Added MediaQuery.textScaleFactorOf() (#17450)

examples/flutter_gallery/lib/gallery/home.dart

@@ -174,7 +174,7 @@ class _DemoItem extends StatelessWidget {
   Widget build(BuildContext context) {
     final ThemeData theme = Theme.of(context);
     final bool isDark = theme.brightness == Brightness.dark;
-    final double textScaleFactor = MediaQuery.of(context)?.textScaleFactor ?? 1.0;
+    final double textScaleFactor = MediaQuery.textScaleFactorOf(context);
 
     final List<Widget> titleChildren = <Widget>[
       new Text(

examples/flutter_gallery/lib/gallery/options.dart

@@ -93,7 +93,7 @@ class _OptionsItem extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final double textScaleFactor = MediaQuery.of(context)?.textScaleFactor ?? 1.0;
+    final double textScaleFactor = MediaQuery.textScaleFactorOf(context);
 
     return new MergeSemantics(
       child: new Container(

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

@@ -270,7 +270,7 @@ class CupertinoDialogAction extends StatelessWidget {
       style = style.copyWith(color: style.color.withOpacity(0.5));
     }
 
-    final double textScaleFactor = MediaQuery.of(context, nullOk: true)?.textScaleFactor ?? 1.0;
+    final double textScaleFactor = MediaQuery.textScaleFactorOf(context);
     return new GestureDetector(
       onTap: onPressed,
       behavior: HitTestBehavior.opaque,

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

@@ -684,7 +684,7 @@ class EditableTextState extends State<EditableText> with AutomaticKeepAliveClien
               hasFocus: _hasFocus,
               maxLines: widget.maxLines,
               selectionColor: widget.selectionColor,
-              textScaleFactor: widget.textScaleFactor ?? MediaQuery.of(context, nullOk: true)?.textScaleFactor ?? 1.0,
+              textScaleFactor: widget.textScaleFactor ?? MediaQuery.textScaleFactorOf(context),
               textAlign: widget.textAlign,
               textDirection: _textDirection,
               obscureText: widget.obscureText,

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

@@ -76,6 +76,11 @@ class MediaQueryData {
   ///
   /// For example, if the text scale factor is 1.5, text will be 50% larger than
   /// the specified font size.
+  ///
+  /// See also:
+  ///
+  ///  * [MediaQuery.textScaleFactorOf], a convenience method which returns the
+  ///    textScaleFactor defined for a [BuildContext].
   final double textScaleFactor;
 
   /// The number of physical pixels on each side of the display rectangle into
@@ -402,6 +407,12 @@ class MediaQuery extends InheritedWidget {
     );
   }
 
+  /// Returns textScaleFactor for the nearest MediaQuery ancestor or 1.0, if
+  /// no such ancestor exists.
+  static double textScaleFactorOf(BuildContext context) {
+    return MediaQuery.of(context, nullOk: true)?.textScaleFactor ?? 1.0;
+  }
+
   @override
   bool updateShouldNotify(MediaQuery oldWidget) => data != oldWidget.data;
 

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

@@ -305,7 +305,7 @@ class Text extends StatelessWidget {
       textDirection: textDirection, // RichText uses Directionality.of to obtain a default if this is null.
       softWrap: softWrap ?? defaultTextStyle.softWrap,
       overflow: overflow ?? defaultTextStyle.overflow,
-      textScaleFactor: textScaleFactor ?? MediaQuery.of(context, nullOk: true)?.textScaleFactor ?? 1.0,
+      textScaleFactor: textScaleFactor ?? MediaQuery.textScaleFactorOf(context),
       maxLines: maxLines ?? defaultTextStyle.maxLines,
       text: new TextSpan(
         style: effectiveTextStyle,

packages/flutter/test/widgets/media_query_test.dart

@@ -165,4 +165,31 @@ void main() {
     expect(unpadded.viewInsets, EdgeInsets.zero);
     expect(unpadded.alwaysUse24HourFormat, true);
   });
+
+ testWidgets('MediaQuery.textScaleFactorOf', (WidgetTester tester) async {
+   double outsideTextScaleFactor;
+   double insideTextScaleFactor;
+
+   await tester.pumpWidget(
+     new Builder(
+       builder: (BuildContext context) {
+         outsideTextScaleFactor = MediaQuery.textScaleFactorOf(context);
+         return new MediaQuery(
+           data: const MediaQueryData(
+             textScaleFactor: 4.0,
+           ),
+           child: new Builder(
+             builder: (BuildContext context) {
+               insideTextScaleFactor = MediaQuery.textScaleFactorOf(context);
+               return new Container();
+             },
+           ),
+         );
+       },
+     ),
+   );
+
+   expect(outsideTextScaleFactor, 1.0);
+   expect(insideTextScaleFactor, 4.0);
+ });
 }