Introduce methods for computing the baseline location of a RenderBox without...

Introduce methods for computing the baseline location of a RenderBox without affecting the current layout (#144655)

Extracted from https://github.com/flutter/flutter/pull/138369

Introduces `RenderBox.{compute,get}DryBaseline` for computing the baseline location in `RenderBox.computeDryLayout`.

packages/flutter/lib/src/material/input_decorator.dart

@@ -1316,7 +1316,10 @@ class _RenderDecoration extends RenderBox with SlottedContainerRenderObjectMixin
 
   @override
   double computeDistanceToActualBaseline(TextBaseline baseline) {
-    return _boxParentData(input!).offset.dy + (input?.computeDistanceToActualBaseline(baseline) ?? 0.0);
+    final RenderBox? input = this.input;
+    return input == null
+      ? 0.0
+      : _boxParentData(input).offset.dy + (input.computeDistanceToActualBaseline(baseline) ?? 0.0);
   }
 
   // Records where the label was painted.

packages/flutter/lib/src/material/list_tile.dart

@@ -1249,10 +1249,12 @@ class _RenderListTile extends RenderBox with SlottedContainerRenderObjectMixin<_
   }
 
   @override
-  double computeDistanceToActualBaseline(TextBaseline baseline) {
+  double? computeDistanceToActualBaseline(TextBaseline baseline) {
     assert(title != null);
     final BoxParentData parentData = title!.parentData! as BoxParentData;
-    return parentData.offset.dy + title!.getDistanceToActualBaseline(baseline)!;
+    final BaselineOffset offset = BaselineOffset(title!.getDistanceToActualBaseline(baseline))
+                                + parentData.offset.dy;
+    return offset.offset;
   }
 
   static double? _boxBaseline(RenderBox box, TextBaseline baseline) {

packages/flutter/lib/src/material/toggle_buttons.dart

@@ -1175,11 +1175,13 @@ class _SelectToggleButtonRenderObject extends RenderShiftedBox {
   }
 
   @override
-  double computeDistanceToActualBaseline(TextBaseline baseline) {
+  double? computeDistanceToActualBaseline(TextBaseline baseline) {
     // The baseline of this widget is the baseline of its child
-    return direction == Axis.horizontal
-      ? child!.computeDistanceToActualBaseline(baseline)! + borderSide.width
-      : child!.computeDistanceToActualBaseline(baseline)! + leadingBorderSide.width;
+    final BaselineOffset childOffset = BaselineOffset(child?.computeDistanceToActualBaseline(baseline));
+    return switch (direction) {
+      Axis.horizontal => childOffset + borderSide.width,
+      Axis.vertical => childOffset + leadingBorderSide.width,
+    }.offset;
   }
 
   @override

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

@@ -2213,16 +2213,16 @@ abstract class RenderObject with DiagnosticableTreeMixin implements HitTestTarge
   @protected
   void debugAssertDoesMeetConstraints();
 
-  /// When true, debugAssertDoesMeetConstraints() is currently
-  /// executing asserts for verifying the consistent behavior of
+  /// When true, a debug method ([debugAssertDoesMeetConstraints], for instance)
+  /// is currently executing asserts for verifying the consistent behavior of
   /// intrinsic dimensions methods.
   ///
-  /// This should only be set by debugAssertDoesMeetConstraints()
-  /// implementations. It is used by tests to selectively ignore
-  /// custom layout callbacks. It should not be set outside of
-  /// debugAssertDoesMeetConstraints(), and should not be checked in
-  /// release mode (where it will always be false).
+  /// This is typically set by framework debug methods. It is read by tests to
+  /// selectively ignore custom layout callbacks. It should not be set outside of
+  /// intrinsic-checking debug methods, and should not be checked in release mode
+  /// (where it will always be false).
   static bool debugCheckingIntrinsics = false;
+
   bool _debugSubtreeRelayoutRootAlreadyMarkedNeedsLayout() {
     if (_relayoutBoundary == null) {
       // We don't know where our relayout boundary is yet.

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

@@ -700,8 +700,9 @@ class RenderIndexedStack extends RenderStack {
 
   @override
   void visitChildrenForSemantics(RenderObjectVisitor visitor) {
-    if (index != null && firstChild != null) {
-      visitor(_childAtIndex());
+    final RenderBox? displayedChild = _childAtIndex();
+    if (displayedChild != null) {
+      visitor(displayedChild);
     }
   }
 
@@ -715,45 +716,55 @@ class RenderIndexedStack extends RenderStack {
     }
   }
 
-  RenderBox _childAtIndex() {
-    assert(index != null);
+  RenderBox? _childAtIndex() {
+    final int? index = this.index;
+    if (index == null) {
+      return null;
+    }
     RenderBox? child = firstChild;
-    int i = 0;
-    while (child != null && i < index!) {
-      final StackParentData childParentData = child.parentData! as StackParentData;
-      child = childParentData.nextSibling;
-      i += 1;
+    for (int i = 0; i < index && child != null; i += 1) {
+      child = childAfter(child);
+    }
+    assert(firstChild == null || child != null);
+    return child;
+  }
+
+  @override
+  double? computeDistanceToActualBaseline(TextBaseline baseline) {
+    final RenderBox? displayedChild = _childAtIndex();
+    if (displayedChild == null) {
+      return null;
     }
-    assert(i == index);
-    assert(child != null);
-    return child!;
+    final StackParentData childParentData = displayedChild.parentData! as StackParentData;
+    final BaselineOffset offset = BaselineOffset(displayedChild.getDistanceToActualBaseline(baseline)) + childParentData.offset.dy;
+    return offset.offset;
   }
 
   @override
   bool hitTestChildren(BoxHitTestResult result, { required Offset position }) {
-    if (firstChild == null || index == null) {
+    final RenderBox? displayedChild = _childAtIndex();
+    if (displayedChild == null) {
       return false;
     }
-    final RenderBox child = _childAtIndex();
-    final StackParentData childParentData = child.parentData! as StackParentData;
+    final StackParentData childParentData = displayedChild.parentData! as StackParentData;
     return result.addWithPaintOffset(
       offset: childParentData.offset,
       position: position,
       hitTest: (BoxHitTestResult result, Offset transformed) {
         assert(transformed == position - childParentData.offset);
-        return child.hitTest(result, position: transformed);
+        return displayedChild.hitTest(result, position: transformed);
       },
     );
   }
 
   @override
   void paintStack(PaintingContext context, Offset offset) {
-    if (firstChild == null || index == null) {
+    final RenderBox? displayedChild = _childAtIndex();
+    if (displayedChild == null) {
       return;
     }
-    final RenderBox child = _childAtIndex();
-    final StackParentData childParentData = child.parentData! as StackParentData;
-    context.paintChild(child, childParentData.offset + offset);
+    final StackParentData childParentData = displayedChild.parentData! as StackParentData;
+    context.paintChild(displayedChild, childParentData.offset + offset);
   }
 
   @override

packages/flutter/test/material/dropdown_form_field_test.dart

@@ -10,11 +10,6 @@ import 'package:flutter_test/flutter_test.dart';
 
 const List<String> menuItems = <String>['one', 'two', 'three', 'four'];
 void onChanged<T>(T _) { }
-final Type dropdownButtonType = DropdownButton<String>(
-  onChanged: (_) { },
-  items: const <DropdownMenuItem<String>>[],
-).runtimeType;
-
 Finder _iconRichText(Key iconKey) {
   return find.descendant(
     of: find.byKey(iconKey),
@@ -566,8 +561,7 @@ void main() {
     }
   });
 
-  testWidgets('DropdownButtonFormField with isDense:true does not clip large scale text',
-      (WidgetTester tester) async {
+  testWidgets('DropdownButtonFormField with isDense:true does not clip large scale text', (WidgetTester tester) async {
     final Key buttonKey = UniqueKey();
     const String value = 'two';
 
@@ -588,9 +582,11 @@ void main() {
                     return DropdownMenuItem<String>(
                       key: ValueKey<String>(item),
                       value: item,
-                      child: Text(item,
-                          key: ValueKey<String>('${item}Text'),
-                          style: const TextStyle(fontSize: 20.0)),
+                      child: Text(
+                        item,
+                        key: ValueKey<String>('${item}Text'),
+                        style: const TextStyle(fontSize: 20.0),
+                      ),
                     );
                   }).toList(),
                 ),
@@ -601,8 +597,7 @@ void main() {
       ),
     );
 
-    final RenderBox box =
-    tester.renderObject<RenderBox>(find.byType(dropdownButtonType));
+    final RenderBox box = tester.renderObject<RenderBox>(find.byType(DropdownButton<String>));
     expect(box.size.height, 64.0);
   });
 
@@ -633,7 +628,7 @@ void main() {
       ),
     );
 
-    final RenderBox box = tester.renderObject<RenderBox>(find.byType(dropdownButtonType));
+    final RenderBox box = tester.renderObject<RenderBox>(find.byType(DropdownButton<String>));
     expect(box.size.height, 48.0);
   });
 
@@ -1077,7 +1072,7 @@ void main() {
     expect(find.text(currentValue), findsOneWidget);
 
     // Tap the DropdownButtonFormField widget
-    await tester.tap(find.byType(dropdownButtonType));
+    await tester.tap(find.byType(DropdownButton<String>));
     await tester.pumpAndSettle();
 
     // Tap the first dropdown menu item.

packages/flutter/test/material/toggle_buttons_test.dart

@@ -1274,7 +1274,7 @@ void main() {
   });
 
   testWidgets('Material2 - ToggleButtons text baseline alignment', (WidgetTester tester) async {
-    // The point size of the fonts must be a multiple of 4 until
+    // The font size must be a multiple of 4 until
     // https://github.com/flutter/flutter/issues/122066 is resolved.
     await tester.pumpWidget(
       boilerplate(

packages/flutter/test/rendering/box_test.dart

@@ -1227,6 +1227,23 @@ void main() {
     );
     layout(goodRoot, onErrors: () { assert(false); });
   });
+
+  group('BaselineOffset', () {
+    test('minOf', () {
+      expect(BaselineOffset.noBaseline.minOf(BaselineOffset.noBaseline), BaselineOffset.noBaseline);
+
+      expect(BaselineOffset.noBaseline.minOf(const BaselineOffset(1)), const BaselineOffset(1));
+      expect(const BaselineOffset(1).minOf(BaselineOffset.noBaseline), const BaselineOffset(1));
+
+      expect(const BaselineOffset(2).minOf(const BaselineOffset(1)), const BaselineOffset(1));
+      expect(const BaselineOffset(1).minOf(const BaselineOffset(2)), const BaselineOffset(1));
+    });
+
+    test('+', () {
+      expect(BaselineOffset.noBaseline + 2, BaselineOffset.noBaseline);
+      expect(const BaselineOffset(1) + 2, const BaselineOffset(3));
+    });
+  });
 }
 
 class _DummyHitTestTarget implements HitTestTarget {

packages/flutter/test/rendering/cached_intrinsics_test.dart

@@ -37,6 +37,34 @@ class RenderTestBox extends RenderBox {
   }
 }
 
+class RenderDryBaselineTestBox extends RenderTestBox {
+  double? baselineOverride;
+
+  @override
+  double? computeDryBaseline(covariant BoxConstraints constraints, TextBaseline baseline) {
+    calls += 1;
+    return baselineOverride ?? constraints.biggest.height / 2.0;
+  }
+}
+
+class RenderBadDryBaselineTestBox extends RenderTestBox {
+  @override
+  double? computeDryBaseline(covariant BoxConstraints constraints, TextBaseline baseline) {
+    return size.height / 2.0;
+  }
+}
+
+class RenderCannotComputeDryBaselineTestBox extends RenderTestBox {
+  bool shouldAssert = true;
+  @override
+  double? computeDryBaseline(covariant BoxConstraints constraints, TextBaseline baseline) {
+    if (shouldAssert) {
+      assert(debugCannotComputeDryLayout(reason: 'no dry baseline for you'));
+    }
+    return null;
+  }
+}
+
 void main() {
   TestRenderingFlutterBinding.ensureInitialized();
 
@@ -134,4 +162,99 @@ void main() {
 
     expect(test.calls, 3); // Use the cached data if the layout constraints do not change.
   });
+
+  group('Dry baseline', () {
+    test('computeDryBaseline results are cached and shared with computeDistanceToActualBaseline', () {
+      const double viewHeight =  200.0;
+      const BoxConstraints constraints = BoxConstraints.tightFor(width: 200.0, height: viewHeight);
+      final RenderDryBaselineTestBox test = RenderDryBaselineTestBox();
+      final RenderBox baseline = RenderBaseline(
+        baseline: 0.0,
+        baselineType: TextBaseline.alphabetic,
+        child: test,
+      );
+
+      final RenderConstrainedBox root = RenderConstrainedBox(
+        additionalConstraints: constraints,
+        child: baseline,
+      );
+
+      layout(RenderPositionedBox(child: root));
+      expect(test.calls, 1);
+
+      // The baseline widget loosens the input constraints when passing on to child.
+      expect(test.getDryBaseline(constraints.loosen(), TextBaseline.alphabetic), test.boxSize.height / 2);
+      // There's cache for the constraints so this should be 1, but we always evaluate
+      // computeDryBaseline in debug mode in case it asserts even if the baseline
+      // cache hits.
+      expect(test.calls, 2);
+
+      const BoxConstraints newConstraints = BoxConstraints.tightFor(width: 10.0, height: 10.0);
+      expect(test.getDryBaseline(newConstraints.loosen(), TextBaseline.alphabetic), 5.0);
+      // Should be 3 but there's an additional computeDryBaseline call in getDryBaseline,
+      // in an assert.
+      expect(test.calls, 4);
+
+      root.additionalConstraints = newConstraints;
+      pumpFrame();
+      expect(test.calls, 4);
+    });
+
+    test('Asserts when a RenderBox cannot compute dry baseline', () {
+      final RenderCannotComputeDryBaselineTestBox test = RenderCannotComputeDryBaselineTestBox();
+      layout(RenderBaseline(baseline: 0.0, baselineType: TextBaseline.alphabetic, child: test));
+
+      final BoxConstraints incomingConstraints = test.constraints;
+      assert(incomingConstraints != const BoxConstraints());
+      expect(
+        () => test.getDryBaseline(const BoxConstraints(), TextBaseline.alphabetic),
+        throwsA(isA<AssertionError>().having((AssertionError e) => e.message, 'message', contains('no dry baseline for you'))),
+      );
+
+      // Still throws when there is cache.
+      expect(
+        () => test.getDryBaseline(incomingConstraints, TextBaseline.alphabetic),
+        throwsA(isA<AssertionError>().having((AssertionError e) => e.message, 'message', contains('no dry baseline for you'))),
+      );
+    });
+
+    test('Cactches inconsistencies between computeDryBaseline and computeDistanceToActualBaseline', () {
+      final RenderDryBaselineTestBox test = RenderDryBaselineTestBox();
+      layout(test, phase: EnginePhase.composite);
+
+      FlutterErrorDetails? error;
+      test.markNeedsLayout();
+      test.baselineOverride = 123;
+      pumpFrame(phase: EnginePhase.composite, onErrors: () {
+        error = TestRenderingFlutterBinding.instance.takeFlutterErrorDetails();
+      });
+
+      expect(error?.exceptionAsString(), contains('differs from the baseline location computed by computeDryBaseline'));
+    });
+
+    test('Accessing RenderBox.size in computeDryBaseline is not allowed', () {
+      final RenderBadDryBaselineTestBox test = RenderBadDryBaselineTestBox();
+      FlutterErrorDetails? error;
+      layout(test, phase: EnginePhase.composite, onErrors: () {
+        error = TestRenderingFlutterBinding.instance.takeFlutterErrorDetails();
+      });
+
+      expect(error?.exceptionAsString(), contains('RenderBox.size accessed in RenderBadDryBaselineTestBox.computeDryBaseline.'));
+    });
+
+    test('debug baseline checks do not freak out when debugCannotComputeDryLayout is called', () {
+      FlutterErrorDetails? error;
+      void onErrors() {
+        error = TestRenderingFlutterBinding.instance.takeFlutterErrorDetails();
+      }
+      final RenderCannotComputeDryBaselineTestBox test = RenderCannotComputeDryBaselineTestBox();
+      layout(test, phase: EnginePhase.composite, onErrors: onErrors);
+      expect(error, isNull);
+
+      test.shouldAssert = false;
+      test.markNeedsLayout();
+      pumpFrame(phase: EnginePhase.composite, onErrors: onErrors);
+      expect(error?.exceptionAsString(), contains('differs from the baseline location computed by computeDryBaseline'));
+    });
+  });
 }

packages/flutter/test/rendering/stack_test.dart

@@ -119,6 +119,7 @@ void main() {
         visitedChildren.add(child);
       }
 
+      layout(stack);
       stack.visitChildrenForSemantics(visitor);
 
       expect(visitedChildren, hasLength(1));

packages/flutter/test/widgets/baseline_test.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
@@ -44,6 +45,8 @@ void main() {
   });
 
   testWidgets('Chip caches baseline', (WidgetTester tester) async {
+    final bool checkIntrinsicSizes = debugCheckIntrinsicSizes;
+    debugCheckIntrinsicSizes = false;
     int calls = 0;
     await tester.pumpWidget(
       MaterialApp(
@@ -53,6 +56,7 @@ void main() {
             baselineType: TextBaseline.alphabetic,
             child: Chip(
               label: BaselineDetector(() {
+                assert(!debugCheckIntrinsicSizes);
                 calls += 1;
               }),
             ),
@@ -66,9 +70,12 @@ void main() {
     tester.renderObject<RenderBaselineDetector>(find.byType(BaselineDetector)).dirty();
     await tester.pump();
     expect(calls, 2);
+    debugCheckIntrinsicSizes = checkIntrinsicSizes;
   });
 
   testWidgets('ListTile caches baseline', (WidgetTester tester) async {
+    final bool checkIntrinsicSizes = debugCheckIntrinsicSizes;
+    debugCheckIntrinsicSizes = false;
     int calls = 0;
     await tester.pumpWidget(
       MaterialApp(
@@ -78,6 +85,7 @@ void main() {
             baselineType: TextBaseline.alphabetic,
             child: ListTile(
               title: BaselineDetector(() {
+                assert(!debugCheckIntrinsicSizes);
                 calls += 1;
               }),
             ),
@@ -91,6 +99,7 @@ void main() {
     tester.renderObject<RenderBaselineDetector>(find.byType(BaselineDetector)).dirty();
     await tester.pump();
     expect(calls, 2);
+    debugCheckIntrinsicSizes = checkIntrinsicSizes;
   });
 
   testWidgets("LayoutBuilder returns child's baseline", (WidgetTester tester) async {