Encapsulate logic for selecting/deselecting further

osu.Game/Rulesets/Edit/Layers/Selection/SelectionLayer.cs

@@ -1,6 +1,7 @@
 // Copyright (c) 2007-2018 ppy Pty Ltd <contact@ppy.sh>.
 // Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
 
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using osu.Framework.Extensions.IEnumerableExtensions;
@@ -16,6 +17,9 @@ namespace osu.Game.Rulesets.Edit.Layers.Selection
 {
     public class SelectionLayer : CompositeDrawable
     {
+        public event Action<DrawableHitObject> ObjectSelected;
+        public event Action<DrawableHitObject> ObjectDeselected;
+
         private readonly Playfield playfield;
 
         public SelectionLayer(Playfield playfield)
@@ -75,15 +79,69 @@ namespace osu.Game.Rulesets.Edit.Layers.Selection
             return true;
         }
 
+        /// <summary>
+        /// Selects a <see cref="DrawableHitObject"/>.
+        /// </summary>
+        /// <param name="hitObject">The <see cref="DrawableHitObject"/> to select.</param>
+        public void Select(DrawableHitObject hitObject)
+        {
+            if (!select(hitObject))
+                return;
+
+            clearCapture();
+            finishSelection();
+        }
+
+        /// <summary>
+        /// Selects a <see cref="DrawableHitObject"/> without performing capture updates.
+        /// </summary>
+        /// <param name="hitObject">The <see cref="DrawableHitObject"/> to select.</param>
+        /// <returns>Whether <paramref name="hitObject"/> was selected.</returns>
+        private bool select(DrawableHitObject hitObject)
+        {
+            if (!selectedHitObjects.Add(hitObject))
+                return false;
+
+            ObjectSelected?.Invoke(hitObject);
+            return true;
+        }
+
+        /// <summary>
+        /// Deselects a <see cref="DrawableHitObject"/>.
+        /// </summary>
+        /// <param name="hitObject">The <see cref="DrawableHitObject"/> to deselect.</param>
+        public void Deselect(DrawableHitObject hitObject)
+        {
+            if (!deselect(hitObject))
+                return;
+
+            clearCapture();
+            finishSelection();
+        }
+
+        /// <summary>
+        /// Deselects a <see cref="DrawableHitObject"/> without performing capture updates.
+        /// </summary>
+        /// <param name="hitObject">The <see cref="DrawableHitObject"/> to deselect.</param>
+        /// <returns>Whether the <see cref="DrawableHitObject"/> was deselected.</returns>
+        private bool deselect(DrawableHitObject hitObject)
+        {
+            if (!selectedHitObjects.Remove(hitObject))
+                return false;
+
+            ObjectDeselected?.Invoke(hitObject);
+            return true;
+        }
+
         /// <summary>
         /// Deselects all selected <see cref="DrawableHitObject"/>s.
         /// </summary>
         public void DeselectAll()
         {
+            selectedHitObjects.ForEach(h => ObjectDeselected?.Invoke(h));
             selectedHitObjects.Clear();
 
-            captureBox?.Hide();
+            clearCapture();
-            captureBox?.Expire();
         }
 
         /// <summary>
@@ -92,8 +150,7 @@ namespace osu.Game.Rulesets.Edit.Layers.Selection
         /// <param name="screenSpaceQuad">The selection <see cref="Quad"/>.</param>
         private void selectQuad(Quad screenSpaceQuad)
         {
-            foreach (var obj in playfield.HitObjects.Objects.Where(h => h.IsAlive && h.IsPresent && screenSpaceQuad.Contains(h.SelectionPoint)))
+            playfield.HitObjects.Objects.Where(h => h.IsAlive && h.IsPresent && screenSpaceQuad.Contains(h.SelectionPoint)).ForEach(h => select(h));
-                selectedHitObjects.Add(obj);
         }
 
         /// <summary>
@@ -102,11 +159,17 @@ namespace osu.Game.Rulesets.Edit.Layers.Selection
         /// <param name="screenSpacePoint">The <see cref="Vector2"/> to select at.</param>
         private void selectPoint(Vector2 screenSpacePoint)
         {
-            var selected = playfield.HitObjects.Objects.Reverse().Where(h => h.IsAlive && h.IsPresent).FirstOrDefault(h => h.ReceiveMouseInputAt(screenSpacePoint));
+            var target = playfield.HitObjects.Objects.Reverse().Where(h => h.IsAlive && h.IsPresent).FirstOrDefault(h => h.ReceiveMouseInputAt(screenSpacePoint));
-            if (selected == null)
+            if (target == null)
                 return;
 
-            selectedHitObjects.Add(selected);
+            select(target);
+        }
+
+        private void clearCapture()
+        {
+            captureBox?.Hide();
+            captureBox?.Expire();
         }
 
         private void finishSelection()