TextPainter throw with stack trace to help track down read-before-layout (#108571)

packages/flutter/lib/src/painting/text_painter.dart

@@ -206,7 +206,18 @@ class TextPainter {
   // rebuilt before painting.
   bool _rebuildParagraphForPaint = true;
 
-  bool get _debugNeedsLayout => _paragraph == null;
+  bool get _debugAssertTextLayoutIsValid {
+    if (_paragraph == null) {
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary('Text layout not available'),
+        if (_debugMarkNeedsLayoutCallStack != null) DiagnosticsStackTrace('The calls that first invalidated the text layout were', _debugMarkNeedsLayoutCallStack)
+        else ErrorDescription('The TextPainter has never been laid out.')
+      ]);
+    }
+    return true;
+  }
+
+  StackTrace? _debugMarkNeedsLayoutCallStack;
 
   /// Marks this text painter's layout information as dirty and removes cached
   /// information.
@@ -215,6 +226,12 @@ class TextPainter {
   /// layout changes in engine. In most cases, updating text painter properties
   /// in framework will automatically invoke this method.
   void markNeedsLayout() {
+    assert(() {
+      if (_paragraph != null) {
+        _debugMarkNeedsLayoutCallStack ??= StackTrace.current;
+      }
+      return true;
+    }());
     _paragraph = null;
     _lineMetricsCache = null;
     _previousCaretPosition = null;
@@ -540,7 +557,7 @@ class TextPainter {
   ///
   /// Valid only after [layout] has been called.
   double get minIntrinsicWidth {
-    assert(!_debugNeedsLayout);
+    assert(_debugAssertTextLayoutIsValid);
     return _applyFloatingPointHack(_paragraph!.minIntrinsicWidth);
   }
 
@@ -548,7 +565,7 @@ class TextPainter {
   ///
   /// Valid only after [layout] has been called.
   double get maxIntrinsicWidth {
-    assert(!_debugNeedsLayout);
+    assert(_debugAssertTextLayoutIsValid);
     return _applyFloatingPointHack(_paragraph!.maxIntrinsicWidth);
   }
 
@@ -556,7 +573,7 @@ class TextPainter {
   ///
   /// Valid only after [layout] has been called.
   double get width {
-    assert(!_debugNeedsLayout);
+    assert(_debugAssertTextLayoutIsValid);
     return _applyFloatingPointHack(
       textWidthBasis == TextWidthBasis.longestLine ? _paragraph!.longestLine : _paragraph!.width,
     );
@@ -566,7 +583,7 @@ class TextPainter {
   ///
   /// Valid only after [layout] has been called.
   double get height {
-    assert(!_debugNeedsLayout);
+    assert(_debugAssertTextLayoutIsValid);
     return _applyFloatingPointHack(_paragraph!.height);
   }
 
@@ -574,7 +591,7 @@ class TextPainter {
   ///
   /// Valid only after [layout] has been called.
   Size get size {
-    assert(!_debugNeedsLayout);
+    assert(_debugAssertTextLayoutIsValid);
     return Size(width, height);
   }
 
@@ -583,7 +600,7 @@ class TextPainter {
   ///
   /// Valid only after [layout] has been called.
   double computeDistanceToActualBaseline(TextBaseline baseline) {
-    assert(!_debugNeedsLayout);
+    assert(_debugAssertTextLayoutIsValid);
     assert(baseline != null);
     switch (baseline) {
       case TextBaseline.alphabetic:
@@ -605,7 +622,7 @@ class TextPainter {
   ///
   /// Valid only after [layout] has been called.
   bool get didExceedMaxLines {
-    assert(!_debugNeedsLayout);
+    assert(_debugAssertTextLayoutIsValid);
     return _paragraph!.didExceedMaxLines;
   }
 
@@ -623,6 +640,10 @@ class TextPainter {
     final ui.ParagraphBuilder builder = ui.ParagraphBuilder(_createParagraphStyle());
     text.build(builder, textScaleFactor: textScaleFactor, dimensions: _placeholderDimensions);
     _inlinePlaceholderScales = builder.placeholderScales;
+    assert(() {
+      _debugMarkNeedsLayoutCallStack = null;
+      return true;
+    }());
     _paragraph = builder.build();
     _rebuildParagraphForPaint = false;
   }
@@ -859,7 +880,7 @@ class TextPainter {
   }
 
   Offset get _emptyOffset {
-    assert(!_debugNeedsLayout); // implies textDirection is non-null
+    assert(_debugAssertTextLayoutIsValid); // implies textDirection is non-null
     assert(textAlign != null);
     switch (textAlign) {
       case TextAlign.left:
@@ -920,7 +941,7 @@ class TextPainter {
   // Checks if the [position] and [caretPrototype] have changed from the cached
   // version and recomputes the metrics required to position the caret.
   void _computeCaretMetrics(TextPosition position, Rect caretPrototype) {
-    assert(!_debugNeedsLayout);
+    assert(_debugAssertTextLayoutIsValid);
     if (position == _previousCaretPosition && caretPrototype == _previousCaretPrototype) {
       return;
     }
@@ -969,7 +990,7 @@ class TextPainter {
     ui.BoxHeightStyle boxHeightStyle = ui.BoxHeightStyle.tight,
     ui.BoxWidthStyle boxWidthStyle = ui.BoxWidthStyle.tight,
   }) {
-    assert(!_debugNeedsLayout);
+    assert(_debugAssertTextLayoutIsValid);
     assert(boxHeightStyle != null);
     assert(boxWidthStyle != null);
     return _paragraph!.getBoxesForRange(
@@ -982,7 +1003,7 @@ class TextPainter {
 
   /// Returns the position within the text for the given pixel offset.
   TextPosition getPositionForOffset(Offset offset) {
-    assert(!_debugNeedsLayout);
+    assert(_debugAssertTextLayoutIsValid);
     return _paragraph!.getPositionForOffset(offset);
   }
 
@@ -996,7 +1017,7 @@ class TextPainter {
   /// <http://www.unicode.org/reports/tr29/#Word_Boundaries>.
   /// {@endtemplate}
   TextRange getWordBoundary(TextPosition position) {
-    assert(!_debugNeedsLayout);
+    assert(_debugAssertTextLayoutIsValid);
     return _paragraph!.getWordBoundary(position);
   }
 
@@ -1004,7 +1025,7 @@ class TextPainter {
   ///
   /// The newline (if any) is not returned as part of the range.
   TextRange getLineBoundary(TextPosition position) {
-    assert(!_debugNeedsLayout);
+    assert(_debugAssertTextLayoutIsValid);
     return _paragraph!.getLineBoundary(position);
   }
 
@@ -1021,7 +1042,7 @@ class TextPainter {
   ///
   /// Valid only after [layout] has been called.
   List<ui.LineMetrics> computeLineMetrics() {
-    assert(!_debugNeedsLayout);
+    assert(_debugAssertTextLayoutIsValid);
     return  _lineMetricsCache ??= _paragraph!.computeLineMetrics();
   }
 }

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

@@ -8,6 +8,7 @@ import 'dart:ui' as ui show BoxHeightStyle, BoxWidthStyle, Gradient, Placeholder
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart';
+import 'package:flutter/scheduler.dart';
 import 'package:flutter/semantics.dart';
 
 import 'box.dart';
@@ -640,11 +641,38 @@ class RenderParagraph extends RenderBox
     );
   }
 
+  bool _systemFontsChangeScheduled = false;
   @override
   void systemFontsDidChange() {
+    final SchedulerPhase phase = SchedulerBinding.instance.schedulerPhase;
+    switch (phase) {
+      case SchedulerPhase.idle:
+      case SchedulerPhase.postFrameCallbacks:
+        if (_systemFontsChangeScheduled) {
+          return;
+        }
+        _systemFontsChangeScheduled = true;
+        SchedulerBinding.instance.scheduleFrameCallback((Duration timeStamp) {
+          assert(_systemFontsChangeScheduled);
+          _systemFontsChangeScheduled = false;
+          assert(
+            attached || (debugDisposed ?? true),
+            '$this is detached during $phase but not disposed.',
+          );
+          if (attached) {
             super.systemFontsDidChange();
             _textPainter.markNeedsLayout();
           }
+        });
+        break;
+      case SchedulerPhase.transientCallbacks:
+      case SchedulerPhase.midFrameMicrotasks:
+      case SchedulerPhase.persistentCallbacks:
+        super.systemFontsDidChange();
+        _textPainter.markNeedsLayout();
+        break;
+    }
+  }
 
   // Placeholder dimensions representing the sizes of child inline widgets.
   //

packages/flutter/test/painting/system_fonts_test.dart

@@ -2,9 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:async';
+
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 
@@ -15,6 +18,8 @@ void main() {
         home: Text('text widget'),
       ),
     );
+    final RenderObject renderObject = tester.renderObject(find.text('text widget'));
+
     const Map<String, dynamic> data = <String, dynamic>{
       'type': 'fontsChange',
     };
@@ -23,8 +28,39 @@ void main() {
       SystemChannels.system.codec.encodeMessage(data),
       (ByteData? data) { },
     );
-    final RenderObject renderObject = tester.renderObject(find.text('text widget'));
-    expect(renderObject.debugNeedsLayout, isTrue);
+
+    final Completer<bool> animation = Completer<bool>();
+    tester.binding.scheduleFrameCallback((Duration timeStamp) {
+      animation.complete(renderObject.debugNeedsLayout);
+    });
+    expect(renderObject.debugNeedsLayout, isFalse);
+    await tester.pump();
+    expect(await animation.future, isTrue);
+  });
+
+  testWidgets('Safe to query RenderParagraph for text layout after system fonts changes', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const MaterialApp(
+        home: Text('text widget'),
+      ),
+    );
+    const Map<String, dynamic> data = <String, dynamic>{
+      'type': 'fontsChange',
+    };
+    await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+      'flutter/system',
+      SystemChannels.system.codec.encodeMessage(data),
+      (ByteData? data) { },
+    );
+    final RenderParagraph paragraph = tester.renderObject<RenderParagraph>(find.text('text widget'));
+    Object? exception;
+    try {
+      paragraph.getPositionForOffset(Offset.zero);
+      paragraph.hitTest(BoxHitTestResult(), position: Offset.zero);
+    } catch (e) {
+      exception = e;
+    }
+    expect(exception, isNull);
   });
 
   testWidgets('RenderEditable relayout upon system fonts changes', (WidgetTester tester) async {

packages/flutter/test/painting/text_painter_test.dart

@@ -1034,6 +1034,41 @@ void main() {
     lines = painter.computeLineMetrics();
     expect(lines.length, 1);
   }, skip: kIsWeb && !isCanvasKit); // https://github.com/flutter/flutter/issues/62819
+
+  test('TextPainter throws with stack trace when accessing text layout', () {
+    final TextPainter painter = TextPainter()
+      ..text = const TextSpan(text: 'TEXT')
+      ..textDirection = TextDirection.ltr;
+
+    FlutterError? exception;
+    try {
+      painter.getPositionForOffset(Offset.zero);
+    } on FlutterError catch (e) {
+      exception = e;
+    }
+    expect(exception?.message, contains('The TextPainter has never been laid out.'));
+    exception = null;
+
+    try {
+      painter.layout();
+      painter.getPositionForOffset(Offset.zero);
+    } on FlutterError catch (e) {
+      exception = e;
+    }
+
+    expect(exception, isNull);
+    exception = null;
+
+    try {
+      painter.markNeedsLayout();
+      painter.getPositionForOffset(Offset.zero);
+    } on FlutterError catch (e) {
+      exception = e;
+    }
+
+    expect(exception?.message, contains('The calls that first invalidated the text layout were:'));
+    exception = null;
+  });
 }
 
 class MockCanvas extends Fake implements Canvas {