Implement getDryBaseline for Stack and Overlay (#146253)

Author: LongCatIsLooong

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

@@ -229,6 +229,29 @@ class StackParentData extends ContainerBoxParentData<RenderBox> {
   /// children in the stack.
   bool get isPositioned => top != null || right != null || bottom != null || left != null || width != null || height != null;
 
+  /// Computes the [BoxConstraints] the stack layout algorithm would give to
+  /// this child, given the [Size] of the stack.
+  ///
+  /// This method should only be called when [isPositioned] is true for the child.
+  BoxConstraints positionedChildConstraints(Size stackSize) {
+    assert(isPositioned);
+    final double? width = switch ((left, right)) {
+      (final double left?, final double right?) => stackSize.width - right - left,
+      (_, _) => this.width,
+    };
+
+    final double? height = switch ((top, bottom)) {
+      (final double top?, final double bottom?) => stackSize.height - bottom - top,
+      (_, _) => this.height,
+    };
+    assert(height == null || !height.isNaN);
+    assert(width == null || !width.isNaN);
+    return BoxConstraints.tightFor(
+      width: width == null ? null : math.max(0.0, width),
+      height: height == null ? null : math.max(0.0, height),
+    );
+  }
+
   @override
   String toString() {
     final List<String> values = <String>[
@@ -354,17 +377,11 @@ class RenderStack extends RenderBox
     }
   }
 
-  Alignment? _resolvedAlignment;
-
-  void _resolve() {
-    if (_resolvedAlignment != null) {
-      return;
-    }
-    _resolvedAlignment = alignment.resolve(textDirection);
-  }
+  Alignment get _resolvedAlignment => _resolvedAlignmentCache ??= alignment.resolve(textDirection);
+  Alignment? _resolvedAlignmentCache;
 
   void _markNeedResolution() {
-    _resolvedAlignment = null;
+    _resolvedAlignmentCache = null;
     markNeedsLayout();
   }
 
@@ -489,53 +506,59 @@ class RenderStack extends RenderBox
   static bool layoutPositionedChild(RenderBox child, StackParentData childParentData, Size size, Alignment alignment) {
     assert(childParentData.isPositioned);
     assert(child.parentData == childParentData);
+    final BoxConstraints childConstraints = childParentData.positionedChildConstraints(size);
+    child.layout(childConstraints, parentUsesSize: true);
 
-    bool hasVisualOverflow = false;
-    BoxConstraints childConstraints = const BoxConstraints();
-
-    if (childParentData.left != null && childParentData.right != null) {
-      childConstraints = childConstraints.tighten(width: size.width - childParentData.right! - childParentData.left!);
-    } else if (childParentData.width != null) {
-      childConstraints = childConstraints.tighten(width: childParentData.width);
-    }
+    final double x = switch (childParentData) {
+      StackParentData(:final double left?) => left,
+      StackParentData(:final double right?) => size.width - right - child.size.width,
+      StackParentData() => alignment.alongOffset(size - child.size as Offset).dx,
+    };
 
-    if (childParentData.top != null && childParentData.bottom != null) {
-      childConstraints = childConstraints.tighten(height: size.height - childParentData.bottom! - childParentData.top!);
-    } else if (childParentData.height != null) {
-      childConstraints = childConstraints.tighten(height: childParentData.height);
-    }
+    final double y = switch (childParentData) {
+      StackParentData(:final double top?) => top,
+      StackParentData(:final double bottom?) => size.height - bottom - child.size.height,
+      StackParentData() => alignment.alongOffset(size - child.size as Offset).dy,
+    };
 
-    child.layout(childConstraints, parentUsesSize: true);
+    childParentData.offset = Offset(x, y);
+    return x < 0.0 || x + child.size.width > size.width
+        || y < 0.0 || y + child.size.height > size.height;
+  }
 
-    final double x;
-    if (childParentData.left != null) {
-      x = childParentData.left!;
-    } else if (childParentData.right != null) {
-      x = size.width - childParentData.right! - child.size.width;
-    } else {
-      x = alignment.alongOffset(size - child.size as Offset).dx;
+  static double? _baselineForChild(RenderBox child, Size stackSize, BoxConstraints nonPositionedChildConstraints, Alignment alignment, TextBaseline baseline) {
+    final StackParentData childParentData = child.parentData! as StackParentData;
+    final BoxConstraints childConstraints = childParentData.isPositioned
+      ? childParentData.positionedChildConstraints(stackSize)
+      : nonPositionedChildConstraints;
+    final double? baselineOffset = child.getDryBaseline(childConstraints, baseline);
+    if (baselineOffset == null) {
+      return null;
     }
+    final double y = switch (childParentData) {
+      StackParentData(:final double top?) => top,
+      StackParentData(:final double bottom?) => stackSize.height - bottom - child.getDryLayout(childConstraints).height,
+      StackParentData() => alignment.alongOffset(stackSize - child.getDryLayout(childConstraints) as Offset).dy,
+    };
+    return baselineOffset + y;
+  }
 
-    if (x < 0.0 || x + child.size.width > size.width) {
-      hasVisualOverflow = true;
-    }
+  @override
+  double? computeDryBaseline(BoxConstraints constraints, TextBaseline baseline) {
+    final BoxConstraints nonPositionedChildConstraints = switch (fit) {
+      StackFit.loose => constraints.loosen(),
+      StackFit.expand => BoxConstraints.tight(constraints.biggest),
+      StackFit.passthrough => constraints,
+    };
 
-    final double y;
-    if (childParentData.top != null) {
-      y = childParentData.top!;
-    } else if (childParentData.bottom != null) {
-      y = size.height - childParentData.bottom! - child.size.height;
-    } else {
-      y = alignment.alongOffset(size - child.size as Offset).dy;
-    }
+    final Alignment alignment = _resolvedAlignment;
+    final Size size = getDryLayout(constraints);
 
-    if (y < 0.0 || y + child.size.height > size.height) {
-      hasVisualOverflow = true;
+    BaselineOffset baselineOffset = BaselineOffset.noBaseline;
+    for (RenderBox? child = firstChild; child != null; child = childAfter(child)) {
+      baselineOffset = baselineOffset.minOf(BaselineOffset(_baselineForChild(child, size, nonPositionedChildConstraints, alignment, baseline)));
     }
-
-    childParentData.offset = Offset(x, y);
-
-    return hasVisualOverflow;
+    return baselineOffset.offset;
   }
 
   @override
@@ -548,8 +571,6 @@ class RenderStack extends RenderBox
   }
 
   Size _computeSize({required BoxConstraints constraints, required ChildLayouter layoutChild}) {
-    _resolve();
-    assert(_resolvedAlignment != null);
     bool hasNonPositionedChildren = false;
     if (childCount == 0) {
       return (constraints.biggest.isFinite) ? constraints.biggest : constraints.smallest;
@@ -603,15 +624,15 @@ class RenderStack extends RenderBox
       layoutChild: ChildLayoutHelper.layoutChild,
     );
 
-    assert(_resolvedAlignment != null);
+    final Alignment resolvedAlignment = _resolvedAlignment;
     RenderBox? child = firstChild;
     while (child != null) {
       final StackParentData childParentData = child.parentData! as StackParentData;
 
       if (!childParentData.isPositioned) {
-        childParentData.offset = _resolvedAlignment!.alongOffset(size - child.size as Offset);
+        childParentData.offset = resolvedAlignment.alongOffset(size - child.size as Offset);
       } else {
-        _hasVisualOverflow = layoutPositionedChild(child, childParentData, size, _resolvedAlignment!) || _hasVisualOverflow;
+        _hasVisualOverflow = layoutPositionedChild(child, childParentData, size, resolvedAlignment) || _hasVisualOverflow;
       }
 
       assert(child.parentData == childParentData);
@@ -740,6 +761,25 @@ class RenderIndexedStack extends RenderStack {
     return offset.offset;
   }
 
+  @override
+  double? computeDryBaseline(BoxConstraints constraints, TextBaseline baseline) {
+    final RenderBox? displayedChild = _childAtIndex();
+    if (displayedChild == null) {
+      return null;
+    }
+    final BoxConstraints nonPositionedChildConstraints = switch (fit) {
+      StackFit.loose => constraints.loosen(),
+      StackFit.expand => BoxConstraints.tight(constraints.biggest),
+      StackFit.passthrough => constraints,
+    };
+
+    final Alignment alignment = _resolvedAlignment;
+    final Size size = getDryLayout(constraints);
+
+    return RenderStack._baselineForChild(displayedChild, size, nonPositionedChildConstraints, alignment, baseline);
+  }
+
+
   @override
   bool hitTestChildren(BoxHitTestResult result, { required Offset position }) {
     final RenderBox? displayedChild = _childAtIndex();

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

@@ -3,7 +3,6 @@
 // found in the LICENSE file.
 
 import 'dart:collection';
-import 'dart:math' as math;
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
@@ -967,6 +966,35 @@ mixin _RenderTheaterMixin on RenderBox {
     }
   }
 
+  @override
+  double? computeDistanceToActualBaseline(TextBaseline baseline) {
+    assert(!debugNeedsLayout);
+    BaselineOffset baselineOffset = BaselineOffset.noBaseline;
+    for (final RenderBox child in _childrenInPaintOrder()) {
+      assert(!child.debugNeedsLayout);
+      final StackParentData childParentData = child.parentData! as StackParentData;
+      baselineOffset = baselineOffset.minOf(BaselineOffset(child.getDistanceToActualBaseline(baseline)) + childParentData.offset.dy);
+    }
+    return baselineOffset.offset;
+  }
+
+  static double? baselineForChild(RenderBox child, Size theaterSize, BoxConstraints nonPositionedChildConstraints, Alignment alignment, TextBaseline baseline) {
+    final StackParentData childParentData = child.parentData! as StackParentData;
+    final BoxConstraints childConstraints = childParentData.isPositioned
+      ? childParentData.positionedChildConstraints(theaterSize)
+      : nonPositionedChildConstraints;
+    final double? baselineOffset = child.getDryBaseline(childConstraints, baseline);
+    if (baselineOffset == null) {
+      return null;
+    }
+    final double y = switch (childParentData) {
+      StackParentData(:final double top?) => top,
+      StackParentData(:final double bottom?) => theaterSize.height - bottom - child.getDryLayout(childConstraints).height,
+      StackParentData() => alignment.alongOffset(theaterSize - child.getDryLayout(childConstraints) as Offset).dy,
+    };
+    return baselineOffset + y;
+  }
+
   void layoutChild(RenderBox child, BoxConstraints nonPositionedChildConstraints) {
     final StackParentData childParentData = child.parentData! as StackParentData;
     final Alignment alignment = theater._resolvedAlignment;
@@ -1201,25 +1229,20 @@ class _RenderTheater extends RenderBox with ContainerRenderObjectMixin<RenderBox
   }
 
   @override
-  double? computeDistanceToActualBaseline(TextBaseline baseline) {
-    assert(!debugNeedsLayout);
-    double? result;
-    RenderBox? child = _firstOnstageChild;
-    while (child != null) {
-      assert(!child.debugNeedsLayout);
-      final StackParentData childParentData = child.parentData! as StackParentData;
-      double? candidate = child.getDistanceToActualBaseline(baseline);
-      if (candidate != null) {
-        candidate += childParentData.offset.dy;
-        if (result != null) {
-          result = math.min(result, candidate);
-        } else {
-          result = candidate;
-        }
-      }
-      child = childParentData.nextSibling;
+  double? computeDryBaseline(BoxConstraints constraints, TextBaseline baseline) {
+    final Size size = constraints.biggest.isFinite
+      ? constraints.biggest
+      : _findSizeDeterminingChild().getDryLayout(constraints);
+    final BoxConstraints nonPositionedChildConstraints = BoxConstraints.tight(size);
+    final Alignment alignment = theater._resolvedAlignment;
+
+    BaselineOffset baselineOffset = BaselineOffset.noBaseline;
+    for (final RenderBox child in _childrenInPaintOrder()) {
+      baselineOffset = baselineOffset.minOf(BaselineOffset(
+        _RenderTheaterMixin.baselineForChild(child, size, nonPositionedChildConstraints, alignment, baseline),
+      ));
     }
-    return result;
+    return baselineOffset.offset;
   }
 
   @override
@@ -2247,6 +2270,15 @@ final class _RenderDeferredLayoutBox extends RenderProxyBox with _RenderTheaterM
     super.markNeedsLayout();
   }
 
+  @override
+  double? computeDryBaseline(BoxConstraints constraints, TextBaseline baseline) {
+    final RenderBox? child = this.child;
+    if (child == null) {
+      return null;
+    }
+    return _RenderTheaterMixin.baselineForChild(child, constraints.biggest, constraints, theater._resolvedAlignment, baseline);
+  }
+
   @override
   RenderObject? get debugLayoutParent => _layoutSurrogate;
 

packages/flutter/test/rendering/stack_test.dart

@@ -10,6 +10,36 @@ import 'rendering_tester.dart';
 void main() {
   TestRenderingFlutterBinding.ensureInitialized();
 
+  test('StackParentData basic test', () {
+    final StackParentData parentData = StackParentData();
+    const Size stackSize = Size(800.0, 600.0);
+    expect(parentData.isPositioned, isFalse);
+
+    parentData.width = -100.0;
+    expect(parentData.isPositioned, isTrue);
+    expect(parentData.positionedChildConstraints(stackSize), const BoxConstraints.tightFor(width: 0.0));
+
+    parentData.width = 100.0;
+    expect(parentData.positionedChildConstraints(stackSize), const BoxConstraints.tightFor(width: 100.0));
+
+    parentData.left = 0.0;
+    parentData.right = 0.0;
+    expect(parentData.positionedChildConstraints(stackSize), const BoxConstraints.tightFor(width: 800.0));
+
+    parentData.height = -100.0;
+    expect(parentData.positionedChildConstraints(stackSize), const BoxConstraints.tightFor(width: 800.0, height: 0.0));
+
+    parentData.height = 100.0;
+    expect(parentData.positionedChildConstraints(stackSize), const BoxConstraints.tightFor(width: 800.0, height: 100.0));
+
+    parentData.top = 0.0;
+    parentData.bottom = 0.0;
+    expect(parentData.positionedChildConstraints(stackSize), const BoxConstraints.tightFor(width: 800.0, height: 600.0));
+
+    parentData.bottom = 1000.0;
+    expect(parentData.positionedChildConstraints(stackSize), const BoxConstraints.tightFor(width: 800.0, height: 0.0));
+  });
+
   test('Stack can layout with top, right, bottom, left 0.0', () {
     final RenderBox size = RenderConstrainedBox(
       additionalConstraints: BoxConstraints.tight(const Size(100.0, 100.0)),