Split transaction handling code out into base class

osu.Game/Screens/Edit/TransactionalCommitComponent.cs

@@ -0,0 +1,45 @@
+// 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;
+
+namespace osu.Game.Screens.Edit
+{
+    /// <summary>
+    /// A component that tracks a batch change, only applying after all active changes are completed.
+    /// </summary>
+    public abstract class TransactionalCommitComponent
+    {
+        public bool TransactionActive => bulkChangesStarted > 0;
+
+        private int bulkChangesStarted;
+
+        /// <summary>
+        /// Signal the beginning of a change.
+        /// </summary>
+        public void BeginChange() => bulkChangesStarted++;
+
+        /// <summary>
+        /// Signal the end of a change.
+        /// </summary>
+        /// <exception cref="InvalidOperationException">Throws if <see cref="BeginChange"/> was not first called.</exception>
+        public void EndChange()
+        {
+            if (bulkChangesStarted == 0)
+                throw new InvalidOperationException($"Cannot call {nameof(EndChange)} without a previous call to {nameof(BeginChange)}.");
+
+            if (--bulkChangesStarted == 0)
+                UpdateState();
+        }
+
+        public void SaveState()
+        {
+            if (bulkChangesStarted > 0)
+                return;
+
+            UpdateState();
+        }
+
+        protected abstract void UpdateState();
+    }
+}