Add change state handling to the editor

osu.Game/Screens/Edit/IEditorChangeHandler.cs

@@ -0,0 +1,33 @@
+// 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.Game.Rulesets.Objects;
+
+namespace osu.Game.Screens.Edit
+{
+    /// <summary>
+    /// Interface for a component that manages changes in the <see cref="Editor"/>.
+    /// </summary>
+    public interface IEditorChangeHandler
+    {
+        /// <summary>
+        /// Begins a bulk state change event. <see cref="EndChange"/> should be invoked soon after.
+        /// </summary>
+        /// <remarks>
+        /// This should be invoked when multiple changes to the <see cref="Editor"/> should be bundled together into one state change event.
+        /// When nested invocations are involved, a state change will not occur until an equal number of invocations of <see cref="EndChange"/> are received.
+        /// </remarks>
+        /// <example>
+        /// When a group of <see cref="HitObject"/>s are deleted, a single undo and redo state change should update the state of all <see cref="HitObject"/>.
+        /// </example>
+        void BeginChange();
+
+        /// <summary>
+        /// Ends a bulk state change event.
+        /// </summary>
+        /// <remarks>
+        /// This should be invoked as soon as possible after <see cref="BeginChange"/> to cause a state change.
+        /// </remarks>
+        void EndChange();
+    }
+}