> or <see cref="SetHeightBasedOnChildren"/> are enabled.
        /// </summary>
        public virtual bool TreatSizeAsMaximum {
            get => this.treatSizeAsMaximum;
            set {
                if (this.treatSizeAsMaximum != value) {
                    this.treatSizeAsMaximum = value;
                    this.SetAreaDirty();
                }
            }
        }
        /// <summary>
        /// Set this field to true to cause this element's final display area to never exceed that of its <see cref="Parent"/>.
        /// If the resulting area is too large, the size of this element is shrunk to fit the target area.
        /// This can be useful if an element should fill the remaining area of a parent exactly.
        /// </summary>
        public virtual bool PreventParentSpill {
            get => this.preventParentSpill;
            set {
                if (this.preventParentSpill != value) {
                    this.preventParentSpill = value;
                    this.SetAreaDirty();
                }
            }
        }
        /// <summary>
        /// The transparency (alpha value) that this element is rendered with.
        /// Note that, when <see cref="Draw(Microsoft.Xna.Framework.GameTime,Microsoft.Xna.Framework.Graphics.SpriteBatch,float,MLEM.Graphics.SpriteBatchContext)"/> is called, this alpha value is multiplied with the <see cref="Parent"/>'s alpha value and passed down to this element's <see cref="Children"/>.
        /// </summary>
        public virtual float DrawAlpha { get; set; } = 1;
        /// <summary>
        /// Stores whether this element is currently being moused over or touched.
        /// </summary>
        public bool IsMouseOver => this.Controls.MousedElement == this || this.Controls.TouchedElement == this;
        /// <summary>
        /// Returns whether this element is its <see cref="Root"/>'s <see cref="RootElement.SelectedElement"/>.
        /// Note that, unlike <see cref="IsSelectedActive"/>, this property will be <see langword="true"/> even if this element's <see cref="Root"/> is not the <see cref="UiControls.ActiveRoot"/>.
        /// </summary>
        public bool IsSelected => this.Root.SelectedElement == this;
        /// <summary>
        /// Returns whether this element is its <see cref="Controls"/>'s <see cref="UiControls.SelectedElement"/>.
        /// Note that <see cref="IsSelected"/> can be used to query whether this element is its <see cref="Root"/>'s <see cref="RootElement.SelectedElement"/> instead.
        /// </summary>
        public bool IsSelectedActive => this.Controls.SelectedElement == this;
        /// <summary>
        /// Returns whether this element's <see cref="SetAreaDirty"/> method has been recently called and its area has not been updated since then using <see cref="UpdateAreaIfDirty"/> or <see cref="ForceUpdateArea"/>.
        /// </summary>
        public bool AreaDirty { get; private set; }
        /// <summary>
        /// An optional string that represents a group of elements for automatic (keyboard and gamepad) navigation.
        /// All elements that share the same auto-nav group will be able to be navigated between, and all other elements will not be reachable from elements of other groups.
        /// Note that, if no element is previously selected and auto-navigation is invoked, this element cannot be chosen as the first element to navigate to if its auto-nav group is non-null.
        /// </summary>
        public virtual string AutoNavGroup { get; set; }

        /// <summary>
        /// This Element's current <see cref="UiStyle"/>.
        /// When this property is set, <see cref="InitStyle"/> is called.
        /// </summary>
        public StyleProp<UiStyle> Style {
            get => this.style;
            set {
                this.style = value;
                if (this.style.HasValue())
                    this.InitStyle(this.style);
            }
        }
        /// <summary>
        /// A style property that contains the selection indicator that is displayed on this element if it is the <see cref="RootElement.SelectedElement"/>
        /// </summary>
        public StyleProp<NinePatch> SelectionIndicator;
        /// <summary>
        /// A style property that contains the sound effect that is played when this element's <see cref="OnPressed"/> is called
        /// </summary>
        public StyleProp<SoundEffectInfo> ActionSound;
        /// <summary>
        /// A style property that contains the sound effect that is played when this element's <see cref="OnSecondaryPressed"/> is called
        /// </summary>
        public StyleProp<SoundEffectInfo> SecondActionSound;
        /// <summary>
        /// The padding that this element has.
        /// The padding is subtracted from the element's <see cref="Size"/>, and it is an area that the element does not extend into. This means that this element's resulting <see cref="DisplayArea"/> does not include this padding.
        /// </summary>
        public StyleProp<Padding> Padding;
        /// <summary>
        /// The child padding that this element has.
        /// The child padding moves any <see cref="Children"/> added to this element inwards by the given amount in each direction.
        /// </summary>
        public StyleProp<Padding> ChildPadding {
            get => this.childPadding;
            set {
                this.childPadding = value;
                this.SetAreaDirty();
            }
        }
        /// <summary>
        /// A <see cref="UiAnimation"/> that is played when the mouse enters this element, in <see cref="OnMouseEnter"/>.
        /// </summary>
        public StyleProp<UiAnimation> MouseEnterAnimation;
        /// <summary>
        /// A <see cref="UiAnimation"/> that is played when the mouse exits this element, in <see cref="OnMouseExit"/>.
        /// </summary>
        public StyleProp<UiAnimation> MouseExitAnimation;

        /// <summary>
        /// Event that is called after this element is drawn, but before its children are drawn
        /// </summary>
        public DrawCallback OnDrawn;
        /// <summary>
        /// Event that is called when this element is updated
        /// </summary>
        public TimeCallback OnUpdated;
        /// <summary>
        /// Event that is called when this element is pressed
        /// </summary>
        public GenericCallback OnPressed;
        /// <summary>
        /// Event that is called when this element is pressed using the secondary action
        /// </summary>
        public GenericCallback OnSecondaryPressed;
        /// <summary>
        /// Event that is called when this element's <see cref="IsSelected"/> is turned true
        /// </summary>
        public GenericCallback OnSelected;
        /// <summary>
        /// Event that is called when this element's <see cref="IsSelected"/> is turned false
        /// </summary>
        public GenericCallback OnDeselected;
        /// <summary>
        /// Event that is called when this element starts being moused over
        /// </summary>
        public GenericCallback OnMouseEnter;
        /// <summary>
        /// Event that is called when this element stops being moused over
        /// </summary>
        public GenericCallback OnMouseExit;
        /// <summary>
        /// Event that is called when this element starts being touched
        /// </summary>
        public GenericCallback OnTouchEnter;
        /// <summary>
        /// Event that is called when this element stops being touched
        /// </summary>
        public GenericCallback OnTouchExit;
        /// <summary>
        /// Event that is called when text input is made.
        /// When an element uses this event, it should call <see cref="MlemPlatform.EnsureExists"/> on construction to ensure that the MLEM platform is initialized.
        /// Note that this event is called for every element, even if it is not selected.
        /// Also note that if the active <see cref="MlemPlatform"/> uses an on-screen keyboard, this event is never called.
        /// </summary>
        public TextInputCallback OnTextInput;
        /// <summary>
        /// Event that is called when this element's <see cref="DisplayArea"/> is changed.
        /// </summary>
        public GenericCallback OnAreaUpdated;
        /// <summary>
        /// Event that is called when this element's <see cref="InitStyle"/> method is called while setting the <see cref="Style"/>.
        /// </summary>
        public GenericCallback OnStyleInit;
        /// <summary>
        /// Event that is called when the element that is currently being moused changes within the ui system.
        /// Note that the event fired doesn't necessarily correlate to this specific element.
        /// </summary>
        public OtherElementCallback OnMousedElementChanged;
        /// <summary>
        /// Event that is called when the element that is currently being touched changes within the ui system.
        /// Note that the event fired doesn't necessarily correlate to this specific element.
        /// </summary>
        public OtherElementCallback OnTouchedElementChanged;
        /// <summary>
        /// Event that is called when the element that is currently selected changes within the ui system.
        /// Note that the event fired doesn't necessarily correlate to this specific element.
        /// </summary>
        public OtherElementCallback OnSelectedElementChanged;
        /// <summary>
        /// Event that is called when the next element to select when pressing tab is calculated.
        /// To cause a different element than the default one to be selected, return it during this event.
        /// </summary>
        public TabNextElementCallback GetTabNextElement;
        /// <summary>
        /// Event that is called when the next element to select when using gamepad input is calculated.
        /// To cause a different element than the default one to be selected, return it during this event.
        /// </summary>
        public GamepadNextElementCallback GetGamepadNextElement;
        /// <summary>
        /// Event that is called when a child is added to this element using <see cref="AddChild{T}"/>
        /// Note that, while this event is only called for immediate children of this element, <see cref="RootElement.OnElementAdded"/> is called for all children and grandchildren.
        /// </summary>
        public OtherElementCallback OnChildAdded;
        /// <summary>
        /// Event that is called when a child is removed from this element using <see cref="RemoveChild"/>.
        /// Note that, while this event is only called for immediate children of this element, <see cref="RootElement.OnElementRemoved"/> is called for all children and grandchildren.
        /// </summary>
        public OtherElementCallback OnChildRemoved;
        /// <summary>
        /// Event that is called when this element is added to a <see cref="UiSystem"/>, that is, when this element's <see cref="System"/> is set to a non-<see langword="null"/> value.
        /// </summary>
        public GenericCallback OnAddedToUi;
        /// <summary>
        /// Event that is called when this element is removed from a <see cref="UiSystem"/>, that is, when this element's <see cref="System"/> is set to <see langword="null"/>.
        /// </summary>
        public GenericCallback OnRemovedFromUi;

        /// <summary>
        /// A list of all of this element's direct children.
        /// Use <see cref="AddChild{T}"/> or <see cref="RemoveChild"/> to manipulate this list while calling all of the necessary callbacks.
        /// </summary>
        public readonly IList<Element> Children;
        /// <summary>
        /// A list of all of the <see cref="UiAnimation"/> instances that are currently playing.
        /// You can modify this collection through <see cref="PlayAnimation"/> and <see cref="StopAnimation"/>.
        /// </summary>
        protected readonly List<UiAnimation> PlayingAnimations = new List<UiAnimation>();

        /// <summary>
        /// A sorted version of <see cref="Children"/>. The children are sorted by their <see cref="Priority"/>.
        /// </summary>
        protected IList<Element> SortedChildren {
            get {
                this.UpdateSortedChildrenIfDirty();
                return this.sortedChildren;
            }
        }
        /// <summary>
        /// The input handler that this element's <see cref="Controls"/> use
        /// </summary>
        protected InputHandler Input => this.Controls.Input;
        /// <summary>
        /// The <see cref="ChildPaddedArea"/> of this element's <see cref="Parent"/>, or the <see cref="UiSystem.Viewport"/> if this element has no parent.
        /// This value is the one that is passed to <see cref="CalcActualSize"/> during <see cref="ForceUpdateArea"/>.
        /// </summary>
        protected RectangleF ParentArea => this.Parent?.ChildPaddedArea ?? (RectangleF) this.System.Viewport;

        private readonly List<Element> children = new List<Element>();
        private readonly Stopwatch stopwatch = new Stopwatch();
        private bool sortedChildrenDirty;
        private IList<Element> sortedChildren;
        private UiSystem system;
        private Anchor anchor;
        private Vector2 size;
        private Vector2 autoSizeAddedAbsolute;
        private Vector2 offset;
        private RectangleF area;
        private bool isHidden;
        private int priority;
        private StyleProp<UiStyle> style;
        private StyleProp<Padding> childPadding;
        private bool canBeSelected = true;
        private bool canAutoAnchorsAttach = true;
        private bool setWidthBasedOnChildren;
        private bool setHeightBasedOnChildren;
        private bool treatSizeAsMinimum;
        private bool treatSizeAsMaximum;
        private bool preventParentSpill;

        /// <summary>
        /// Creates a new element with the given anchor and size and sets up some default event reactions.
        /// </summary>
        /// <param name="anchor">This element's <see cref="Anchor"/></param>
        /// <param name="size">This element's default <see cref="Size"/></param>
        protected Element(Anchor anchor, Vector2 size) {
            this.anchor = anchor;
            this.size = size;

            this.Children = new ReadOnlyCollection<Element>(this.children);
            this.GetTabNextElement += (backward, next) => next;
            this.GetGamepadNextElement += (dir, next) => next;
            this.OnMouseEnter += e => {
                if (e.MouseEnterAnimation.HasValue())
                    e.PlayAnimation(e.MouseEnterAnimation);
            };
            this.OnMouseExit += e => {
                if (e.MouseExitAnimation.HasValue())
                    e.PlayAnimation(e.MouseExitAnimation);
            };

            this.SetAreaDirty();
            this.SetSortedChildrenDirty();
        }

        /// <summary>
        /// Adds a child to this element.
        /// </summary>
        /// <param name="element">The child element to add</param>
        /// <param name="index">The index to add the child at, or -1 to add it to the end of the <see cref="Children"/> list</param>
        /// <typeparam name="T">The type of child to add</typeparam>
        /// <returns>This element, for chaining</returns>
        public virtual T AddChild<T>(T element, int index = -1) where T : Element {
            if (index < 0 || index > this.children.Count)
                index = this.children.Count;
            this.children.Insert(index, element);
            element.Parent = this;
            element.AndChildren(e => e.AddedToUi(this.System, this.Root));
            this.OnChildAdded?.Invoke(this, element);
            this.SetSortedChildrenDirty();
            element.SetAreaDirty();
            return element;
        }

        /// <summary>
        /// Removes the given child from this element.
        /// </summary>
        /// <param name="element">The child element to remove</param>
        public virtual void RemoveChild(Element element) {
            this.children.Remove(element);
            if (this.Root?.SelectedElement == element)
                this.Root.SelectElement(null);
            // set area dirty here so that a dirty call is made
            // upwards to us if the element is auto-positioned
            element.SetAreaDirty();
            element.Parent = null;
            element.AndChildren(e => e.RemovedFromUi());
            this.OnChildRemoved?.Invoke(this, element);
            this.SetSortedChildrenDirty();
        }

        /// <summary>
        /// Removes all children from this element that match the given condition.
        /// </summary>
        /// <param name="condition">The condition that determines if a child should be removed</param>
        public virtual void RemoveChildren(Func<Element, bool> condition = null) {
            for (var i = this.Children.Count - 1; i >= 0; i--) {
                var child = this.Children[i];
                if (condition == null || condition(child)) {
                    this.RemoveChild(child);
                }
            }
        }

        /// <summary>
        /// Causes <see cref="SortedChildren"/> to be recalculated as soon as possible.
        /// </summary>
        public void SetSortedChildrenDirty() {
            this.sortedChildrenDirty = true;
        }

        /// <summary>
        /// Updates the <see cref="SortedChildren"/> list if <see cref="SetSortedChildrenDirty"/> is true.
        /// </summary>
        public void UpdateSortedChildrenIfDirty() {
            if (this.sortedChildrenDirty)
                this.ForceUpdateSortedChildren();
        }

        /// <summary>
        /// Forces an update of the <see cref="SortedChildren"/> list.
        /// </summary>
        public virtual void ForceUpdateSortedChildren() {
            this.sortedChildrenDirty = false;
            this.sortedChildren = new ReadOnlyCollection<Element>(this.Children.OrderBy(e => e.Priority).ToArray());
        }

        /// <summary>
        /// Causes this element's <see cref="Area"/> to be recalculated as soon as possible.
        /// If this element is auto-anchored or its parent automatically changes its size based on its children, this element's parent's area is also marked dirty.
        /// </summary>
        public void SetAreaDirty() {
            this.AreaDirty = true;
            this.Parent?.OnChildAreaDirty(this, false);
        }

        /// <summary>
        /// Updates this element's <see cref="Area"/> and all of its <see cref="Children"/> by calling <see cref="ForceUpdateArea"/> if <see cref="AreaDirty"/> is true.
        /// </summary>
        /// <returns>Whether <see cref="AreaDirty"/> was true and <see cref="ForceUpdateArea"/> was called</returns>
        public bool UpdateAreaIfDirty() {
            if (this.AreaDirty) {
                this.ForceUpdateArea();
                return true;
            }
            return false;
        }

        /// <summary>
        /// Forces this element's <see cref="Area"/> to be updated if it is not <see cref="IsHidden"/>.
        /// This method also updates all of this element's <see cref="Children"/>'s areas.
        /// </summary>
        public virtual void ForceUpdateArea() {
            this.AreaDirty = false;
            if (this.IsHidden || this.System == null)
                return;
            // if the parent's area is dirty, it would get updated anyway when querying its ChildPaddedArea,
            // which would cause our ForceUpdateArea code to be run twice, so we only update our parent instead
            if (this.Parent != null && this.Parent.UpdateAreaIfDirty())
                return;
            this.stopwatch.Restart();

            var recursion = 0;
            UpdateDisplayArea();

            this.stopwatch.Stop();
            this.System.Metrics.ForceAreaUpdateTime += this.stopwatch.Elapsed;
            this.System.Metrics.ForceAreaUpdates++;

            void UpdateDisplayArea(Vector2? overrideSize = null) {
                var parentArea = this.ParentArea;
                var parentCenterX = parentArea.X + parentArea.Width / 2;
                var parentCenterY = parentArea.Y + parentArea.Height / 2;

                var intendedSize = this.CalcActualSize(parentArea);
                var newSize = overrideSize ?? intendedSize;
                var pos = new Vector2();

                switch (this.anchor) {
                    case Anchor.TopLeft:
                    case Anchor.AutoLeft:
                    case Anchor.AutoInline:
                    case Anchor.AutoInlineCenter:
                    case Anchor.AutoInlineBottom:
                    case Anchor.AutoInlineIgnoreOverflow:
                    case Anchor.AutoInlineCenterIgnoreOverflow:
                    case Anchor.AutoInlineBottomIgnoreOverflow:
                        pos.X = parentArea.X + this.ScaledOffset.X;
                        pos.Y = parentArea.Y + this.ScaledOffset.Y;
                        break;
                    case Anchor.TopCenter:
                    case Anchor.AutoCenter:
                        pos.X = parentCenterX - newSize.X / 2 + this.ScaledOffset.X;
                        pos.Y = parentArea.Y + this.ScaledOffset.Y;
                        break;
                    case Anchor.TopRight:
                    case Anchor.AutoRight:
                        pos.X = parentArea.Right - newSize.X - this.ScaledOffset.X;
                        pos.Y = parentArea.Y + this.ScaledOffset.Y;
                        break;
                    case Anchor.CenterLeft:
                        pos.X = parentArea.X + this.ScaledOffset.X;
                        pos.Y = parentCenterY - newSize.Y / 2 + this.ScaledOffset.Y;
                        break;
                    case Anchor.Center:
                        pos.X = parentCenterX - newSize.X / 2 + this.ScaledOffset.X;
                        pos.Y = parentCenterY - newSize.Y / 2 + this.ScaledOffset.Y;
                        break;
                    case Anchor.CenterRight:
                        pos.X = parentArea.Right - newSize.X - this.ScaledOffset.X;
                        pos.Y = parentCenterY - newSize.Y / 2 + this.ScaledOffset.Y;
                        break;
                    case Anchor.BottomLeft:
                        pos.X = parentArea.X + this.ScaledOffset.X;
                        pos.Y = parentArea.Bottom - newSize.Y - this.ScaledOffset.Y;
                        break;
                    case Anchor.BottomCenter:
                        pos.X = parentCenterX - newSize.X / 2 + this.ScaledOffset.X;
                        pos.Y = parentArea.Bottom - newSize.Y - this.ScaledOffset.Y;
                        break;
                    case Anchor.BottomRight:
                        pos.X = parentArea.Right - newSize.X - this.ScaledOffset.X;
                        pos.Y = parentArea.Bottom - newSize.Y - this.ScaledOffset.Y;
                        break;
                }

                if (this.Anchor.IsAuto()) {
                    if (this.Anchor.IsInline()) {
                        var anchorEl = this.GetOlderSibling(e => !e.IsHidden && e.CanAutoAnchorsAttach);
                        if (anchorEl != null) {
                            var anchorElArea = anchorEl.GetAreaForAutoAnchors();
                            var newX = anchorElArea.Right + this.ScaledOffset.X;
                            // with awkward ui scale values, floating point rounding can cause an element that would usually
                            // be positioned correctly to be pushed into the next line due to a very small deviation
                            if (this.Anchor.IsIgnoreOverflow() || newX + newSize.X <= parentArea.Right + Element.Epsilon) {
                                pos.X = newX;
                                pos.Y = anchorElArea.Y + this.ScaledOffset.Y;
                                if (this.Anchor == Anchor.AutoInlineCenter || this.Anchor == Anchor.AutoInlineCenterIgnoreOverflow) {
                                    pos.Y += (anchorElArea.Height - newSize.Y) / 2;
                                } else if (this.Anchor == Anchor.AutoInlineBottom || this.Anchor == Anchor.AutoInlineBottomIgnoreOverflow) {
                                    pos.Y += anchorElArea.Height - newSize.Y;
                                }
                            } else {
                                // inline anchors that overflow into the next line act like AutoLeft
                                var newlineAnchorEl = this.GetLowestOlderSibling(e => !e.IsHidden && e.CanAutoAnchorsAttach);
                                if (newlineAnchorEl != null)
                                    pos.Y = newlineAnchorEl.GetAreaForAutoAnchors().Bottom + this.ScaledOffset.Y;
                            }
                        }
                    } else {
                        // auto anchors keep their x coordinates from the switch above
                        var anchorEl = this.GetLowestOlderSibling(e => !e.IsHidden && e.CanAutoAnchorsAttach);
                        if (anchorEl != null)
                            pos.Y = anchorEl.GetAreaForAutoAnchors().Bottom + this.ScaledOffset.Y;
                    }
                }

                if (this.PreventParentSpill) {
                    if (pos.X < parentArea.X)
                        pos.X = parentArea.X;
                    if (pos.Y < parentArea.Y)
                        pos.Y = parentArea.Y;
                    if (pos.X + newSize.X > parentArea.Right)
                        newSize.X = parentArea.Right - pos.X;
                    if (pos.Y + newSize.Y > parentArea.Bottom)
                        newSize.Y = parentArea.Bottom - pos.Y;
                }

                this.SetAreaAndUpdateChildren(new RectangleF(pos, newSize));

                if (this.SetWidthBasedOnChildren || this.SetHeightBasedOnChildren) {
                    Element foundChild = null;
                    var autoSize = this.UnscrolledArea.Size;

                    if (this.SetHeightBasedOnChildren) {
                        var lowest = this.GetLowestChild(e => !e.IsHidden);
                        if (lowest != null) {
                            if (lowest.Anchor.IsTopAligned()) {
                                autoSize.Y = lowest.UnscrolledArea.Bottom - pos.Y + this.ScaledChildPadding.Bottom;
                            } else {
                                autoSize.Y = lowest.UnscrolledArea.Height + this.ScaledChildPadding.Height;
                            }
                            foundChild = lowest;
                        } else {
                            autoSize.Y = 0;
                        }
                    }

                    if (this.SetWidthBasedOnChildren) {
                        var rightmost = this.GetRightmostChild(e => !e.IsHidden);
                        if (rightmost != null) {
                            if (rightmost.Anchor.IsLeftAligned()) {
                                autoSize.X = rightmost.UnscrolledArea.Right - pos.X + this.ScaledChildPadding.Right;
                            } else {
                                autoSize.X = rightmost.UnscrolledArea.Width + this.ScaledChildPadding.Width;
                            }
                            foundChild = rightmost;
                        } else {
                            autoSize.X = 0;
                        }
                    }

                    if (this.TreatSizeAsMinimum) {
                        autoSize = Vector2.Max(autoSize, intendedSize);
                    } else if (this.TreatSizeAsMaximum) {
                        autoSize = Vector2.Min(autoSize, intendedSize);
                    }

                    // we want to leave some leeway to prevent float rounding causing an infinite loop
                    if (!autoSize.Equals(this.UnscrolledArea.Size, Element.Epsilon)) {
                        recursion++;

                        this.System.Metrics.SummedRecursionDepth++;
                        if (recursion > this.System.Metrics.MaxRecursionDepth)
                            this.System.Metrics.MaxRecursionDepth = recursion;

                        if (recursion >= 64)
                            throw new ArithmeticException($"The area of {this} has recursively updated too often. Does its child {foundChild} contain any conflicting auto-sizing settings?");
                        UpdateDisplayArea(autoSize);
                    }
                }
            }
        }

        /// <summary>
        /// Sets this element's <see cref="Area"/> to the given <see cref="RectangleF"/> and invokes the <see cref="UiSystem.OnElementAreaUpdated"/> event.
        /// This method also updates all of this element's <see cref="Children"/>'s areas.
        /// Note that this method does not take into account any auto-sizing, anchoring or positioning, and so it should be used sparingly, if at all.
        /// </summary>
        /// <seealso cref="ForceUpdateArea"/>
        public virtual void SetAreaAndUpdateChildren(RectangleF area) {
            this.area = area;
            this.System.InvokeOnElementAreaUpdated(this);
            foreach (var child in this.Children)
                child.ForceUpdateArea();
            this.System.Metrics.ActualAreaUpdates++;
        }

        /// <summary>
        /// Calculates the actual size that this element should take up, based on the area that its parent encompasses.
        /// By default, this is based on the information specified in <see cref="Size"/>'s documentation.
        /// </summary>
        /// <param name="parentArea">This parent's area, or the ui system's viewport if it has no parent</param>
        /// <returns>The actual size of this element, taking <see cref="Scale"/> into account</returns>
        protected virtual Vector2 CalcActualSize(RectangleF parentArea) {
            var ret = new Vector2(
                this.size.X > 1 ? this.ScaledSize.X : parentArea.Width * this.size.X + this.ScaledAutoSizeAddedAbsolute.X,
                this.size.Y > 1 ? this.ScaledSize.Y : parentArea.Height * this.size.Y + this.ScaledAutoSizeAddedAbsolute.Y);
            if (this.size.X < 0)
                ret.X = -this.size.X * ret.Y + this.ScaledAutoSizeAddedAbsolute.X;
            if (this.size.Y < 0)
                ret.Y = -this.size.Y * ret.X + this.ScaledAutoSizeAddedAbsolute.Y;
            return ret;
        }

        /// <summary>
        /// Returns the area that should be used for determining where auto-anchoring children should attach.
        /// </summary>
        /// <returns>The area for auto anchors</returns>
        protected virtual RectangleF GetAreaForAutoAnchors() {
            return this.UnscrolledArea;
        }

        /// <summary>
        /// Returns this element's lowest child element (in terms of y position) that matches the given condition.
        /// </summary>
        /// <param name="condition">The condition to match</param>
        /// <param name="total">Whether to evaluate based on the child's <see cref="GetTotalCoveredArea"/>, rather than its <see cref="UnscrolledArea"/>.</param>
        /// <returns>The lowest element, or null if no such element exists</returns>
        public Element GetLowestChild(Func<Element, bool> condition = null, bool total = false) {
            Element lowest = null;
            var lowestX = float.MinValue;
            foreach (var child in this.Children) {
                if (condition != null && !condition(child))
                    continue;
                var covered = total ? child.GetTotalCoveredArea(true) : child.UnscrolledArea;
                var x = !child.Anchor.IsTopAligned() ? covered.Height : covered.Bottom;
                if (x >= lowestX) {
                    lowest = child;
                    lowestX = x;
                }
            }
            return lowest;
        }

        /// <summary>
        /// Returns this element's rightmost child (in terms of x position) that matches the given condition.
        /// </summary>
        /// <param name="condition">The condition to match</param>
        /// <param name="total">Whether to evaluate based on the child's <see cref="GetTotalCoveredArea"/>, rather than its <see cref="UnscrolledArea"/>.</param>
        /// <returns>The rightmost element, or null if no such element exists</returns>
        public Element GetRightmostChild(Func<Element, bool> condition = null, bool total = false) {
            Element rightmost = null;
            var rightmostX = float.MinValue;
            foreach (var child in this.Children) {
                if (condition != null && !condition(child))
                    continue;
                var covered = total ? child.GetTotalCoveredArea(true) : child.UnscrolledArea;
                var x = !child.Anchor.IsLeftAligned() ? covered.Width : covered.Right;
                if (x >= rightmostX) {
                    rightmost = child;
                    rightmostX = x;
                }
            }
            return rightmost;
        }

        /// <summary>
        /// Returns this element's lowest sibling that is also older (lower in its parent's <see cref="Children"/> list) than this element that also matches the given condition.
        /// The returned element's <see cref="Parent"/> will always be equal to this element's <see cref="Parent"/>.
        /// </summary>
        /// <param name="condition">The condition to match</param>
        /// <param name="total">Whether to evaluate based on the child's <see cref="GetTotalCoveredArea"/>, rather than its <see cref="UnscrolledArea"/>.</param>
        /// <returns>The lowest older sibling of this element, or null if no such element exists</returns>
        public Element GetLowestOlderSibling(Func<Element, bool> condition = null, bool total = false) {
            if (this.Parent == null)
                return null;
            Element lowest = null;
            foreach (var child in this.Parent.Children) {
                if (child == this)
                    break;
                if (condition != null && !condition(child))
                    continue;
                if (lowest == null || (total ? child.GetTotalCoveredArea(true) : child.UnscrolledArea).Bottom >= lowest.UnscrolledArea.Bottom)
                    lowest = child;
            }
            return lowest;
        }

        /// <summary>
        /// Returns this element's first older sibling that matches the given condition.
        /// The returned element's <see cref="Parent"/> will always be equal to this element's <see cref="Parent"/>.
        /// </summary>
        /// <param name="condition">The condition to match</param>
        /// <returns>The older sibling, or null if no such element exists</returns>
        public Element GetOlderSibling(Func<Element, bool> condition = null) {
            if (this.Parent == null)
                return null;
            Element older = null;
            foreach (var child in this.Parent.Children) {
                if (child == this)
                    break;
                if (condition != null && !condition(child))
                    continue;
                older = child;
            }
            return older;
        }

        /// <summary>
        /// Returns all of this element's siblings that match the given condition.
        /// Siblings are elements that have the same <see cref="Parent"/> as this element.
        /// </summary>
        /// <param name="condition">The condition to match</param>
        /// <returns>This element's siblings</returns>
        public IEnumerable<Element> GetSiblings(Func<Element, bool> condition = null) {
            if (this.Parent == null)
                yield break;
            foreach (var child in this.Parent.Children) {
                if (condition != null && !condition(child))
                    continue;
                if (child != this)
                    yield return child;
            }
        }

        /// <summary>
        /// Returns all of this element's children of the given type that match the given condition.
        /// Optionally, the entire tree of children (grandchildren) can be searched.
        /// </summary>
        /// <param name="condition">The condition to match</param>
        /// <param name="regardGrandchildren">If this value is true, children of children of this element are also searched</param>
        /// <param name="ignoreFalseGrandchildren">If this value is true, children for which the condition does not match will not have their children searched</param>
        /// <typeparam name="T">The type of children to search for</typeparam>
        /// <returns>All children that match the condition</returns>
        public IEnumerable<T> GetChildren<T>(Func<T, bool> condition = null, bool regardGrandchildren = false, bool ignoreFalseGrandchildren = false) where T : Element {
            foreach (var child in this.Children) {
                var applies = child is T t && (condition == null || condition(t));
                if (applies)
                    yield return (T) child;
                if (regardGrandchildren && (!ignoreFalseGrandchildren || applies)) {
                    foreach (var cc in child.GetChildren(condition, true, ignoreFalseGrandchildren))
                        yield return cc;
                }
            }
        }

        /// <inheritdoc cref="GetChildren{T}"/>
        public IEnumerable<Element> GetChildren(Func<Element, bool> condition = null, bool regardGrandchildren = false, bool ignoreFalseGrandchildren = false) {
            return this.GetChildren<Element>(condition, regardGrandchildren, ignoreFalseGrandchildren);
        }

        /// <summary>
        /// Returns the parent tree of this element.
        /// The parent tree is this element's <see cref="Parent"/>, followed by its parent, and so on, up until the <see cref="RootElement"/>'s <see cref="RootElement.Element"/>.
        /// </summary>
        /// <returns>This element's parent tree</returns>
        public IEnumerable<Element> GetParentTree() {
            if (this.Parent == null)
                yield break;
            yield return this.Parent;
            foreach (var parent in this.Parent.GetParentTree())
                yield return parent;
        }

        /// <summary>
        /// Returns the total covered area of this element, which is its <see cref="Area"/> (or <see cref="UnscrolledArea"/>), unioned with all of the total covered areas of its <see cref="Children"/>.
        /// The returned area is only different from this element's <see cref="Area"/> (or <see cref="UnscrolledArea"/>) if it has any <see cref="Children"/> that are outside of this element's area, or are bigger than this element.
        /// </summary>
        /// <param name="unscrolled">Whether to use elements' <see cref="UnscrolledArea"/> (instead of their <see cref="Area"/>).</param>
        /// <returns>This element's total covered area.</returns>
        public RectangleF GetTotalCoveredArea(bool unscrolled) {
            var ret = unscrolled ? this.UnscrolledArea : this.Area;
            foreach (var child in this.Children) {
                if (!child.IsHidden)
                    ret = RectangleF.Union(ret, child.GetTotalCoveredArea(unscrolled));
            }
            return ret;
        }

        /// <summary>
        /// Returns a subset of <see cref="Children"/> that are currently relevant in terms of drawing and input querying.
        /// A <see cref="Panel"/> only returns elements that are currently in view here.
        /// </summary>
        /// <returns>This element's relevant children</returns>
        protected virtual IList<Element> GetRelevantChildren() {
            return this.SortedChildren;
        }

        /// <summary>
        /// Updates this element and all of its <see cref="SortedChildren"/>
        /// </summary>
        /// <param name="time">The game's time</param>
        public virtual void Update(GameTime time) {
            this.System.InvokeOnElementUpdated(this, time);

            for (var i = this.PlayingAnimations.Count - 1; i >= 0; i--) {
                var anim = this.PlayingAnimations[i];
                if (anim.Update(this, time)) {
                    anim.OnFinished(this);
                    this.PlayingAnimations.RemoveAt(i);
                }
            }

            // update all sorted children, not just relevant ones, because they might become relevant or irrelevant through updates
            foreach (var child in this.SortedChildren) {
                if (child.System != null)
                    child.Update(time);
            }

            if (this.System != null)
                this.System.Metrics.Updates++;
        }

        /// <summary>
        /// Draws this element by calling <see cref="Draw(Microsoft.Xna.Framework.GameTime,Microsoft.Xna.Framework.Graphics.SpriteBatch,float,MLEM.Graphics.SpriteBatchContext)"/> internally.
        /// If <see cref="Transform"/> is set, a new <c>SpriteBatch.Begin</c> call is also started.
        /// </summary>
        /// <param name="time">The game's time</param>
        /// <param name="batch">The sprite batch to use for drawing</param>
        /// <param name="alpha">The alpha to draw this element and its children with</param>
        /// <param name="context">The sprite batch context to use for drawing</param>
        public void DrawTransformed(GameTime time, SpriteBatch batch, float alpha, SpriteBatchContext context) {
            var customDraw = this.Transform != Matrix.Identity;
            var transformed = context;
            transformed.TransformMatrix = this.Transform * transformed.TransformMatrix;
            // TODO ending and beginning again when the matrix changes isn't ideal (https://github.com/MonoGame/MonoGame/issues/3156)
            if (customDraw) {
                // end the usual draw so that we can begin our own
                batch.End();
                // begin our own draw call
                batch.Begin(transformed);
            }

            // draw content in custom begin call
            this.Draw(time, batch, alpha, transformed);
            if (this.System != null)
                this.System.Metrics.Draws++;

            if (customDraw) {
                // end our draw
                batch.End();
                // begin the usual draw again for other elements
                batch.Begin(context);
            }
        }

        /// <summary>
        /// Draws this element and all of its children. Override this method to draw the content of custom elements.
        /// Note that, when this is called, <c>SpriteBatch.Begin</c> has already been called with custom <see cref="Transform"/> etc. applied.
        /// </summary>
        /// <param name="time">The game's time</param>
        /// <param name="batch">The sprite batch to use for drawing</param>
        /// <param name="alpha">The alpha to draw this element and its children with</param>
        /// <param name="context">The sprite batch context to use for drawing</param>
        public virtual void Draw(GameTime time, SpriteBatch batch, float alpha, SpriteBatchContext context) {
            this.System.InvokeOnElementDrawn(this, time, batch, alpha, context);
            if (this.IsSelected)
                this.System.InvokeOnSelectedElementDrawn(this, time, batch, alpha, context);

            foreach (var child in this.GetRelevantChildren()) {
                if (!child.IsHidden)
                    child.DrawTransformed(time, batch, alpha * child.DrawAlpha, context);
            }
        }

        /// <summary>
        /// Returns the element under the given position, searching the current element and all of its <see cref="GetRelevantChildren"/>.
        /// </summary>
        /// <param name="position">The position to query</param>
        /// <returns>The element under the position, or null if no such element exists</returns>
        public virtual Element GetElementUnderPos(Vector2 position) {
            if (this.IsHidden)
                return null;
            position = this.TransformInverse(position);
            var relevant = this.GetRelevantChildren();
            for (var i = relevant.Count - 1; i >= 0; i--) {
                var element = relevant[i].GetElementUnderPos(position);
                if (element != null)
                    return element;
            }
            return this.CanBeMoused && this.DisplayArea.Contains(position) ? this : null;
        }

        /// <summary>
        /// Plays the given <see cref="UiAnimation"/> on this element, causing it to be added to the <see cref="PlayingAnimations"/> and updated in <see cref="Update"/>.
        /// If the given <paramref name="animation"/> is already playing on this element, it will be restarted.
        /// </summary>
        /// <param name="animation">The animation to play.</param>
        public virtual void PlayAnimation(UiAnimation animation) {
            if (this.PlayingAnimations.Contains(animation)) {
                // if we're already playing this animation, just restart it
                animation.OnFinished(this);
            } else {
                this.PlayingAnimations.Add(animation);
            }
        }

        /// <summary>
        /// Stops the given <see cref="UiAnimation"/> on this element, causing it to be removed from the <see cref="PlayingAnimations"/> and <see cref="UiAnimation.OnFinished"/> to be invoked.
        /// </summary>
        /// <param name="animation">The animation to stop.</param>
        /// <returns>Whether the animation was present in this element's <see cref="PlayingAnimations"/>.</returns>
        public virtual bool StopAnimation(UiAnimation animation) {
            if (this.PlayingAnimations.Remove(animation)) {
                animation.OnFinished(this);
                return true;
            }
            return false;
        }

        /// <inheritdoc />
        public override string ToString() {
            var ret = this.GetType().Name;
            // elements will contain their path up to the root and their index in each parent
            // eg Paragraph 2 @ Panel 3 @ ... @ Group RootName
            if (this.Parent != null) {
                ret += $" {this.Parent.Children.IndexOf(this)} @ {this.Parent}";
            } else if (this.Root?.Element == this) {
                ret += $" {this.Root.Name}";
            }
            return ret;
        }

        /// <summary>
        /// Performs the specified action on this element and all of its <see cref="Children"/>
        /// </summary>
        /// <param name="action">The action to perform</param>
        public void AndChildren(Action<Element> action) {
            action(this);
            foreach (var child in this.Children)
                child.AndChildren(action);
        }

        /// <summary>
        /// Sorts this element's <see cref="Children"/> using the given comparison.
        /// </summary>
        /// <param name="comparison">The comparison to sort by</param>
        public void ReorderChildren(Comparison<Element> comparison) {
            this.children.Sort(comparison);
        }

        /// <summary>
        /// Reverses this element's <see cref="Children"/> list in the given range.
        /// </summary>
        /// <param name="index">The index to start reversing at</param>
        /// <param name="count">The amount of elements to reverse</param>
        public void ReverseChildren(int index = 0, int? count = null) {
            this.children.Reverse(index, count ?? this.Children.Count);
        }

        /// <summary>
        /// Scales this element's <see cref="Transform"/> matrix based on the given scale and origin.
        /// </summary>
        /// <param name="scale">The scale to use</param>
        /// <param name="origin">The origin to use for scaling, or null to use this element's center point</param>
        public void ScaleTransform(float scale, Vector2? origin = null) {
            this.Transform = Matrix.CreateScale(scale, scale, 1) * Matrix.CreateTranslation(new Vector3((1 - scale) * (origin ?? this.DisplayArea.Center), 0));
        }

        /// <summary>
        /// Initializes this element's <see cref="StyleProp{T}"/> instances using the ui system's <see cref="UiStyle"/>.
        /// </summary>
        /// <param name="style">The new style</param>
        protected virtual void InitStyle(UiStyle style) {
            this.SelectionIndicator = this.SelectionIndicator.OrStyle(style.SelectionIndicator);
            this.ActionSound = this.ActionSound.OrStyle(style.ActionSound);
            this.SecondActionSound = this.SecondActionSound.OrStyle(style.ActionSound);
            this.MouseEnterAnimation = this.MouseEnterAnimation.OrStyle(style.MouseEnterAnimation);
            this.MouseExitAnimation = this.MouseExitAnimation.OrStyle(style.MouseExitAnimation);

            this.System?.InvokeOnElementStyleInit(this);
            style.ApplyCustomStyle(this);
        }

        /// <summary>
        /// A method that gets called by this element's <see cref="Children"/> or any of its grandchildren when their <see cref="SetAreaDirty"/> methods get called.
        /// Note that the element's area might already be dirty, which will not stop this method from being called.
        /// </summary>
        /// <param name="child">The child whose area is being set dirty.</param>
        /// <param name="grandchild">Whether the <paramref name="child"/> is a grandchild of this element, rather than a direct child.</param>
        protected virtual void OnChildAreaDirty(Element child, bool grandchild) {
            if (!grandchild) {
                if (child.Anchor.IsAuto() || child.PreventParentSpill || this.SetWidthBasedOnChildren || this.SetHeightBasedOnChildren)
                    this.SetAreaDirty();
            }
            this.Parent?.OnChildAreaDirty(child, true);
        }

        /// <summary>
        /// Transforms the given <paramref name="position"/> by the inverse of this element's <see cref="Transform"/> matrix.
        /// </summary>
        /// <param name="position">The position to transform</param>
        /// <returns>The transformed position</returns>
        protected Vector2 TransformInverse(Vector2 position) {
            return this.Transform != Matrix.Identity ? Vector2.Transform(position, Matrix.Invert(this.Transform)) : position;
        }

        /// <summary>
        /// Transforms the given <paramref name="position"/> by this element's <see cref="Root"/>'s <see cref="RootElement.InvTransform"/>, the inverses of all of the <see cref="Transform"/> matrices of this element's parent tree (<see cref="GetParentTree"/>), and the inverse of this element's <see cref="Transform"/> matrix.
        /// Note that, when using <see cref="UiControls.GetElementUnderPos"/>, this operation is done recursively, which is more efficient.
        /// </summary>
        /// <param name="position">The position to transform</param>
        /// <returns>The transformed position</returns>
        protected Vector2 TransformInverseAll(Vector2 position) {
            position = Vector2.Transform(position, this.Root.InvTransform);
            foreach (var parent in this.GetParentTree().Reverse())
                position = parent.TransformInverse(position);
            return this.TransformInverse(position);
        }

        /// <summary>
        /// Called when this element is added to a <see cref="UiSystem"/> and, optionally, a given <see cref="RootElement"/>.
        /// This method is called in <see cref="AddChild{T}"/> and <see cref="UiSystem.Add"/>.
        /// </summary>
        /// <param name="system">The ui system to add to.</param>
        /// <param name="root">The root element to add to.</param>
        protected internal virtual void AddedToUi(UiSystem system, RootElement root) {
            this.Root = root;
            this.System = system;
            this.OnAddedToUi?.Invoke(this);
            root?.InvokeOnElementAdded(this);
        }

        /// <summary>
        /// Called when this element is removed from a <see cref="UiSystem"/> and <see cref="RootElement"/>.
        /// This method is called in <see cref="RemoveChild"/> and <see cref="UiSystem.Remove"/>.
        /// </summary>
        protected internal virtual void RemovedFromUi() {
            var root = this.Root;
            this.Root = null;
            this.System = null;
            this.OnRemovedFromUi?.Invoke(this);
            root?.InvokeOnElementRemoved(this);
        }

        /// <summary>
        /// A delegate used for the <see cref="Element.OnTextInput"/> event.
        /// </summary>
        /// <param name="element">The current element</param>
        /// <param name="key">The key that was pressed</param>
        /// <param name="character">The character that was input</param>
        public delegate void TextInputCallback(Element element, Keys key, char character);

        /// <summary>
        /// A generic element-specific delegate.
        /// </summary>
        /// <param name="element">The current element</param>
        public delegate void GenericCallback(Element element);

        /// <summary>
        /// A generic element-specific delegate that includes a second element.
        /// </summary>
        /// <param name="thisElement">The current element</param>
        /// <param name="otherElement">The other element</param>
        public delegate void OtherElementCallback(Element thisElement, Element otherElement);

        /// <summary>
        /// A delegate used inside of <see cref="Element.Draw(Microsoft.Xna.Framework.GameTime,Microsoft.Xna.Framework.Graphics.SpriteBatch,float,MLEM.Graphics.SpriteBatchContext)"/>
        /// </summary>
        /// <param name="element">The element that is being drawn</param>
        /// <param name="time">The game's time</param>
        /// <param name="batch">The sprite batch used for drawing</param>
        /// <param name="alpha">The alpha this element is drawn with</param>
        /// <param name="context">The sprite batch context to use for drawing</param>
        public delegate void DrawCallback(Element element, GameTime time, SpriteBatch batch, float alpha, SpriteBatchContext context);

        /// <summary>
        /// A generic delegate used inside of <see cref="Element.Update"/>
        /// </summary>
        /// <param name="element">The current element</param>
        /// <param name="time">The game's time</param>
        public delegate void TimeCallback(Element element, GameTime time);

        /// <summary>
        /// A delegate used by <see cref="Element.GetTabNextElement"/>.
        /// </summary>
        /// <param name="backward">If this value is true, <see cref="ModifierKey.Shift"/> is being held</param>
        /// <param name="usualNext">The element that is considered to be the next element by default</param>
        public delegate Element TabNextElementCallback(bool backward, Element usualNext);

        /// <summary>
        /// A delegate used by <see cref="Element.GetGamepadNextElement"/>.
        /// </summary>
        /// <param name="dir">The direction of the gamepad button that was pressed</param>
        /// <param name="usualNext">The element that is considered to be the next element by default</param>
        public delegate Element GamepadNextElementCallback(Direction2 dir, Element usualNext);

    }
}