feat: provide a way to set the initial child's alignment (#114745)

InteractiveViewer.alignment parameter

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

@@ -87,6 +87,7 @@ class InteractiveViewer extends StatefulWidget {
     this.scaleEnabled = true,
     this.scaleFactor = 200.0,
     this.transformationController,
+    this.alignment,
     required Widget this.child,
   }) : assert(alignPanAxis != null),
        assert(panAxis != null),
@@ -141,6 +142,7 @@ class InteractiveViewer extends StatefulWidget {
     this.scaleEnabled = true,
     this.scaleFactor = 200.0,
     this.transformationController,
+    this.alignment,
     required InteractiveViewerWidgetBuilder this.builder,
   }) : assert(panAxis != null),
        assert(builder != null),
@@ -166,6 +168,9 @@ class InteractiveViewer extends StatefulWidget {
        constrained = false,
        child = null;
 
+  /// The alignment of the child's origin, relative to the size of the box.
+  final Alignment? alignment;
+
   /// If set to [Clip.none], the child may extend beyond the size of the InteractiveViewer,
   /// but it will not receive gestures in these areas.
   /// Be sure that the InteractiveViewer is the desired size when using [Clip.none].
@@ -1096,6 +1101,7 @@ class _InteractiveViewerState extends State<InteractiveViewer> with TickerProvid
         clipBehavior: widget.clipBehavior,
         constrained: widget.constrained,
         matrix: _transformationController!.value,
+        alignment: widget.alignment,
         child: widget.child!,
       );
     } else {
@@ -1110,6 +1116,7 @@ class _InteractiveViewerState extends State<InteractiveViewer> with TickerProvid
             childKey: _childKey,
             clipBehavior: widget.clipBehavior,
             constrained: widget.constrained,
+            alignment: widget.alignment,
             matrix: matrix,
             child: widget.builder!(
               context,
@@ -1143,6 +1150,7 @@ class _InteractiveViewerBuilt extends StatelessWidget {
     required this.clipBehavior,
     required this.constrained,
     required this.matrix,
+    required this.alignment,
   });
 
   final Widget child;
@@ -1150,11 +1158,13 @@ class _InteractiveViewerBuilt extends StatelessWidget {
   final Clip clipBehavior;
   final bool constrained;
   final Matrix4 matrix;
+  final Alignment? alignment;
 
   @override
   Widget build(BuildContext context) {
     Widget child = Transform(
       transform: matrix,
+      alignment: alignment,
       child: KeyedSubtree(
         key: childKey,
         child: this.child,

packages/flutter/test/widgets/interactive_viewer_test.dart

@@ -1650,6 +1650,22 @@ void main() {
       expect(scaleHighZoomedIn - scaleHighZoomedOut, lessThan(scaleZoomedIn - scaleZoomedOut));
     });
 
+    testWidgets('alignment argument is used properly', (WidgetTester tester) async {
+      const Alignment alignment = Alignment.center;
+
+      await tester.pumpWidget(MaterialApp(
+        home: Scaffold(
+          body: InteractiveViewer(
+            alignment: alignment,
+            child: Container(),
+          ),
+        ),
+      ));
+
+      final Transform transform = tester.firstWidget(find.byType(Transform));
+      expect(transform.alignment, alignment);
+    });
+
     testWidgets('interactionEndFrictionCoefficient', (WidgetTester tester) async {
       // Use the default interactionEndFrictionCoefficient.
       final TransformationController transformationController1 = TransformationController();