CustomSingleChildLayout should be able to resize (#7528)

It's not correct to set sizedByParent for
RenderCustomSingleChildLayoutBox because the delegate's size function
might depend on information other than the incoming constraints.

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

@@ -736,18 +736,54 @@ class RenderFractionallySizedOverflowBox extends RenderAligningShiftedBox {
 }
 
 /// A delegate for computing the layout of a render object with a single child.
-class SingleChildLayoutDelegate {
-  /// Returns the size of this object given the incoming constraints.
+abstract class SingleChildLayoutDelegate {
+  // TODO(abarth): This class should take a Listenable to drive relayout.
+
+  /// The size of this object given the incoming constraints.
+  ///
+  /// Defaults to the biggest size that satifies the given constraints.
   Size getSize(BoxConstraints constraints) => constraints.biggest;
 
-  /// Returns the box constraints for the child given the incoming constraints.
+  /// The constraints for the child given the incoming constraints.
+  ///
+  /// During layout, the child is given the layout constraints returned by this
+  /// function. The child is required to pick a size for itself that satisfies
+  /// these constraints.
+  ///
+  /// Defaults to the given constraints.
   BoxConstraints getConstraintsForChild(BoxConstraints constraints) => constraints;
 
-  /// Returns the position where the child should be placed given the size of this object and the size of the child.
+  /// The position where the child should be placed.
+  ///
+  /// The `size` argument is the size of the parent, which might be different
+  /// from the value returned by [getSize] if that size doesn't satisfy the
+  /// constraints passed to [getSize]. The `childSize` argument is the size of
+  /// the child, which will satisfy the constraints returned by
+  /// [getConstraintsForChild].
+  ///
+  /// Defaults to positioning the child in the upper left corner of the parent.
   Offset getPositionForChild(Size size, Size childSize) => Offset.zero;
 
-  /// Override this method to return true when the child needs to be laid out.
-  bool shouldRelayout(@checked SingleChildLayoutDelegate oldDelegate) => true;
+  /// Called whenever a new instance of the custom layout delegate class is
+  /// provided to the [RenderCustomSingleChildLayoutBox] object, or any time
+  /// that a new [CustomSingleChildLayout] object is created with a new instance
+  /// of the custom layout delegate class (which amounts to the same thing,
+  /// because the latter is implemented in terms of the former).
+  ///
+  /// If the new instance represents different information than the old
+  /// instance, then the method should return `true`, otherwise it should return
+  /// `false`.
+  ///
+  /// If the method returns `false`, then the [getSize],
+  /// [getConstraintsForChild], and [getPositionForChild] calls might be
+  /// optimized away.
+  ///
+  /// It's possible that the layout methods will get called even if
+  /// [shouldRelayout] returns `false` (e.g. if an ancestor changed its layout).
+  /// It's also possible that the layout method will get called
+  /// without [shouldRelayout] being called at all (e.g. if the parent changes
+  /// size).
+  bool shouldRelayout(@checked SingleChildLayoutDelegate oldDelegate);
 }
 
 /// Defers the layout of its single child to a delegate.
@@ -819,16 +855,9 @@ class RenderCustomSingleChildLayoutBox extends RenderShiftedBox {
     return 0.0;
   }
 
-  @override
-  bool get sizedByParent => true;
-
-  @override
-  void performResize() {
-    size = _getSize(constraints);
-  }
-
   @override
   void performLayout() {
+    size = _getSize(constraints);
     if (child != null) {
       BoxConstraints childConstraints = delegate.getConstraintsForChild(constraints);
       assert(childConstraints.debugAssertIsValid(isAppliedConstraint: true));

packages/flutter/test/widgets/custom_one_child_layout_test.dart → packages/flutter/test/widgets/custom_single_child_layout_test.dart

@@ -24,11 +24,7 @@ class TestSingleChildLayoutDelegate extends SingleChildLayoutDelegate {
     assert(!RenderObject.debugCheckingIntrinsics);
     constraintsFromGetConstraintsForChild = constraints;
     return const BoxConstraints(
-      minWidth: 100.0,
-      maxWidth: 150.0,
-      minHeight: 200.0,
-      maxHeight: 400.0
-    );
+        minWidth: 100.0, maxWidth: 150.0, minHeight: 200.0, maxHeight: 400.0);
   }
 
   @override
@@ -50,13 +46,39 @@ class TestSingleChildLayoutDelegate extends SingleChildLayoutDelegate {
   }
 }
 
+class FixedSizeLayoutDelegate extends SingleChildLayoutDelegate {
+  FixedSizeLayoutDelegate(this.size);
+
+  final Size size;
+
+  @override
+  Size getSize(BoxConstraints constraints) => size;
+
+  @override
+  BoxConstraints getConstraintsForChild(BoxConstraints constraints) {
+    return new BoxConstraints.tight(size);
+  }
+
+  @override
+  bool shouldRelayout(FixedSizeLayoutDelegate oldDelegate) {
+    return size != oldDelegate.size;
+  }
+}
+
 Widget buildFrame(SingleChildLayoutDelegate delegate) {
-  return new Center(child: new CustomSingleChildLayout(delegate: delegate, child: new Container()));
+  return new Center(
+    child: new CustomSingleChildLayout(
+      delegate: delegate,
+      child: new Container(),
+    ),
+  );
 }
 
 void main() {
-  testWidgets('Control test for CustomSingleChildLayout', (WidgetTester tester) async {
-      TestSingleChildLayoutDelegate delegate = new TestSingleChildLayoutDelegate();
+  testWidgets('Control test for CustomSingleChildLayout',
+      (WidgetTester tester) async {
+    TestSingleChildLayoutDelegate delegate =
+        new TestSingleChildLayoutDelegate();
     await tester.pumpWidget(buildFrame(delegate));
 
     expect(delegate.constraintsFromGetSize.minWidth, 0.0);
@@ -76,12 +98,15 @@ void main() {
     expect(delegate.childSizeFromGetPositionForChild.height, 400.0);
   });
 
-  testWidgets('Test SingleChildDelegate shouldRelayout method', (WidgetTester tester) async {
-      TestSingleChildLayoutDelegate delegate = new TestSingleChildLayoutDelegate();
+  testWidgets('Test SingleChildDelegate shouldRelayout method',
+      (WidgetTester tester) async {
+    TestSingleChildLayoutDelegate delegate =
+        new TestSingleChildLayoutDelegate();
     await tester.pumpWidget(buildFrame(delegate));
 
     // Layout happened because the delegate was set.
-      expect(delegate.constraintsFromGetConstraintsForChild, isNotNull); // i.e. layout happened
+    expect(delegate.constraintsFromGetConstraintsForChild,
+        isNotNull); // i.e. layout happened
     expect(delegate.shouldRelayoutCalled, isFalse);
 
     // Layout did not happen because shouldRelayout() returned false.
@@ -99,4 +124,17 @@ void main() {
     expect(delegate.constraintsFromGetConstraintsForChild, isNotNull);
   });
 
+  testWidgets('Delegate can change size', (WidgetTester tester) async {
+    await tester.pumpWidget(
+        buildFrame(new FixedSizeLayoutDelegate(const Size(100.0, 200.0))));
+
+    RenderBox box = tester.renderObject(find.byType(CustomSingleChildLayout));
+    expect(box.size, equals(const Size(100.0, 200.0)));
+
+    await tester.pumpWidget(
+        buildFrame(new FixedSizeLayoutDelegate(const Size(150.0, 240.0))));
+
+    box = tester.renderObject(find.byType(CustomSingleChildLayout));
+    expect(box.size, equals(const Size(150.0, 240.0)));
+  });
 }