要素を分割したとき、動画の再生位置を調整するようにした

src/Beutl.Operators/Source/SourceVideoOperator.cs

@@ -73,4 +73,16 @@ public override bool TryGetOriginalLength(out TimeSpan timeSpan)
             return false;
         }
     }
+
+    public override IRecordableCommand? OnSplit(bool backward, TimeSpan startDelta, TimeSpan lengthDelta)
+    {
+        if (backward)
+        {
+            return new ChangeSetterValueCommand<TimeSpan>(OffsetPosition, OffsetPosition.Value, OffsetPosition.Value + startDelta);
+        }
+        else
+        {
+            return base.OnSplit(backward, startDelta, lengthDelta);
+        }
+    }
 }

src/Beutl.ProjectSystem/ChangeSetterValueCommand.cs

@@ -0,0 +1,29 @@
+using Beutl.Styling;
+
+namespace Beutl;
+
+public sealed class ChangeSetterValueCommand<T> : IRecordableCommand
+{
+    private readonly Setter<T> _setter;
+    private readonly T _oldValue;
+    private readonly T _newValue;
+
+    public ChangeSetterValueCommand(Setter<T> setter, T oldValue, T newValue)
+    {
+        _setter = setter;
+        _oldValue = oldValue;
+        _newValue = newValue;
+    }
+
+    public void Do()
+    {
+        _setter.Value = _newValue;
+    }
+
+    public void Redo() => Do();
+
+    public void Undo()
+    {
+        _setter.Value = _oldValue;
+    }
+}

src/Beutl.ProjectSystem/Operation/SourceOperation.cs

@@ -40,6 +40,14 @@ public SourceOperation()
     [NotAutoSerialized]
     public ICoreList<SourceOperator> Children => _children;
 
+    public IRecordableCommand OnSplit(bool backward, TimeSpan startDelta,TimeSpan lengthDelta)
+    {
+        return _children.Select(v => v.OnSplit(backward, startDelta, lengthDelta))
+            .Where(v => v != null)
+            .ToArray()!
+            .ToCommand();
+    }
+
     public override void ReadFromJson(JsonObject json)
     {
         base.ReadFromJson(json);

src/Beutl.ProjectSystem/Operation/SourceOperator.cs

@@ -97,6 +97,11 @@ public virtual bool TryGetOriginalLength(out TimeSpan timeSpan)
         return false;
     }
 
+    public virtual IRecordableCommand? OnSplit(bool backward, TimeSpan startDelta, TimeSpan lengthDelta)
+    {
+        return null;
+    }
+
     protected void RaiseInvalidated(RenderInvalidatedEventArgs args)
     {
         Invalidated?.Invoke(this, args);

src/Beutl/ViewModels/ElementViewModel.cs

@@ -364,8 +364,12 @@ private void OnSplit(TimeSpan timeSpan)
 
         backwardLayer.Save(RandomFileNameGenerator.Generate(Path.GetDirectoryName(Scene.FileName)!, Constants.ElementFileExtension));
         IRecordableCommand command2 = Scene.AddChild(backwardLayer);
+        IRecordableCommand command3 = backwardLayer.Operation.OnSplit(true, forwardLength, -forwardLength);
+        IRecordableCommand command4 = Model.Operation.OnSplit(false, TimeSpan.Zero, -backwardLength);
 
         command1.Append(command2)
+            .Append(command3)
+            .Append(command4)
             .DoAndRecord(CommandRecorder.Default);
     }