Adds relayout option to CustomMultiChildLayout. (#39252)

AUTHORS

@@ -40,4 +40,5 @@ Frederik Schweiger <mail@flschweiger.net>
 Martin Staadecker <machstg@gmail.com>
 Igor Katsuba <katsuba.igor@gmail.com>
 Diego Velásquez <diego.velasquez.lopez@gmail.com>
-Sarbagya Dhaubanjar <mail@sarbagyastha.com.np>
+Simon Lightfoot <simon@devangels.london>
\ No newline at end of file
+Sarbagya Dhaubanjar <mail@sarbagyastha.com.np>

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

@@ -20,6 +20,9 @@ class MultiChildLayoutParentData extends ContainerBoxParentData<RenderBox> {
 
 /// A delegate that controls the layout of multiple children.
 ///
+/// Used with [CustomMultiChildLayout] (in the widgets library) and
+/// [RenderCustomMultiChildLayoutBox] (in the rendering library).
+///
 /// Delegates must be idempotent. Specifically, if two delegates are equal, then
 /// they must produce the same layout. To change the layout, replace the
 /// delegate with a different instance whose [shouldRelayout] returns true when
@@ -38,8 +41,11 @@ class MultiChildLayoutParentData extends ContainerBoxParentData<RenderBox> {
 /// Override [shouldRelayout] to determine when the layout of the children needs
 /// to be recomputed when the delegate changes.
 ///
-/// Used with [CustomMultiChildLayout], the widget for the
-/// [RenderCustomMultiChildLayoutBox] render object.
+/// The most efficient way to trigger a relayout is to supply a `relayout`
+/// argument to the constructor of the [MultiChildLayoutDelegate]. The custom
+/// layout will listen to this value and relayout whenever the Listenable
+/// notifies its listeners, such as when an [Animation] ticks. This allows
+/// the custom layout to avoid the build phase of the pipeline.
 ///
 /// Each child must be wrapped in a [LayoutId] widget to assign the id that
 /// identifies it to the delegate. The [LayoutId.id] needs to be unique among
@@ -94,7 +100,20 @@ class MultiChildLayoutParentData extends ContainerBoxParentData<RenderBox> {
 /// The leader and follower widget will paint in the order they appear in the
 /// child list, regardless of the order in which [layoutChild] is called on
 /// them.
+///
+/// See also:
+///
+///  * [CustomMultiChildLayout], the widget that uses this delegate.
+///  * [RenderCustomMultiChildLayoutBox], render object that uses this
+///    delegate.
 abstract class MultiChildLayoutDelegate {
+  /// Creates a layout delegate.
+  ///
+  /// The layout will update whenever [relayout] notifies its listeners.
+  MultiChildLayoutDelegate({ Listenable relayout }) : _relayout = relayout;
+
+  final Listenable _relayout;
+
   Map<Object, RenderBox> _idToChild;
   Set<RenderBox> _debugChildrenNeedingLayout;
 
@@ -300,13 +319,30 @@ class RenderCustomMultiChildLayoutBox extends RenderBox
   /// The delegate that controls the layout of the children.
   MultiChildLayoutDelegate get delegate => _delegate;
   MultiChildLayoutDelegate _delegate;
-  set delegate(MultiChildLayoutDelegate value) {
-    assert(value != null);
-    if (_delegate == value)
+  set delegate(MultiChildLayoutDelegate newDelegate) {
+    assert(newDelegate != null);
+    if (_delegate == newDelegate)
       return;
-    if (value.runtimeType != _delegate.runtimeType || value.shouldRelayout(_delegate))
+    final MultiChildLayoutDelegate oldDelegate = _delegate;
+    if (newDelegate.runtimeType != oldDelegate.runtimeType || newDelegate.shouldRelayout(oldDelegate))
       markNeedsLayout();
-    _delegate = value;
+    _delegate = newDelegate;
+    if (attached) {
+      oldDelegate?._relayout?.removeListener(markNeedsLayout);
+      newDelegate?._relayout?.addListener(markNeedsLayout);
+    }
+  }
+
+  @override
+  void attach(PipelineOwner owner) {
+    super.attach(owner);
+    _delegate?._relayout?.addListener(markNeedsLayout);
+  }
+
+  @override
+  void detach() {
+    _delegate?._relayout?.removeListener(markNeedsLayout);
+    super.detach();
   }
 
   Size _getSize(BoxConstraints constraints) {

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

@@ -962,10 +962,11 @@ class RenderFractionallySizedOverflowBox extends RenderAligningShiftedBox {
 /// is provided, to check if the new instance actually represents different
 /// information.
 ///
-/// The most efficient way to trigger a relayout is to supply a relayout
+/// The most efficient way to trigger a relayout is to supply a `relayout`
 /// argument to the constructor of the [SingleChildLayoutDelegate]. The custom
-/// object will listen to this value and relayout whenever the animation
-/// ticks, avoiding both the build phase of the pipeline.
+/// layout will listen to this value and relayout whenever the Listenable
+/// notifies its listeners, such as when an [Animation] ticks. This allows
+/// the custom layout to avoid the build phase of the pipeline.
 ///
 /// See also:
 ///

packages/flutter/test/widgets/custom_multi_child_layout_test.dart

@@ -73,6 +73,23 @@ class PreferredSizeDelegate extends MultiChildLayoutDelegate {
   }
 }
 
+class NotifierLayoutDelegate extends MultiChildLayoutDelegate {
+  NotifierLayoutDelegate(this.size) : super(relayout: size);
+
+  final ValueNotifier<Size> size;
+
+  @override
+  Size getSize(BoxConstraints constraints) => size.value;
+
+  @override
+  void performLayout(Size size) { }
+
+  @override
+  bool shouldRelayout(NotifierLayoutDelegate oldDelegate) {
+    return size != oldDelegate.size;
+  }
+}
+
 void main() {
   testWidgets('Control test for CustomMultiChildLayout', (WidgetTester tester) async {
     final TestMultiChildLayoutDelegate delegate = TestMultiChildLayoutDelegate();
@@ -161,4 +178,25 @@ void main() {
     expect(box.size.width, equals(350.0));
     expect(box.size.height, equals(250.0));
   });
+
+  testWidgets('Can use listener for relayout', (WidgetTester tester) async {
+    final ValueNotifier<Size> size = ValueNotifier<Size>(const Size(100.0, 200.0));
+
+    await tester.pumpWidget(
+      Center(
+        child: CustomMultiChildLayout(
+          delegate: NotifierLayoutDelegate(size),
+        ),
+      ),
+    );
+
+    RenderBox box = tester.renderObject(find.byType(CustomMultiChildLayout));
+    expect(box.size, equals(const Size(100.0, 200.0)));
+
+    size.value = const Size(150.0, 240.0);
+    await tester.pump();
+
+    box = tester.renderObject(find.byType(CustomMultiChildLayout));
+    expect(box.size, equals(const Size(150.0, 240.0)));
+  });
 }