LayoutBuilder: skip calling builder when constraints are the same (flutter#55414)

Avoid calling `builder` in `ConstrainedLayoutBuilder` when layout constraints are the same.

[Design doc](flutter.dev/go/layout-builder-optimization).

## Related Issues

Fixes flutter#6469

Author: yjbanov

dev/integration_tests/flutter_gallery/lib/gallery/backdrop.dart

@@ -215,6 +215,14 @@ class _BackdropState extends State<Backdrop> with SingleTickerProviderStateMixin
       value: 1.0,
       vsync: this,
     );
+    _controller.addStatusListener((AnimationStatus status) {
+      setState(() {
+        // This is intentionally left empty. The state change itself takes
+        // place inside the AnimationController, so there's nothing to update.
+        // All we want is for the widget to rebuild and read the new animation
+        // state from the AnimationController.
+      });
+    });
     _frontOpacity = _controller.drive(_frontOpacityTween);
   }
 

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

@@ -17,6 +17,18 @@ typedef LayoutWidgetBuilder = Widget Function(BuildContext context, BoxConstrain
 /// function at layout time and provides the constraints that this widget should
 /// adhere to. This is useful when the parent constrains the child's size and layout,
 /// and doesn't depend on the child's intrinsic size.
+///
+/// {@template flutter.widgets.layoutBuilder.builderFunctionInvocation}
+/// The [builder] function is called in the following situations:
+///
+/// * The first time the widget is laid out.
+/// * When the parent widget passes different layout constraints.
+/// * When the parent widget updates this widget.
+/// * When the depedencies that the [builder] function subscribes to change.
+///
+/// The [builder] function is _not_ called during layout if the parent passes
+/// the same constraints repeatedly.
+/// {@endtemplate}
 abstract class ConstrainedLayoutBuilder<ConstraintType extends Constraints> extends RenderObjectWidget {
   /// Creates a widget that defers its building until layout.
   ///
@@ -74,15 +86,22 @@ class _LayoutBuilderElement<ConstraintType extends Constraints> extends RenderOb
     assert(widget != newWidget);
     super.update(newWidget);
     assert(widget == newWidget);
+
     renderObject.updateCallback(_layout);
-    renderObject.markNeedsLayout();
+    // Force the callback to be called, even if the layout constraints are the
+    // same, because the logic in the callback might have changed.
+    renderObject.markNeedsBuild();
   }
 
   @override
   void performRebuild() {
     // This gets called if markNeedsBuild() is called on us.
     // That might happen if, e.g., our builder uses Inherited widgets.
-    renderObject.markNeedsLayout();
+
+    // Force the callback to be called, even if the layout constraints are the
+    // same. This is because that callback may depend on the updated widget
+    // configuration, or an inherited widget.
+    renderObject.markNeedsBuild();
     super.performRebuild(); // Calls widget.updateRenderObject (a no-op in this case).
   }
 
@@ -168,10 +187,42 @@ mixin RenderConstrainedLayoutBuilder<ConstraintType extends Constraints, ChildTy
     markNeedsLayout();
   }
 
-  /// Invoke the layout callback.
-  void layoutAndBuildChild() {
+  bool _needsBuild = true;
+
+  /// Marks this layout builder as needing to rebuild.
+  ///
+  /// The layout build rebuilds automatically when layout constraints change.
+  /// However, we must also rebuild when the widget updates, e.g. after
+  /// [State.setState], or [State.didChangeDependencies], even when the layout
+  /// constraints remain unchanged.
+  ///
+  /// See also:
+  ///
+  ///  * [ConstrainedLayoutBuilder.builder], which is called during the rebuild.
+  void markNeedsBuild() {
+    // Do not call the callback directly. It must be called during the layout
+    // phase, when parent constraints are available. Calling `markNeedsLayout`
+    // will cause it to be called at the right time.
+    _needsBuild = true;
+    markNeedsLayout();
+  }
+
+  // The constraints that were passed to this class last time it was laid out.
+  // These constraints are compared to the new constraints to determine whether
+  // [ConstrainedLayoutBuilder.builder] needs to be called.
+  Constraints _previousConstraints;
+
+  /// Invoke the callback supplied via [updateCallback].
+  ///
+  /// Typically this results in [ConstrainedLayoutBuilder.builder] being called
+  /// during layout.
+  void rebuildIfNecessary() {
     assert(_callback != null);
-    invokeLayoutCallback(_callback);
+    if (_needsBuild || constraints != _previousConstraints) {
+      _previousConstraints = constraints;
+      _needsBuild = false;
+      invokeLayoutCallback(_callback);
+    }
   }
 }
 
@@ -183,11 +234,13 @@ mixin RenderConstrainedLayoutBuilder<ConstraintType extends Constraints, ChildTy
 /// the child's intrinsic size. The [LayoutBuilder]'s final size will match its
 /// child's size.
 ///
+/// {@macro flutter.widgets.layoutBuilder.builderFunctionInvocation}
+///
 /// {@youtube 560 315 https://www.youtube.com/watch?v=IYDVcriKjsw}
 ///
 /// If the child should be smaller than the parent, consider wrapping the child
 /// in an [Align] widget. If the child might want to be bigger, consider
-/// wrapping it in a [SingleChildScrollView].
+/// wrapping it in a [SingleChildScrollView] or [OverflowBox].
 ///
 /// See also:
 ///
@@ -241,7 +294,7 @@ class _RenderLayoutBuilder extends RenderBox with RenderObjectWithChildMixin<Ren
   @override
   void performLayout() {
     final BoxConstraints constraints = this.constraints;
-    layoutAndBuildChild();
+    rebuildIfNecessary();
     if (child != null) {
       child.layout(constraints, parentUsesSize: true);
       size = constraints.constrain(child.size);

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

@@ -19,6 +19,7 @@ typedef SliverLayoutWidgetBuilder = Widget Function(BuildContext context, Sliver
 /// The [SliverLayoutBuilder]'s final [SliverGeometry] will match the [SliverGeometry]
 /// of its child.
 ///
+/// {@macro flutter.widgets.layoutBuilder.builderFunctionInvocation}
 ///
 /// See also:
 ///
@@ -52,7 +53,7 @@ class _RenderSliverLayoutBuilder extends RenderSliver with RenderObjectWithChild
 
   @override
   void performLayout() {
-    layoutAndBuildChild();
+    rebuildIfNecessary();
     child?.layout(constraints, parentUsesSize: true);
     geometry = child?.geometry ?? SliverGeometry.zero;
   }

packages/flutter/test/widgets/layout_builder_test.dart

@@ -587,4 +587,111 @@ void main() {
     await tester.pump();
     expect(hitCounts, const <int> [0, 0, 0]);
   });
+
+  testWidgets('LayoutBuilder does not call builder when layout happens but layout constraints do not change', (WidgetTester tester) async {
+    int builderInvocationCount = 0;
+
+    Future<void> pumpTestWidget(Size size) async {
+      await tester.pumpWidget(
+        // Center is used to give the SizedBox the power to determine constraints for LayoutBuilder
+        Center(
+          child: SizedBox.fromSize(
+            size: size,
+            child: LayoutBuilder(builder: (BuildContext context, BoxConstraints constraints) {
+              builderInvocationCount += 1;
+              return _LayoutSpy();
+            }),
+          ),
+        ),
+      );
+    }
+
+    await pumpTestWidget(const Size(10, 10));
+
+    final _RenderLayoutSpy spy = tester.renderObject(find.byType(_LayoutSpy));
+
+    // The child is laid out once the first time.
+    expect(spy.performLayoutCount, 1);
+    expect(spy.performResizeCount, 1);
+
+    // The initial `pumpWidget` will trigger `performRebuild`, asking for
+    // builder invocation.
+    expect(builderInvocationCount, 1);
+
+    // Invalidate the layout without chaning the constraints.
+    tester.renderObject(find.byType(LayoutBuilder)).markNeedsLayout();
+
+    // The second pump will not go through the `performRebuild` or `update`, and
+    // only judge the need for builder invocation based on constraints, which
+    // didn't change, so we don't expect any counters to go up.
+    await tester.pump();
+    expect(builderInvocationCount, 1);
+    expect(spy.performLayoutCount, 1);
+    expect(spy.performResizeCount, 1);
+
+    // Cause the `update` to be called (but not `performRebuild`), triggering
+    // builder invocation.
+    await pumpTestWidget(const Size(10, 10));
+    expect(builderInvocationCount, 2);
+
+    // The spy does not invalidate its layout on widget update, so no
+    // layout-related methods should be called.
+    expect(spy.performLayoutCount, 1);
+    expect(spy.performResizeCount, 1);
+
+    // Have the child request layout and verify that the child gets laid out
+    // despite layout constraints remaining constant.
+    spy.markNeedsLayout();
+    await tester.pump();
+
+    // Builder is not invoked. This was a layout-only pump with the same parent
+    // constraints.
+    expect(builderInvocationCount, 2);
+
+    // Expect performLayout to be called.
+    expect(spy.performLayoutCount, 2);
+
+    // performResize should not be called because the spy sets sizedByParent,
+    // and the constraints did not change.
+    expect(spy.performResizeCount, 1);
+
+    // Change the parent size, triggering constraint change.
+    await pumpTestWidget(const Size(20, 20));
+
+    // We should see everything invoked once.
+    expect(builderInvocationCount, 3);
+    expect(spy.performLayoutCount, 3);
+    expect(spy.performResizeCount, 2);
+  });
+}
+
+class _LayoutSpy extends LeafRenderObjectWidget {
+  @override
+  LeafRenderObjectElement createElement() => _LayoutSpyElement(this);
+
+  @override
+  RenderObject createRenderObject(BuildContext context) => _RenderLayoutSpy();
+}
+
+class _LayoutSpyElement extends LeafRenderObjectElement {
+  _LayoutSpyElement(LeafRenderObjectWidget widget) : super(widget);
+}
+
+class _RenderLayoutSpy extends RenderBox {
+  int performLayoutCount = 0;
+  int performResizeCount = 0;
+
+  @override
+  bool get sizedByParent => true;
+
+  @override
+  void performResize() {
+    performResizeCount += 1;
+    size = constraints.biggest;
+  }
+
+  @override
+  void performLayout() {
+    performLayoutCount += 1;
+  }
 }