Create a new collection via a placeholder item

osu.Game/Collections/DrawableCollectionList.cs

@@ -1,6 +1,8 @@
 // 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 System.Linq;
+using osu.Framework.Bindables;
 using osu.Framework.Graphics;
 using osu.Framework.Graphics.Containers;
 using osu.Game.Graphics.Containers;
@@ -10,19 +12,94 @@ namespace osu.Game.Collections
 {
     public class DrawableCollectionList : OsuRearrangeableListContainer<BeatmapCollection>
     {
-        protected override ScrollContainer<Drawable> CreateScrollContainer() => base.CreateScrollContainer().With(d =>
-        {
-            d.ScrollbarVisible = false;
-            d.Padding = new MarginPadding(10);
-        });
+        private Scroll scroll;
 
-        protected override FillFlowContainer<RearrangeableListItem<BeatmapCollection>> CreateListFillFlowContainer() => new FillFlowContainer<RearrangeableListItem<BeatmapCollection>>
+        protected override ScrollContainer<Drawable> CreateScrollContainer() => scroll = new Scroll();
+
+        protected override FillFlowContainer<RearrangeableListItem<BeatmapCollection>> CreateListFillFlowContainer() => new Flow
         {
-            LayoutDuration = 200,
-            LayoutEasing = Easing.OutQuint,
-            Spacing = new Vector2(0, 5)
+            DragActive = { BindTarget = DragActive }
         };
 
-        protected override OsuRearrangeableListItem<BeatmapCollection> CreateOsuDrawable(BeatmapCollection item) => new DrawableCollectionListItem(item);
+        protected override OsuRearrangeableListItem<BeatmapCollection> CreateOsuDrawable(BeatmapCollection item)
+        {
+            if (item == scroll.PlaceholderItem.Model)
+                return scroll.ReplacePlaceholder();
+
+            return new DrawableCollectionListItem(item, true);
+        }
+
+        private class Scroll : OsuScrollContainer
+        {
+            public DrawableCollectionListItem PlaceholderItem { get; private set; }
+
+            protected override Container<Drawable> Content => content;
+            private readonly Container content;
+
+            private readonly Container<DrawableCollectionListItem> placeholderContainer;
+
+            public Scroll()
+            {
+                ScrollbarVisible = false;
+                Padding = new MarginPadding(10);
+
+                base.Content.Add(new FillFlowContainer
+                {
+                    RelativeSizeAxes = Axes.X,
+                    AutoSizeAxes = Axes.Y,
+                    LayoutDuration = 200,
+                    LayoutEasing = Easing.OutQuint,
+                    Children = new Drawable[]
+                    {
+                        content = new Container { RelativeSizeAxes = Axes.X },
+                        placeholderContainer = new Container<DrawableCollectionListItem>
+                        {
+                            RelativeSizeAxes = Axes.X,
+                            AutoSizeAxes = Axes.Y
+                        }
+                    }
+                });
+
+                ReplacePlaceholder();
+            }
+
+            protected override void Update()
+            {
+                base.Update();
+
+                // AutoSizeAxes cannot be used as the height should represent the post-layout-transform height at all times, so that the placeholder doesn't bounce around.
+                content.Height = ((Flow)Child).Children.Sum(c => c.DrawHeight + 5);
+            }
+
+            /// <summary>
+            /// Replaces the current <see cref="PlaceholderItem"/> with a new one, and returns the previous.
+            /// </summary>
+            public DrawableCollectionListItem ReplacePlaceholder()
+            {
+                var previous = PlaceholderItem;
+
+                placeholderContainer.Clear(false);
+                placeholderContainer.Add(PlaceholderItem = new DrawableCollectionListItem(new BeatmapCollection(), false));
+
+                return previous;
+            }
+        }
+
+        private class Flow : FillFlowContainer<RearrangeableListItem<BeatmapCollection>>
+        {
+            public readonly IBindable<bool> DragActive = new Bindable<bool>();
+
+            public Flow()
+            {
+                Spacing = new Vector2(0, 5);
+                LayoutEasing = Easing.OutQuint;
+            }
+
+            protected override void LoadComplete()
+            {
+                base.LoadComplete();
+                DragActive.BindValueChanged(active => LayoutDuration = active.NewValue ? 200 : 0);
+            }
+        }
     }
 }