Add some Transform constructors (#16553)

See https://github.com/flutter/flutter/pull/15383

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

@@ -819,6 +819,8 @@ class PhysicalShape extends SingleChildRenderObjectWidget {
 ///
 ///  * [RotatedBox], which rotates the child widget during layout, not just
 ///    during painting.
+///  * [FractionalTranslation], which applies a translation to the child
+///    that is relative to the child's size.
 ///  * [FittedBox], which sizes and positions its child widget to fit the parent
 ///    according to a given [BoxFit] discipline.
 class Transform extends SingleChildRenderObjectWidget {
@@ -866,6 +868,67 @@ class Transform extends SingleChildRenderObjectWidget {
   }) : transform = new Matrix4.rotationZ(angle),
        super(key: key, child: child);
 
+  /// Creates a widget that transforms its child using a translation.
+  ///
+  /// The `offset` argument must not be null. It specifies the translation.
+  ///
+  /// ## Sample code
+  ///
+  /// This example shifts the silver-colored child down by fifteen pixels.
+  ///
+  /// ```dart
+  /// new Transform.translate(
+  ///   offset: const Offset(0.0, 15.0),
+  ///   child: new Container(
+  ///     padding: const EdgeInsets.all(8.0),
+  ///     color: const Color(0xFF7F7F7F),
+  ///     child: const Text('Quarter'),
+  ///   ),
+  /// )
+  /// ```
+  Transform.translate({
+    Key key,
+    @required Offset offset,
+    this.transformHitTests: true,
+    Widget child,
+  }) : transform = new Matrix4.translationValues(offset.dx, offset.dy, 0.0),
+       origin = null,
+       alignment = null,
+       super(key: key, child: child);
+
+  /// Creates a widget that scales its child uniformly.
+  ///
+  /// The `scale` argument must not be null. It gives the scalar by which
+  /// to multiply the `x` and `y` axes.
+  ///
+  /// The [alignment] controls the origin of the scale; by default, this is
+  /// the center of the box.
+  ///
+  /// ## Sample code
+  ///
+  /// This example shrinks an orange box containing text such that each dimension
+  /// is half the size it would otherwise be.
+  ///
+  /// ```dart
+  /// new Transform.scale(
+  ///   scale: 0.5,
+  ///   child: new Container(
+  ///     padding: const EdgeInsets.all(8.0),
+  ///     color: const Color(0xFFE8581C),
+  ///     child: const Text('Bad Ideas'),
+  ///   ),
+  /// )
+  /// ```
+  Transform.scale({
+    Key key,
+    @required double scale,
+    this.origin,
+    this.alignment: Alignment.center,
+    this.transformHitTests: true,
+    Widget child,
+  }) : transform = new Matrix4.diagonal3Values(scale, scale, 1.0),
+       super(key: key, child: child);
+
   /// The matrix to transform the child by during painting.
   final Matrix4 transform;
 
@@ -1124,6 +1187,10 @@ class FittedBox extends SingleChildRenderObjectWidget {
 ///
 /// See also:
 ///
+///  * [Transform], which applies an arbitrary transform to its child widget at
+///    paint time.
+///  * [new Transform.translate], which applies an absolute offset translation
+///    transformation instead of an offset scaled to the child.
 ///  * The [catalog of layout widgets](https://flutter.io/widgets/layout/).
 class FractionalTranslation extends SingleChildRenderObjectWidget {
   /// Creates a widget that translates its child's painting.
@@ -1133,7 +1200,7 @@ class FractionalTranslation extends SingleChildRenderObjectWidget {
     Key key,
     @required this.translation,
     this.transformHitTests: true,
-    Widget child
+    Widget child,
   }) : assert(translation != null),
        super(key: key, child: child);
 
@@ -1184,6 +1251,7 @@ class FractionalTranslation extends SingleChildRenderObjectWidget {
 ///
 ///  * [Transform], which is a paint effect that allows you to apply an
 ///    arbitrary transform to a child.
+///  * [new Transform.rotate], which applies a rotation paint effect.
 ///  * The [catalog of layout widgets](https://flutter.io/widgets/layout/).
 class RotatedBox extends SingleChildRenderObjectWidget {
   /// A widget that rotates its child.

packages/flutter/test/widgets/transform_test.dart

@@ -295,4 +295,42 @@ void main() {
     );
     expect(tester.getTopLeft(find.byType(Placeholder)), const Offset(30.0, 20.0));
   });
+
+  testWidgets('Transform.translate', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new Transform.translate(
+        offset: const Offset(100.0, 50.0),
+        child: new Opacity(opacity: 0.5, child: new Container()),
+      ),
+    );
+
+    // This should not cause a transform layer to be inserted.
+    final List<Layer> layers = tester.layers
+      ..retainWhere((Layer layer) => layer is TransformLayer);
+    expect(layers.length, 1); // only the render view
+    expect(tester.getTopLeft(find.byType(Container)), const Offset(100.0, 50.0));
+  });
+
+  testWidgets('Transform.scale', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new Transform.scale(
+        scale: 2.0,
+        child: new Opacity(opacity: 0.5, child: new Container()),
+      ),
+    );
+
+    final List<Layer> layers = tester.layers
+      ..retainWhere((Layer layer) => layer is TransformLayer);
+    expect(layers.length, 2);
+    // The first transform is from the render view.
+    final TransformLayer layer = layers[1];
+    final Matrix4 transform = layer.transform;
+    expect(transform.storage, <dynamic>[
+      // These are column-major, not row-major.
+      2.0, 0.0, 0.0, 0.0,
+      0.0, 2.0, 0.0, 0.0,
+      0.0, 0.0, 1.0, 0.0,
+      -400.0, -300.0, 0.0, 1.0, // it's 1600x1200, centered in an 800x600 square
+    ]);
+  });
 }