using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Audio;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;
using MLEM.Extensions;
using MLEM.Input;
using MLEM.Misc;
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>
        /// 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>
        protected readonly List<Element> Children = new List<Element>();
        private readonly List<Element> sortedChildren = new List<Element>();
        /// <summary>
        /// A sorted version of <see cref="Children"/>. The children are sorted by their <see cref="Priority"/>.
        /// </summary>
        protected List<Element> SortedChildren {
            get {
                this.UpdateSortedChildrenIfDirty();
                return this.sortedChildren;
            }
        }
        private bool sortedChildrenDirty;

        private UiSystem system;
        /// <summary>
        /// The ui system that this element is currently a part of
        /// </summary>
        public UiSystem System {
            get => this.system;
            internal set {
                this.system = value;
                this.Controls = value?.Controls;
                if (this.system != null)
                    this.InitStyle(this.system.Style);
            }
        }
        /// <summary>
        /// The controls that this element's <see cref="System"/> uses
        /// </summary>
        public UiControls Controls;
        /// <summary>
        /// The input handler that this element's <see cref="Controls"/> use
        /// </summary>
        protected InputHandler Input => this.Controls.Input;
        /// <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; internal set; }
        /// <summary>
        /// The scale that this ui element renders with
        /// </summary>
        public float Scale => this.Root.ActualScale;

        private Anchor anchor;
        /// <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();
            }
        }

        private Vector2 size;
        /// <summary>
        /// The size of this element.
        /// If the x or y value of the size is lower than or equal to 1, the size will be seen as a percentage of its parent's size.
        /// </summary>
        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;

        private Vector2 offset;
        /// <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 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 Padding Padding;
        /// <summary>
        /// The <see cref="Padding"/>, but with <see cref="Scale"/> applied.
        /// </summary>
        public Padding ScaledPadding => this.Padding * this.Scale;

        private Padding childPadding;
        /// <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 Padding ChildPadding {
            get => this.childPadding;
            set {
                if (this.childPadding == value)
                    return;
                this.childPadding = value;
                this.SetAreaDirty();
            }
        }
        /// <summary>
        /// The <see cref="ChildPadding"/>, but with <see cref="Scale"/> applied.
        /// </summary>
        public Padding ScaledChildPadding => this.childPadding * 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);

        private RectangleF area;
        /// <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;
            }
        }
        private bool areaDirty;
        /// <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;

        private bool isHidden;
        /// <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 bool IsHidden {
            get => this.isHidden;
            set {
                if (this.isHidden == value)
                    return;
                this.isHidden = value;
                this.SetAreaDirty();
            }
        }

        private int priority;
        /// <summary>
        /// The priority of this element as part of its <see cref="Parent"/> element.
        /// A higher priority means the element will be drawn first and, if auto-anchoring is used, anchored higher up within its parent.
        /// </summary>
        public int Priority {
            get => this.priority;
            set {
                this.priority = value;
                if (this.Parent != null)
                    this.Parent.SetSortedChildrenDirty();
            }
        }

        /// <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 bool CanBeSelected = true;
        /// <summary>
        /// Set this field to false to disallow the element from reacting to being moused over.
        /// </summary>
        public bool CanBeMoused = 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 bool CanBePressed = true;
        /// <summary>
        /// Set this field to false to cause auto-anchored siblings to ignore this element as a possible anchor point.
        /// </summary>
        public bool CanAutoAnchorsAttach = true;
        /// <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.
        /// </summary>
        public bool SetHeightBasedOnChildren;
        /// <summary>
        /// Set this field to true to cause this element's width to be automatically calculated based on the area that is <see cref="Children"/> take up.
        /// </summary>
        public bool SetWidthBasedOnChildren;
        /// <summary>
        /// The transparency (alpha value) that this element is rendered with.
        /// Note that, when <see cref="Draw"/> 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 float DrawAlpha = 1;

        /// <summary>
        /// Stores whether this element is currently being moused over.
        /// </summary>
        public bool IsMouseOver { get; private set; }
        /// <summary>
        /// Stores whether this element is its <see cref="Root"/>'s <see cref="RootElement.SelectedElement"/>.
        /// </summary>
        public bool IsSelected { get; private set; }

        /// <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 text input is made.
        /// Note that this event is called for every element, even if it is not selected.
        /// Also note that if <see cref="TextInputWrapper.RequiresOnScreenKeyboard"/> is true, 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 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 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}"/>
        /// </summary>
        public OtherElementCallback OnChildAdded;
        /// <summary>
        /// Event that is called when a child is removed from this element using <see cref="RemoveChild"/>
        /// </summary>
        public OtherElementCallback OnChildRemoved;

        /// <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<SoundEffectInstance> 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<SoundEffectInstance> SecondActionSound;

        /// <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.OnMouseEnter += element => this.IsMouseOver = true;
            this.OnMouseExit += element => this.IsMouseOver = false;
            this.OnSelected += element => this.IsSelected = true;
            this.OnDeselected += element => this.IsSelected = false;
            this.GetTabNextElement += (backward, next) => next;
            this.GetGamepadNextElement += (dir, next) => next;

            this.SetAreaDirty();
        }

        /// <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 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.Root = this.Root;
                e.System = this.System;
                this.Root?.OnElementAdded(e);
                this.OnChildAdded?.Invoke(this, e);
            });
            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 void RemoveChild(Element element) {
            this.Children.Remove(element);
            // 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.Root = null;
                e.System = null;
                this.Root?.OnElementRemoved(e);
                this.OnChildRemoved?.Invoke(this, e);
            });
            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 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.Clear();
            this.sortedChildren.AddRange(this.Children);
            this.sortedChildren.Sort((e1, e2) => e1.Priority.CompareTo(e2.Priority));
        }

        /// <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;
            if (this.Parent != null && (this.Anchor >= Anchor.AutoLeft || this.Parent.SetWidthBasedOnChildren || this.Parent.SetHeightBasedOnChildren))
                this.Parent.SetAreaDirty();
        }

        /// <summary>
        /// Updates this element's <see cref="Area"/> list if <see cref="areaDirty"/> is true.
        /// </summary>
        public void UpdateAreaIfDirty() {
            if (this.areaDirty)
                this.ForceUpdateArea();
        }

        /// <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)
                return;

            var parentArea = this.Parent != null ? this.Parent.ChildPaddedArea : (RectangleF) this.system.Viewport;
            var parentCenterX = parentArea.X + parentArea.Width / 2;
            var parentCenterY = parentArea.Y + parentArea.Height / 2;

            var actualSize = this.CalcActualSize(parentArea);
            var pos = new Vector2();

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

            if (this.Anchor >= Anchor.AutoLeft) {
                Element previousChild;
                if (this.Anchor == Anchor.AutoInline || this.Anchor == Anchor.AutoInlineIgnoreOverflow) {
                    previousChild = this.GetOlderSibling(e => !e.IsHidden && e.CanAutoAnchorsAttach);
                } else {
                    previousChild = this.GetLowestOlderSibling(e => !e.IsHidden && e.CanAutoAnchorsAttach);
                }
                if (previousChild != null) {
                    var prevArea = previousChild.GetAreaForAutoAnchors();
                    switch (this.Anchor) {
                        case Anchor.AutoLeft:
                        case Anchor.AutoCenter:
                        case Anchor.AutoRight:
                            pos.Y = prevArea.Bottom + this.ScaledOffset.Y;
                            break;
                        case Anchor.AutoInline:
                            var newX = prevArea.Right + this.ScaledOffset.X;
                            if (newX + actualSize.X <= parentArea.Right) {
                                pos.X = newX;
                                pos.Y = prevArea.Y + this.ScaledOffset.Y;
                            } else {
                                pos.Y = prevArea.Bottom + this.ScaledOffset.Y;
                            }
                            break;
                        case Anchor.AutoInlineIgnoreOverflow:
                            pos.X = prevArea.Right + this.ScaledOffset.X;
                            pos.Y = prevArea.Y;
                            break;
                    }
                }
            }

            this.area = new RectangleF(pos, actualSize);
            this.System.OnElementAreaUpdated?.Invoke(this);

            foreach (var child in this.Children)
                child.ForceUpdateArea();

            if (this.Children.Count > 0) {
                var changed = false;
                if (this.SetHeightBasedOnChildren) {
                    var lowest = this.GetLowestChild(e => !e.IsHidden);
                    if (lowest != null) {
                        var newHeight = (lowest.UnscrolledArea.Bottom - pos.Y + this.ScaledChildPadding.Bottom) / this.Scale;
                        if (!newHeight.Equals(this.size.Y, 0.01F)) {
                            this.size.Y = newHeight;
                            changed = true;
                        }
                    }
                }
                if (this.SetWidthBasedOnChildren) {
                    var rightmost = this.GetRightmostChild(e => !e.IsHidden);
                    if (rightmost != null) {
                        var newWidth = (rightmost.UnscrolledArea.Right - pos.X + this.ScaledChildPadding.Right) / this.Scale;
                        if (!newWidth.Equals(this.size.X, 0.01F)) {
                            this.size.X = newWidth;
                            changed = true;
                        }
                    }
                }
                if (changed)
                    this.ForceUpdateArea();
            }
        }

        /// <summary>
        /// Calculates the actual size that this element should take up, based on the area that its parent encompasses.
        /// </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) {
            return new Vector2(
                this.size.X > 1 ? this.ScaledSize.X : parentArea.Width * this.size.X,
                this.size.Y > 1 ? this.ScaledSize.Y : parentArea.Height * this.size.Y);
        }

        /// <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>
        /// <returns>The lowest element, or null if no such element exists</returns>
        public Element GetLowestChild(Func<Element, bool> condition = null) {
            Element lowest = null;
            foreach (var child in this.Children) {
                if (condition != null && !condition(child))
                    continue;
                if (child.Anchor > Anchor.TopRight && child.Anchor < Anchor.AutoLeft)
                    continue;
                if (lowest == null || child.UnscrolledArea.Bottom >= lowest.UnscrolledArea.Bottom)
                    lowest = child;
            }
            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>
        /// <returns>The rightmost element, or null if no such element exists</returns>
        public Element GetRightmostChild(Func<Element, bool> condition = null) {
            Element rightmost = null;
            foreach (var child in this.Children) {
                if (condition != null && !condition(child))
                    continue;
                if (child.Anchor < Anchor.AutoLeft && child.Anchor != Anchor.TopLeft && child.Anchor != Anchor.CenterLeft && child.Anchor != Anchor.BottomLeft)
                    continue;
                if (rightmost == null || child.UnscrolledArea.Right >= rightmost.UnscrolledArea.Right)
                    rightmost = child;
            }
            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>
        /// <returns>The lowest older sibling of this element, or null if no such element exists</returns>
        public Element GetLowestOlderSibling(Func<Element, bool> condition = null) {
            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 || 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 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 List<Element> GetRelevantChildren() {
            return this.SortedChildren;
        }

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

            foreach (var child in this.GetRelevantChildren())
                if (child.System != null)
                    child.Update(time);
        }

        /// <summary>
        /// Draws this element and all of its <see cref="GetRelevantChildren"/>
        /// Note that, when this is called, <see cref="SpriteBatch.Begin"/> has already been called.
        /// </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="blendState">The blend state that is used for drawing</param>
        /// <param name="samplerState">The sampler state that is used for drawing</param>
        /// <param name="matrix">The transformation matrix that is used for drawing</param>
        public virtual void Draw(GameTime time, SpriteBatch batch, float alpha, BlendState blendState, SamplerState samplerState, Matrix matrix) {
            this.System.OnElementDrawn?.Invoke(this, time, batch, alpha);
            if (this.Controls.SelectedElement == this)
                this.System.OnSelectedElementDrawn?.Invoke(this, time, batch, alpha);

            foreach (var child in this.GetRelevantChildren()) {
                if (!child.IsHidden)
                    child.Draw(time, batch, alpha * child.DrawAlpha, blendState, samplerState, matrix);
            }
        }

        /// <summary>
        /// Draws this element and all of its <see cref="GetRelevantChildren"/> early.
        /// Drawing early involves drawing onto <see cref="RenderTarget2D"/> instances rather than onto the screen.
        /// Note that, when this is called, <see cref="SpriteBatch.Begin"/> has not yet been called.
        /// </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="blendState">The blend state that is used for drawing</param>
        /// <param name="samplerState">The sampler state that is used for drawing</param>
        /// <param name="matrix">The transformation matrix that is used for drawing</param>
        public virtual void DrawEarly(GameTime time, SpriteBatch batch, float alpha, BlendState blendState, SamplerState samplerState, Matrix matrix) {
            foreach (var child in this.GetRelevantChildren()) {
                if (!child.IsHidden)
                    child.DrawEarly(time, batch, alpha * child.DrawAlpha, blendState, samplerState, matrix);
            }
        }

        /// <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;
            var children = this.GetRelevantChildren();
            for (var i = children.Count - 1; i >= 0; i--) {
                var element = children[i].GetElementUnderPos(position);
                if (element != null)
                    return element;
            }
            return this.CanBeMoused && this.DisplayArea.Contains(position) ? this : null;
        }

        /// <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>
        /// 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.SetFromStyle(style.SelectionIndicator);
            this.ActionSound.SetFromStyle(style.ActionSound?.CreateInstance());
            this.SecondActionSound.SetFromStyle(style.ActionSound?.CreateInstance());
        }

        /// <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"/>
        /// </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>
        public delegate void DrawCallback(Element element, GameTime time, SpriteBatch batch, float alpha);

        /// <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);

    }
}