From 6766d769f4a07c87064a96419587d9a72e36a89b Mon Sep 17 00:00:00 2001
From: Ellpeck <megamaximal@gmail.com>
Date: Sat, 24 Aug 2019 22:27:47 +0200
Subject: [PATCH] made scrolling a lot less intensive

---
 MLEM.Ui/Elements/Element.cs | 15 +++++++++------
 MLEM.Ui/Elements/Panel.cs   | 20 ++++++++++----------
 2 files changed, 19 insertions(+), 16 deletions(-)

diff --git a/MLEM.Ui/Elements/Element.cs b/MLEM.Ui/Elements/Element.cs
index c8d55f9..f0e811a 100644
--- a/MLEM.Ui/Elements/Element.cs
+++ b/MLEM.Ui/Elements/Element.cs
@@ -66,7 +66,7 @@ namespace MLEM.Ui.Elements {
         }
         public Rectangle ChildPaddedArea {
             get {
-                var padded = this.Area;
+                var padded = this.UnscrolledArea;
                 padded.Location += this.ScaledChildPadding;
                 padded.Width -= this.ScaledChildPadding.X * 2;
                 padded.Height -= this.ScaledChildPadding.Y * 2;
@@ -120,12 +120,15 @@ namespace MLEM.Ui.Elements {
         public bool CanAutoAnchorsAttach = true;
 
         private Rectangle area;
-        public Rectangle Area {
+        public Rectangle UnscrolledArea {
             get {
                 this.UpdateAreaIfDirty();
                 return this.area;
             }
         }
+        public Rectangle Area => this.UnscrolledArea.OffsetCopy(this.ScaledScrollOffset);
+        public Point ScrollOffset;
+        public Point ScaledScrollOffset => this.ScrollOffset.Multiply(this.Scale);
         public Rectangle DisplayArea {
             get {
                 var padded = this.Area;
@@ -302,7 +305,7 @@ namespace MLEM.Ui.Elements {
 
             if (this.SetHeightBasedOnChildren && this.Children.Count > 0) {
                 var lowest = this.GetLowestChild(false, true);
-                var newHeight = (lowest.area.Bottom - pos.Y + this.ScaledChildPadding.Y) / this.Scale;
+                var newHeight = (lowest.UnscrolledArea.Bottom - pos.Y + this.ScaledChildPadding.Y) / this.Scale;
                 if (newHeight != this.size.Y) {
                     this.size.Y = newHeight;
                     if (this.Anchor > Anchor.TopRight)
@@ -318,7 +321,7 @@ namespace MLEM.Ui.Elements {
         }
 
         protected virtual Rectangle GetAreaForAutoAnchors() {
-            return this.Area;
+            return this.UnscrolledArea;
         }
 
         public Element GetLowestChild(bool hiddenAlso, bool unattachableAlso) {
@@ -330,7 +333,7 @@ namespace MLEM.Ui.Elements {
                     continue;
                 if (child.Anchor > Anchor.TopRight && child.Anchor < Anchor.AutoLeft)
                     continue;
-                if (lowest == null || child.Area.Bottom > lowest.Area.Bottom)
+                if (lowest == null || child.UnscrolledArea.Bottom > lowest.UnscrolledArea.Bottom)
                     lowest = child;
             }
             return lowest;
@@ -345,7 +348,7 @@ namespace MLEM.Ui.Elements {
                     break;
                 if (!hiddenAlso && child.IsHidden || !unattachableAlso && !child.CanAutoAnchorsAttach)
                     continue;
-                if (lowest == null || child.Area.Bottom >= lowest.Area.Bottom)
+                if (lowest == null || child.UnscrolledArea.Bottom >= lowest.UnscrolledArea.Bottom)
                     lowest = child;
             }
             return lowest;
diff --git a/MLEM.Ui/Elements/Panel.cs b/MLEM.Ui/Elements/Panel.cs
index bfde385..54327e3 100644
--- a/MLEM.Ui/Elements/Panel.cs
+++ b/MLEM.Ui/Elements/Panel.cs
@@ -24,16 +24,7 @@ namespace MLEM.Ui.Elements {
                 var scrollSize = scrollerSize ?? Point.Zero;
                 this.ScrollBar = new ScrollBar(Anchor.TopRight, new Vector2(scrollSize.X, 1), scrollSize.Y, 0) {
                     StepPerScroll = 10,
-                    OnValueChanged = (element, value) => {
-                        // if there is only one child, then we have just the scroll bar
-                        if (this.Children.Count == 1)
-                            return;
-                        // the "real" first child is the scroll bar, which we want to ignore
-                        var firstChild = this.Children[1];
-                        // 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.Floor());
-                        this.ForceUpdateArea();
-                    },
+                    OnValueChanged = (element, value) => this.ForceChildrenScroll(),
                     CanAutoAnchorsAttach = false
                 };
                 this.AddChild(this.ScrollBar);
@@ -58,6 +49,7 @@ namespace MLEM.Ui.Elements {
             }
 
             base.ForceUpdateArea();
+            this.ForceChildrenScroll();
 
             if (this.scrollOverflow) {
                 // if there is only one child, then we have just the scroll bar
@@ -80,6 +72,14 @@ namespace MLEM.Ui.Elements {
             }
         }
 
+        private void ForceChildrenScroll() {
+            if (!this.scrollOverflow)
+                return;
+            var offset = -this.ScrollBar.CurrentValue.Floor();
+            foreach (var child in this.GetChildren(c => c != this.ScrollBar, true))
+                child.ScrollOffset = new Point(0, offset);
+        }
+
         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