Text wrap width (#31987)

Add `textWidthBasis` param to Text to allow calculating width according to longest line.

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

@@ -15,6 +15,21 @@ import 'text_span.dart';
 
 export 'package:flutter/services.dart' show TextRange, TextSelection;
 
+/// The different ways of considering the width of one or more lines of text.
+///
+/// See [Text.widthType].
+enum TextWidthBasis {
+  /// Multiline text will take up the full width given by the parent. For single
+  /// line text, only the minimum amount of width needed to contain the text
+  /// will be used. A common use case for this is a standard series of
+  /// paragraphs.
+  parent,
+
+  /// The width will be exactly enough to contain the longest line and no
+  /// longer. A common use case for this is chat bubbles.
+  longestLine,
+}
+
 class _CaretMetrics {
   const _CaretMetrics({this.offset, this.fullHeight});
   /// The offset of the top left corner of the caret from the top left
@@ -60,10 +75,12 @@ class TextPainter {
     String ellipsis,
     Locale locale,
     StrutStyle strutStyle,
+    TextWidthBasis textWidthBasis = TextWidthBasis.parent,
   }) : assert(text == null || text.debugAssertIsValid()),
        assert(textAlign != null),
        assert(textScaleFactor != null),
        assert(maxLines == null || maxLines > 0),
+       assert(textWidthBasis != null),
        _text = text,
        _textAlign = textAlign,
        _textDirection = textDirection,
@@ -71,7 +88,8 @@ class TextPainter {
        _maxLines = maxLines,
        _ellipsis = ellipsis,
        _locale = locale,
-       _strutStyle = strutStyle;
+       _strutStyle = strutStyle,
+       _textWidthBasis = textWidthBasis;
 
   ui.Paragraph _paragraph;
   bool _needsLayout = true;
@@ -233,6 +251,18 @@ class TextPainter {
     _needsLayout = true;
   }
 
+  /// {@macro flutter.dart:ui.text.TextWidthBasis}
+  TextWidthBasis get textWidthBasis => _textWidthBasis;
+  TextWidthBasis _textWidthBasis;
+  set textWidthBasis(TextWidthBasis value) {
+    assert(value != null);
+    if (_textWidthBasis == value)
+      return;
+    _textWidthBasis = value;
+    _paragraph = null;
+    _needsLayout = true;
+  }
+
 
   ui.Paragraph _layoutTemplate;
 
@@ -317,7 +347,9 @@ class TextPainter {
   /// Valid only after [layout] has been called.
   double get width {
     assert(!_needsLayout);
-    return _applyFloatingPointHack(_paragraph.width);
+    return _applyFloatingPointHack(
+      textWidthBasis == TextWidthBasis.longestLine ? _paragraph.longestLine : _paragraph.width,
+    );
   }
 
   /// The vertical space required to paint this text.

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

@@ -49,6 +49,7 @@ class RenderParagraph extends RenderBox {
     TextOverflow overflow = TextOverflow.clip,
     double textScaleFactor = 1.0,
     int maxLines,
+    TextWidthBasis textWidthBasis = TextWidthBasis.parent,
     Locale locale,
     StrutStyle strutStyle,
   }) : assert(text != null),
@@ -59,6 +60,7 @@ class RenderParagraph extends RenderBox {
        assert(overflow != null),
        assert(textScaleFactor != null),
        assert(maxLines == null || maxLines > 0),
+       assert(textWidthBasis != null),
        _softWrap = softWrap,
        _overflow = overflow,
        _textPainter = TextPainter(
@@ -70,6 +72,7 @@ class RenderParagraph extends RenderBox {
          ellipsis: overflow == TextOverflow.ellipsis ? _kEllipsis : null,
          locale: locale,
          strutStyle: strutStyle,
+         textWidthBasis: textWidthBasis,
        );
 
   final TextPainter _textPainter;
@@ -212,6 +215,17 @@ class RenderParagraph extends RenderBox {
     markNeedsLayout();
   }
 
+  /// {@macro flutter.widgets.basic.TextWidthBasis}
+  TextWidthBasis get textWidthBasis => _textPainter.textWidthBasis;
+  set textWidthBasis(TextWidthBasis value) {
+    assert(value != null);
+    if (_textPainter.textWidthBasis == value)
+      return;
+    _textPainter.textWidthBasis = value;
+    _overflowShader = null;
+    markNeedsLayout();
+  }
+
   void _layoutText({ double minWidth = 0.0, double maxWidth = double.infinity }) {
     final bool widthMatters = softWrap || overflow == TextOverflow.ellipsis;
     _textPainter.layout(minWidth: minWidth, maxWidth: widthMatters ? maxWidth : double.infinity);

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

@@ -4811,12 +4811,14 @@ class RichText extends LeafRenderObjectWidget {
     this.maxLines,
     this.locale,
     this.strutStyle,
+    this.textWidthBasis = TextWidthBasis.parent,
   }) : assert(text != null),
        assert(textAlign != null),
        assert(softWrap != null),
        assert(overflow != null),
        assert(textScaleFactor != null),
        assert(maxLines == null || maxLines > 0),
+       assert(textWidthBasis != null),
        super(key: key);
 
   /// The text to display in this widget.
@@ -4875,6 +4877,9 @@ class RichText extends LeafRenderObjectWidget {
   /// {@macro flutter.painting.textPainter.strutStyle}
   final StrutStyle strutStyle;
 
+  /// {@macro flutter.widgets.text.DefaultTextStyle.textWidthBasis}
+  final TextWidthBasis textWidthBasis;
+
   @override
   RenderParagraph createRenderObject(BuildContext context) {
     assert(textDirection != null || debugCheckHasDirectionality(context));
@@ -4886,6 +4891,7 @@ class RichText extends LeafRenderObjectWidget {
       textScaleFactor: textScaleFactor,
       maxLines: maxLines,
       strutStyle: strutStyle,
+      textWidthBasis: textWidthBasis,
       locale: locale ?? Localizations.localeOf(context, nullOk: true),
     );
   }
@@ -4902,6 +4908,7 @@ class RichText extends LeafRenderObjectWidget {
       ..textScaleFactor = textScaleFactor
       ..maxLines = maxLines
       ..strutStyle = strutStyle
+      ..textWidthBasis = textWidthBasis
       ..locale = locale ?? Localizations.localeOf(context, nullOk: true);
   }
 
@@ -4914,6 +4921,7 @@ class RichText extends LeafRenderObjectWidget {
     properties.add(EnumProperty<TextOverflow>('overflow', overflow, defaultValue: TextOverflow.clip));
     properties.add(DoubleProperty('textScaleFactor', textScaleFactor, defaultValue: 1.0));
     properties.add(IntProperty('maxLines', maxLines, ifNull: 'unlimited'));
+    properties.add(EnumProperty<TextWidthBasis>('textWidthBasis', textWidthBasis, defaultValue: TextWidthBasis.parent));
     properties.add(StringProperty('text', text.toPlainText()));
   }
 }

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

@@ -33,12 +33,14 @@ class DefaultTextStyle extends InheritedWidget {
     this.softWrap = true,
     this.overflow = TextOverflow.clip,
     this.maxLines,
+    this.textWidthBasis = TextWidthBasis.parent,
     @required Widget child,
   }) : assert(style != null),
        assert(softWrap != null),
        assert(overflow != null),
        assert(maxLines == null || maxLines > 0),
        assert(child != null),
+       assert(textWidthBasis != null),
        super(key: key, child: child);
 
   /// A const-constructible default text style that provides fallback values.
@@ -52,7 +54,8 @@ class DefaultTextStyle extends InheritedWidget {
       textAlign = null,
       softWrap = true,
       maxLines = null,
-      overflow = TextOverflow.clip;
+      overflow = TextOverflow.clip,
+      textWidthBasis = TextWidthBasis.parent;
 
   /// Creates a default text style that overrides the text styles in scope at
   /// this point in the widget tree.
@@ -77,6 +80,7 @@ class DefaultTextStyle extends InheritedWidget {
     bool softWrap,
     TextOverflow overflow,
     int maxLines,
+    TextWidthBasis textWidthBasis,
     @required Widget child,
   }) {
     assert(child != null);
@@ -90,6 +94,7 @@ class DefaultTextStyle extends InheritedWidget {
           softWrap: softWrap ?? parent.softWrap,
           overflow: overflow ?? parent.overflow,
           maxLines: maxLines ?? parent.maxLines,
+          textWidthBasis: textWidthBasis ?? parent.textWidthBasis,
           child: child,
         );
       },
@@ -121,6 +126,10 @@ class DefaultTextStyle extends InheritedWidget {
   /// [Text.maxLines].
   final int maxLines;
 
+  /// The strategy to use when calculating the width of the Text.
+  /// See [TextWidthBasis] for possible values and their implications.
+  final TextWidthBasis textWidthBasis;
+
   /// The closest instance of this class that encloses the given context.
   ///
   /// If no such instance exists, returns an instance created by
@@ -141,7 +150,8 @@ class DefaultTextStyle extends InheritedWidget {
         textAlign != oldWidget.textAlign ||
         softWrap != oldWidget.softWrap ||
         overflow != oldWidget.overflow ||
-        maxLines != oldWidget.maxLines;
+        maxLines != oldWidget.maxLines ||
+        textWidthBasis != oldWidget.textWidthBasis;
   }
 
   @override
@@ -152,6 +162,7 @@ class DefaultTextStyle extends InheritedWidget {
     properties.add(FlagProperty('softWrap', value: softWrap, ifTrue: 'wrapping at box width', ifFalse: 'no wrapping except at line break characters', showName: true));
     properties.add(EnumProperty<TextOverflow>('overflow', overflow, defaultValue: null));
     properties.add(IntProperty('maxLines', maxLines, defaultValue: null));
+    properties.add(EnumProperty<TextWidthBasis>('textWidthBasis', textWidthBasis, defaultValue: TextWidthBasis.parent));
   }
 }
 
@@ -237,6 +248,7 @@ class Text extends StatelessWidget {
     this.textScaleFactor,
     this.maxLines,
     this.semanticsLabel,
+    this.textWidthBasis,
   }) : assert(
          data != null,
          'A non-null String must be provided to a Text widget.',
@@ -260,6 +272,7 @@ class Text extends StatelessWidget {
     this.textScaleFactor,
     this.maxLines,
     this.semanticsLabel,
+    this.textWidthBasis,
   }) : assert(
          textSpan != null,
          'A non-null TextSpan must be provided to a Text.rich widget.',
@@ -359,6 +372,9 @@ class Text extends StatelessWidget {
   /// ```
   final String semanticsLabel;
 
+  /// {@macro flutter.dart:ui.text.TextWidthBasis}
+  final TextWidthBasis textWidthBasis;
+
   @override
   Widget build(BuildContext context) {
     final DefaultTextStyle defaultTextStyle = DefaultTextStyle.of(context);
@@ -376,6 +392,7 @@ class Text extends StatelessWidget {
       textScaleFactor: textScaleFactor ?? MediaQuery.textScaleFactorOf(context),
       maxLines: maxLines ?? defaultTextStyle.maxLines,
       strutStyle: strutStyle,
+      textWidthBasis: textWidthBasis ?? defaultTextStyle.textWidthBasis,
       text: TextSpan(
         style: effectiveTextStyle,
         text: data,

packages/flutter/test/widgets/text_test.dart

@@ -6,6 +6,7 @@ import 'package:flutter/gestures.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter/foundation.dart';
+import 'package:flutter/material.dart';
 
 import '../rendering/mock_canvas.dart';
 import 'semantics_tester.dart';
@@ -363,6 +364,43 @@ void main() {
 
     expect(find.byType(Text), isNot(paints..clipRect()));
   });
+
+  testWidgets('textWidthBasis affects the width of a Text widget', (WidgetTester tester) async {
+    Future<void> createText(TextWidthBasis textWidthBasis) {
+      return tester.pumpWidget(
+        MaterialApp(
+          home: Scaffold(
+            body: Center(
+              child: Container(
+                // Each word takes up more than a half of a line. Together they
+                // wrap onto two lines, but leave a lot of extra space.
+                child: Text('twowordsthateachtakeupmorethanhalfof alineoftextsothattheywrapwithlotsofextraspace',
+                  textDirection: TextDirection.ltr,
+                  textWidthBasis: textWidthBasis,
+                ),
+              ),
+            ),
+          ),
+        ),
+      );
+    }
+
+    const double fontHeight = 14.0;
+    const double screenWidth = 800.0;
+
+    // When textWidthBasis is parent, takes up full screen width.
+    await createText(TextWidthBasis.parent);
+    final Size textSizeParent = tester.getSize(find.byType(Text));
+    expect(textSizeParent.width, equals(screenWidth));
+    expect(textSizeParent.height, equals(fontHeight * 2));
+
+    // When textWidthBasis is longestLine, sets the width to as small as
+    // possible for the two lines.
+    await createText(TextWidthBasis.longestLine);
+    final Size textSizeLongestLine = tester.getSize(find.byType(Text));
+    expect(textSizeLongestLine.width, equals(630.0));
+    expect(textSizeLongestLine.height, equals(fontHeight * 2));
+  });
 }
 
 Future<void> _pumpTextWidget({ WidgetTester tester, String text, TextOverflow overflow }) {