Added transformAlignment for container (#66201)

packages/flutter/lib/src/widgets/container.dart

@@ -268,6 +268,7 @@ class Container extends StatelessWidget {
     BoxConstraints? constraints,
     this.margin,
     this.transform,
+    this.transformAlignment,
     this.child,
     this.clipBehavior = Clip.none,
   }) : assert(margin == null || margin.isNonNegative),
@@ -356,6 +357,15 @@ class Container extends StatelessWidget {
   /// The transformation matrix to apply before painting the container.
   final Matrix4? transform;
 
+ /// The alignment of the origin, relative to the size of the container, if [transform] is specified.
+ ///
+ /// When [transform] is null, the value of this property is ignored.
+ ///
+ /// See also:
+ ///
+ ///  * [Transform.alignment], which is set by this property.
+  final Alignment? transformAlignment;
+
   /// The clip behavior when [Container.decoration] is not null.
   ///
   /// Defaults to [Clip.none]. Must be [Clip.none] if [decoration] is null.
@@ -427,7 +437,7 @@ class Container extends StatelessWidget {
       current = Padding(padding: margin!, child: current);
 
     if (transform != null)
-      current = Transform(transform: transform!, child: current);
+      current = Transform(transform: transform!, child: current, alignment: transformAlignment);
 
     return current!;
   }

packages/flutter/test/widgets/container_test.dart

@@ -470,6 +470,53 @@ void main() {
     );
   });
 
+  testWidgets('Container transformAlignment', (WidgetTester tester) async {
+    final Key key = UniqueKey();
+
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: Stack(
+          children: <Widget>[
+            Positioned(
+              top: 100.0,
+              left: 100.0,
+              child: Container(
+                width: 100.0,
+                height: 100.0,
+                color: const Color(0xFF0000FF),
+              ),
+            ),
+            Positioned(
+              top: 100.0,
+              left: 100.0,
+              child: Container(
+                width: 100.0,
+                height: 100.0,
+                key: key,
+                transform: Matrix4.diagonal3Values(0.5, 0.5, 1.0),
+                transformAlignment: const Alignment(1.0, 0.0),
+                child: Container(
+                  color: const Color(0xFF00FFFF),
+                ),
+              ),
+            ),
+          ],
+        ),
+      ),
+    );
+
+    final Finder finder = find.byKey(key);
+
+    expect(tester.getSize(finder), equals(const Size(100, 100)));
+
+    expect(tester.getTopLeft(finder), equals(const Offset(100, 100)));
+    expect(tester.getTopRight(finder), equals(const Offset(200, 100)));
+
+    expect(tester.getBottomLeft(finder), equals(const Offset(100, 200)));
+    expect(tester.getBottomRight(finder), equals(const Offset(200, 200)));
+  });
+
   testWidgets('giving clipBehaviour Clip.None, will not add a ClipPath to the tree', (WidgetTester tester) async {
     await tester.pumpWidget(Container(
       clipBehavior: Clip.none,