Allow RenderObject.getTransformTo to take an arbitrary RenderObject in the same tree (#148897)

This is https://github.com/flutter/flutter/pull/130192 but without the additional parameter.

Fixes https://github.com/flutter/flutter/issues/146764, https://github.com/flutter/flutter/issues/148410

packages/flutter/lib/src/rendering/object.dart

@@ -3317,12 +3317,21 @@ abstract class RenderObject with DiagnosticableTreeMixin implements HitTestTarge
   }
 
   /// {@template flutter.rendering.RenderObject.getTransformTo}
-  /// Applies the paint transform up the tree to `ancestor`.
+  /// Applies the paint transform from this [RenderObject] to the `target`
+  /// [RenderObject].
   ///
   /// Returns a matrix that maps the local paint coordinate system to the
-  /// coordinate system of `ancestor`.
+  /// coordinate system of `target`, or a [Matrix4.zero] if the paint transform
+  /// can not be computed.
   ///
-  /// If `ancestor` is null, this method returns a matrix that maps from the
+  /// This method throws an exception when the `target` is not in the same render
+  /// tree as this [RenderObject], as the behavior is undefined.
+  ///
+  /// This method ignores [RenderObject.paintsChild]. This means it will still
+  /// try to compute the paint transform even if [this] or `target` is currently
+  /// not visible.
+  ///
+  /// If `target` is null, this method returns a matrix that maps from the
   /// local paint coordinate system to the coordinate system of the
   /// [PipelineOwner.rootNode].
   /// {@endtemplate}
@@ -3332,30 +3341,61 @@ abstract class RenderObject with DiagnosticableTreeMixin implements HitTestTarge
   /// the global coordinate system in logical pixels. To get physical pixels,
   /// use [applyPaintTransform] from the [RenderView] to further transform the
   /// coordinate.
-  Matrix4 getTransformTo(RenderObject? ancestor) {
+  Matrix4 getTransformTo(RenderObject? target) {
-    final bool ancestorSpecified = ancestor != null;
     assert(attached);
-    if (ancestor == null) {
+    // The paths from to fromRenderObject and toRenderObject's common ancestor.
-      final RenderObject? rootNode = owner!.rootNode;
+    // Each list's length is greater than 1 if not null.
-      if (rootNode is RenderObject) {
+    //
-        ancestor = rootNode;
+    // [this, ...., commonAncestorRenderObject], or null if `this` is the common
+    // ancestor.
+    List<RenderObject>? fromPath;
+    // [target, ...., commonAncestorRenderObject], or null if `target` is the
+    // common ancestor.
+    List<RenderObject>? toPath;
+
+    RenderObject from = this;
+    RenderObject to = target ?? owner!.rootNode!;
+
+    while (!identical(from, to)) {
+      final int fromDepth = from.depth;
+      final int toDepth = to.depth;
+
+      if (fromDepth >= toDepth) {
+        final RenderObject fromParent = from.parent ?? (throw FlutterError('$target and $this are not in the same render tree.'));
+        (fromPath ??= <RenderObject>[this]).add(fromParent);
+        from = fromParent;
+      }
+      if (fromDepth <= toDepth) {
+        final RenderObject toParent = to.parent ?? (throw FlutterError('$target and $this are not in the same render tree.'));
+        assert(target != null, '$this has a depth that is less than or equal to ${owner?.rootNode}');
+        (toPath ??= <RenderObject>[target!]).add(toParent);
+        to = toParent;
       }
     }
-    final List<RenderObject> renderers = <RenderObject>[];
-    for (RenderObject renderer = this; renderer != ancestor; renderer = renderer.parent!) {
-      renderers.add(renderer);
-      assert(renderer.parent != null); // Failed to find ancestor in parent chain.
-    }
-    if (ancestorSpecified) {
-      renderers.add(ancestor!);
-    }
-    final Matrix4 transform = Matrix4.identity();
-    for (int index = renderers.length - 1; index > 0; index -= 1) {
-      renderers[index].applyPaintTransform(renderers[index - 1], transform);
-    }
-    return transform;
-  }
 
+    Matrix4? fromTransform;
+    if (fromPath != null) {
+      assert(fromPath.length > 1);
+      fromTransform = Matrix4.identity();
+      final int lastIndex = target == null ? fromPath.length - 2 : fromPath.length - 1;
+      for (int index = lastIndex; index > 0; index -= 1) {
+        fromPath[index].applyPaintTransform(fromPath[index - 1], fromTransform);
+      }
+    }
+    if (toPath == null) {
+      return fromTransform ?? Matrix4.identity();
+    }
+
+    assert(toPath.length > 1);
+    final Matrix4 toTransform = Matrix4.identity();
+    for (int index = toPath.length - 1; index > 0; index -= 1) {
+      toPath[index].applyPaintTransform(toPath[index - 1], toTransform);
+    }
+    if (toTransform.invert() == 0) {      // If the matrix is singular then `invert()` doesn't do anything.
+      return Matrix4.zero();
+    }
+    return (fromTransform?..multiply(toTransform)) ?? toTransform;
+  }
 
   /// Returns a rect in this object's coordinate system that describes
   /// the approximate bounding box of the clip rect that would be

packages/flutter/test/rendering/object_test.dart

@@ -156,7 +156,7 @@ void main() {
     expect(() => data3.detach(), throwsAssertionError);
   });
 
-  test('RenderObject.getTransformTo asserts is argument is not descendant', () {
+  test('RenderObject.getTransformTo asserts if target not in the same render tree', () {
     final PipelineOwner owner = PipelineOwner();
     final TestRenderObject renderObject1 = TestRenderObject();
     renderObject1.attach(owner);
@@ -165,6 +165,64 @@ void main() {
     expect(() => renderObject1.getTransformTo(renderObject2), throwsAssertionError);
   });
 
+  test('RenderObject.getTransformTo works for siblings and descendants', () {
+    final PipelineOwner owner = PipelineOwner();
+    final TestRenderObject renderObject1 = TestRenderObject()..attach(owner);
+    final TestRenderObject renderObject11 = TestRenderObject();
+    final TestRenderObject renderObject12 = TestRenderObject();
+
+    renderObject1
+      ..add(renderObject11)
+      ..add(renderObject12);
+    expect(renderObject11.getTransformTo(renderObject12), equals(Matrix4.identity()));
+    expect(renderObject1.getTransformTo(renderObject11), equals(Matrix4.identity()));
+    expect(renderObject1.getTransformTo(renderObject12), equals(Matrix4.identity()));
+    expect(renderObject11.getTransformTo(renderObject1), equals(Matrix4.identity()));
+    expect(renderObject12.getTransformTo(renderObject1), equals(Matrix4.identity()));
+
+    expect(renderObject1.getTransformTo(renderObject1), equals(Matrix4.identity()));
+    expect(renderObject11.getTransformTo(renderObject11), equals(Matrix4.identity()));
+    expect(renderObject12.getTransformTo(renderObject12), equals(Matrix4.identity()));
+  });
+
+  test('RenderObject.getTransformTo gets the correct paint transform', () {
+    final PipelineOwner owner = PipelineOwner();
+    final TestRenderObject renderObject0 = TestRenderObject()
+      ..attach(owner);
+    final TestRenderObject renderObject1 = TestRenderObject();
+    final TestRenderObject renderObject2 = TestRenderObject();
+    renderObject0
+      ..add(renderObject1)
+      ..add(renderObject2)
+      ..paintTransform = Matrix4.diagonal3Values(9, 4, 1);
+
+    final TestRenderObject renderObject11 = TestRenderObject();
+    final TestRenderObject renderObject21 = TestRenderObject();
+    renderObject1
+      ..add(renderObject11)
+      ..paintTransform = Matrix4.translationValues(8, 16, 32);
+    renderObject2
+      ..add(renderObject21)
+      ..paintTransform = Matrix4.translationValues(32, 64, 128);
+
+    expect(
+      renderObject11.getTransformTo(renderObject21),
+      equals(Matrix4.translationValues((8 - 32) * 9, (16 - 64) * 4, 32 - 128)),
+    );
+    // Turn one of the paint transforms into a singular matrix and getTransformTo
+    // should return Matrix4.zero().
+    renderObject0.paintTransform = Matrix4(
+      1, 1, 1 ,1,
+      2, 2, 2, 2,
+      3, 3, 3, 3,
+      4, 4, 4, 4,
+    ); // Not a full rank matrix, so it has to be singular.
+    expect(
+      renderObject11.getTransformTo(renderObject21),
+      equals(Matrix4.zero()),
+    );
+  });
+
   test('PaintingContext.pushClipRect reuses the layer', () {
     _testPaintingContextLayerReuse<ClipRectLayer>((PaintingContextCallback painter, PaintingContext context, Offset offset, Layer? oldLayer) {
       return context.pushClipRect(true, offset, Rect.zero, painter, oldLayer: oldLayer as ClipRectLayer?);
@@ -376,7 +434,8 @@ class _TestCustomLayerBox extends RenderBox {
 
 class TestParentData extends ParentData with ContainerParentDataMixin<RenderBox> { }
 
-class TestRenderObject extends RenderObject {
+class TestRenderObjectParentData extends ParentData with ContainerParentDataMixin<TestRenderObject> { }
+class TestRenderObject extends RenderObject with ContainerRenderObjectMixin<TestRenderObject, TestRenderObjectParentData> {
   TestRenderObject({this.allowPaintBounds = false});
 
   final bool allowPaintBounds;
@@ -393,6 +452,20 @@ class TestRenderObject extends RenderObject {
     return Rect.zero;
   }
 
+  Matrix4 paintTransform = Matrix4.identity();
+  @override
+  void applyPaintTransform(covariant RenderObject child, Matrix4 transform) {
+    super.applyPaintTransform(child, transform);
+    transform.multiply(paintTransform);
+  }
+
+  @override
+  void setupParentData(RenderObject child) {
+    if (child.parentData is! TestRenderObjectParentData) {
+      child.parentData = TestRenderObjectParentData();
+    }
+  }
+
   @override
   void performLayout() { }