MLEM/MLEM.Ui/Elements/Panel.cs

using System;
using System.Diagnostics;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;
using MLEM.Extensions;
using MLEM.Textures;
using MLEM.Ui.Style;

namespace MLEM.Ui.Elements {
    public class Panel : Element {

        public NinePatch Texture;
        public readonly ScrollBar ScrollBar;
        private readonly bool scrollOverflow;
        private RenderTarget2D renderTarget;

        public Panel(Anchor anchor, Vector2 size, Vector2 positionOffset, bool setHeightBasedOnChildren = false, bool scrollOverflow = false, Point? scrollerSize = null) : base(anchor, size) {
            this.PositionOffset = positionOffset;
            this.SetHeightBasedOnChildren = setHeightBasedOnChildren;
            this.scrollOverflow = scrollOverflow;
            this.ChildPadding = new Point(5);

            if (scrollOverflow) {
                var scrollSize = scrollerSize ?? Point.Zero;
                this.ScrollBar = new ScrollBar(Anchor.TopRight, new Vector2(scrollSize.X, 1), scrollSize.Y, 0) {
                    StepPerScroll = 10,
                    OnValueChanged = (element, value) => {
                        var firstChild = this.Children[0];
                        // if the first child is the scrollbar, there are no other children
                        if (firstChild == element)
                            return;
                        // as all children have to be auto-aligned, moving the first one up will move all others
                        firstChild.PositionOffset = new Vector2(firstChild.PositionOffset.X, -value.Ceil());
                        this.ForceUpdateArea();
                    }
                };
                this.AddChild(this.ScrollBar);

                // modify the padding so that the scroll bar isn't over top of something else
                this.ScrollBar.PositionOffset -= new Vector2(scrollSize.X + 1, 0);
                this.ChildPadding += new Point(scrollSize.X, 0);
            }
        }

        public override void ForceUpdateArea() {
            if (this.scrollOverflow) {
                // sanity check
                if (this.SetHeightBasedOnChildren)
                    throw new NotSupportedException("A panel can't both set height based on children and scroll overflow");
                foreach (var child in this.Children) {
                    if (child != this.ScrollBar && child.Anchor < Anchor.AutoLeft)
                        throw new NotSupportedException($"A panel that handles overflow can't contain non-automatic anchors ({child})");
                    if (child is Panel panel && panel.scrollOverflow)
                        throw new NotSupportedException($"A panel that scrolls overflow cannot contain another panel that scrolls overflow ({child})");
                }

                // move the scrollbar to the front so it isn't used for auto-aligning
                this.ScrollBar.MoveToFront();
            }

            base.ForceUpdateArea();

            if (this.scrollOverflow) {
                var firstChild = this.Children[0];
                // if the first child is the scrollbar, then we know there's no other children
                if (firstChild == this.ScrollBar)
                    return;
                var lastChild = this.Children[this.Children.Count - 2];
                // the max value of the scrollbar is the amount of non-scaled pixels taken up by overflowing components
                var childrenHeight = lastChild.Area.Bottom - firstChild.Area.Top;
                this.ScrollBar.MaxValue = (childrenHeight - this.Area.Height) / this.Scale + this.ChildPadding.Y * 2;

                // update the render target
                var targetArea = this.GetRenderTargetArea();
                if (this.renderTarget == null || targetArea.Width != this.renderTarget.Width || targetArea.Height != this.renderTarget.Height) {
                    if (this.renderTarget != null)
                        this.renderTarget.Dispose();
                    this.renderTarget = new RenderTarget2D(this.System.GraphicsDevice, targetArea.Width, targetArea.Height);
                }
            }
        }

        public override void Draw(GameTime time, SpriteBatch batch, float alpha, Point offset) {
            batch.Draw(this.Texture, this.DisplayArea.OffsetCopy(offset), Color.White * alpha, this.Scale);
            // if we handle overflow, draw using the render target in DrawUnbound
            if (!this.scrollOverflow) {
                base.Draw(time, batch, alpha, offset);
            } else {
                // draw the actual render target (don't apply the alpha here because it's already drawn onto with alpha)
                batch.Draw(this.renderTarget, this.GetRenderTargetArea().OffsetCopy(offset), Color.White);
            }
        }

        public override void DrawEarly(GameTime time, SpriteBatch batch, float alpha, BlendState blendState = null, SamplerState samplerState = null) {
            if (this.scrollOverflow) {
                // draw children onto the render target
                batch.GraphicsDevice.SetRenderTarget(this.renderTarget);
                batch.GraphicsDevice.Clear(Color.Transparent);
                batch.Begin(SpriteSortMode.Deferred, blendState, samplerState);
                // offset children by the render target's location
                var area = this.GetRenderTargetArea();
                base.Draw(time, batch, alpha, new Point(-area.X, -area.Y));
                batch.End();
                batch.GraphicsDevice.SetRenderTarget(null);
            }
            base.DrawEarly(time, batch, alpha, blendState, samplerState);
        }

        private Rectangle GetRenderTargetArea() {
            var area = this.ChildPaddedArea;
            area.X = this.DisplayArea.X;
            area.Width = this.DisplayArea.Width;
            return area;
        }

        protected override void InitStyle(UiStyle style) {
            base.InitStyle(style);
            this.Texture = style.PanelTexture;
        }

    }
}