Further refactor and simplify ChannelScrollContainer

osu.Game/Overlays/Chat/ChannelScrollContainer.cs

@@ -0,0 +1,78 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using osu.Framework.Graphics;
+using osu.Framework.Utils;
+using osu.Game.Graphics.Containers;
+
+namespace osu.Game.Overlays.Chat
+{
+    /// <summary>
+    /// An <see cref="OsuScrollContainer"/> with functionality to automatically scroll whenever the maximum scrollable distance increases.
+    /// </summary>
+    public class ChannelScrollContainer : OsuScrollContainer
+    {
+        /// <summary>
+        /// The chat will be automatically scrolled to end if and only if
+        /// the distance between the current scroll position and the end of the scroll
+        /// is less than this value.
+        /// </summary>
+        private const float auto_scroll_leniency = 10f;
+
+        private bool trackNewContent = true;
+
+        protected override void Update()
+        {
+            base.Update();
+
+            // If our behaviour hasn't been overriden and there has been new content added to the container, we should update our scroll position to track it.
+            bool requiresScrollUpdate = trackNewContent && !IsScrolledToEnd();
+
+            if (requiresScrollUpdate)
+            {
+                // Schedule required to allow FillFlow to be the correct size.
+                Schedule(() =>
+                {
+                    if (trackNewContent)
+                    {
+                        if (Current < ScrollableExtent)
+                            ScrollToEnd();
+                    }
+                });
+            }
+        }
+
+        private void updateTrackState() => trackNewContent = IsScrolledToEnd(auto_scroll_leniency);
+
+        // todo: we may eventually want this encapsulated in a "OnScrollChange" event handler method provided by ScrollContainer.
+        // important to note that this intentionally doesn't consider OffsetScrollPosition, but could make it do so with side changes.
+
+        #region Scroll handling
+
+        protected override void OnUserScroll(float value, bool animated = true, double? distanceDecay = null)
+        {
+            base.OnUserScroll(value, animated, distanceDecay);
+            updateTrackState();
+        }
+
+        public new void ScrollIntoView(Drawable d, bool animated = true)
+        {
+            base.ScrollIntoView(d, animated);
+            updateTrackState();
+        }
+
+        public new void ScrollToStart(bool animated = true, bool allowDuringDrag = false)
+        {
+            base.ScrollToStart(animated, allowDuringDrag);
+            updateTrackState();
+        }
+
+        public new void ScrollToEnd(bool animated = true, bool allowDuringDrag = false)
+        {
+            base.ScrollToEnd(animated, allowDuringDrag);
+            updateTrackState();
+        }
+
+        #endregion
+    }
+}