Utility methods for measuring text (#111493)

Author: dnfield

packages/flutter/lib/src/cupertino/date_picker.dart

@@ -470,23 +470,13 @@ class CupertinoDatePicker extends StatefulWidget {
 
     assert(longestText != '', 'column type is not appropriate');
 
-    final TextPainter painter = TextPainter(
+    return TextPainter.computeMaxIntrinsicWidth(
       text: TextSpan(
         style: _themeTextStyle(context),
         text: longestText,
       ),
       textDirection: Directionality.of(context),
     );
-
-    // This operation is expensive and should be avoided. It is called here only
-    // because there's no other way to get the information we want without
-    // laying out the text.
-    painter.layout();
-    try {
-      return painter.maxIntrinsicWidth;
-    } finally {
-      painter.dispose();
-    }
   }
 }
 

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

@@ -217,6 +217,86 @@ class TextPainter {
        _textWidthBasis = textWidthBasis,
        _textHeightBehavior = textHeightBehavior;
 
+  /// Computes the width of a configured [TextPainter].
+  ///
+  /// This is a convenience method that creates a text painter with the supplied
+  /// parameters, lays it out with the supplied [minWidth] and [maxWidth], and
+  /// returns its [TextPainter.width] making sure to dispose the underlying
+  /// resources.
+  static double computeWidth({
+    required InlineSpan text,
+    TextAlign textAlign = TextAlign.start,
+    TextDirection? textDirection,
+    double textScaleFactor = 1.0,
+    int? maxLines,
+    String? ellipsis,
+    Locale? locale,
+    StrutStyle? strutStyle,
+    TextWidthBasis textWidthBasis = TextWidthBasis.parent,
+    ui.TextHeightBehavior? textHeightBehavior,
+    double minWidth = 0.0,
+    double maxWidth = double.infinity,
+  }) {
+    final TextPainter painter = TextPainter(
+      text: text,
+      textAlign: textAlign,
+      textDirection: textDirection,
+      textScaleFactor: textScaleFactor,
+      maxLines: maxLines,
+      ellipsis: ellipsis,
+      locale: locale,
+      strutStyle: strutStyle,
+      textWidthBasis: textWidthBasis,
+      textHeightBehavior: textHeightBehavior,
+    )..layout(minWidth: minWidth, maxWidth: maxWidth);
+
+    try {
+      return painter.width;
+    } finally {
+      painter.dispose();
+    }
+  }
+
+  /// Computes the max intrinsic width of a configured [TextPainter].
+  ///
+  /// This is a convenience method that creates a text painter with the supplied
+  /// parameters, lays it out with the supplied [minWidth] and [maxWidth], and
+  /// returns its [TextPainter.maxIntrinsicWidth] making sure to dispose the
+  /// underlying resources.
+  static double computeMaxIntrinsicWidth({
+    required InlineSpan text,
+    TextAlign textAlign = TextAlign.start,
+    TextDirection? textDirection,
+    double textScaleFactor = 1.0,
+    int? maxLines,
+    String? ellipsis,
+    Locale? locale,
+    StrutStyle? strutStyle,
+    TextWidthBasis textWidthBasis = TextWidthBasis.parent,
+    ui.TextHeightBehavior? textHeightBehavior,
+    double minWidth = 0.0,
+    double maxWidth = double.infinity,
+  }) {
+    final TextPainter painter = TextPainter(
+      text: text,
+      textAlign: textAlign,
+      textDirection: textDirection,
+      textScaleFactor: textScaleFactor,
+      maxLines: maxLines,
+      ellipsis: ellipsis,
+      locale: locale,
+      strutStyle: strutStyle,
+      textWidthBasis: textWidthBasis,
+      textHeightBehavior: textHeightBehavior,
+    )..layout(minWidth: minWidth, maxWidth: maxWidth);
+
+    try {
+      return painter.maxIntrinsicWidth;
+    } finally {
+      painter.dispose();
+    }
+  }
+
   // _paragraph being null means the text needs layout because of style changes.
   // Setting _paragraph to null invalidates all the layout cache.
   //

packages/flutter/test/painting/text_painter_test.dart

@@ -1184,6 +1184,30 @@ void main() {
     painter.dispose();
     expect(painter.debugDisposed, true);
   });
+
+  test('TextPainter computeWidth', () {
+    const InlineSpan text = TextSpan(text: 'foobar');
+    final TextPainter painter = TextPainter(text: text, textDirection: TextDirection.ltr);
+    painter.layout();
+    expect(painter.width, TextPainter.computeWidth(text: text, textDirection: TextDirection.ltr));
+
+    painter.layout(minWidth: 500);
+    expect(painter.width, TextPainter.computeWidth(text: text, textDirection: TextDirection.ltr, minWidth: 500));
+
+    painter.dispose();
+  });
+
+  test('TextPainter computeMaxIntrinsicWidth', () {
+    const InlineSpan text = TextSpan(text: 'foobar');
+    final TextPainter painter = TextPainter(text: text, textDirection: TextDirection.ltr);
+    painter.layout();
+    expect(painter.maxIntrinsicWidth, TextPainter.computeMaxIntrinsicWidth(text: text, textDirection: TextDirection.ltr));
+
+    painter.layout(minWidth: 500);
+    expect(painter.maxIntrinsicWidth, TextPainter.computeMaxIntrinsicWidth(text: text, textDirection: TextDirection.ltr, minWidth: 500));
+
+    painter.dispose();
+  });
 }
 
 class MockCanvas extends Fake implements Canvas {