Add font features (such as tabular numbers) as an option in text styl…

…es (flutter#8823)

Fixes flutter/flutter#31691

ci/licenses_golden/licenses_flutter

@@ -1060,6 +1060,8 @@ FILE: ../../../flutter/third_party/txt/src/txt/font_asset_provider.cc
 FILE: ../../../flutter/third_party/txt/src/txt/font_asset_provider.h
 FILE: ../../../flutter/third_party/txt/src/txt/font_collection.cc
 FILE: ../../../flutter/third_party/txt/src/txt/font_collection.h
+FILE: ../../../flutter/third_party/txt/src/txt/font_features.cc
+FILE: ../../../flutter/third_party/txt/src/txt/font_features.h
 FILE: ../../../flutter/third_party/txt/src/txt/font_skia.cc
 FILE: ../../../flutter/third_party/txt/src/txt/font_skia.h
 FILE: ../../../flutter/third_party/txt/src/txt/font_style.h

lib/stub_ui/lib/src/ui/text.dart

@@ -110,6 +110,145 @@ class FontWeight {
   }
 }
 
+/// A feature tag and value that affect the selection of glyphs in a font.
+class FontFeature {
+  /// Creates a [FontFeature] object, which can be added to a [TextStyle] to
+  /// change how the engine selects glyphs when rendering text.
+  ///
+  /// `feature` is the four-character tag that identifies the feature.
+  /// These tags are specified by font formats such as OpenType.
+  ///
+  /// `value` is the value that the feature will be set to.  The behavior
+  /// of the value depends on the specific feature.  Many features are
+  /// flags whose value can be 1 (when enabled) or 0 (when disabled).
+  ///
+  /// See <https://docs.microsoft.com/en-us/typography/opentype/spec/featuretags>
+  const FontFeature(this.feature, [ this.value = 1 ]) : assert(feature != null), assert(feature.length == 4), assert(value != null), assert(value >= 0);
+
+  /// Create a [FontFeature] object that enables the feature with the given tag.
+  const FontFeature.enable(String feature) : this(feature, 1);
+
+  /// Create a [FontFeature] object that disables the feature with the given tag.
+  const FontFeature.disable(String feature) : this(feature, 0);
+
+  /// Randomize the alternate forms used in text.
+  ///
+  /// For example, this can be used with suitably-prepared handwriting fonts to
+  /// vary the forms used for each character, so that, for instance, the word
+  /// "cross-section" would be rendered with two different "c"s, two different "o"s,
+  /// and three different "s"s.
+  ///
+  /// See also:
+  ///
+  ///  * <https://docs.microsoft.com/en-us/typography/opentype/spec/features_pt#rand>
+  const FontFeature.randomize() : feature = 'rand', value = 1;
+
+  /// Select a stylistic set.
+  ///
+  /// Fonts may have up to 20 stylistic sets, numbered 1 through 20.
+  ///
+  /// See also:
+  ///
+  ///  * <https://docs.microsoft.com/en-us/typography/opentype/spec/features_pt#ssxx>
+  factory FontFeature.stylisticSet(int value) {
+    assert(value >= 1);
+    assert(value <= 20);
+    return FontFeature('ss${value.toString().padLeft(2, "0")}');
+  }
+
+  /// Use the slashed zero.
+  ///
+  /// Some fonts contain both a circular zero and a zero with a slash. This
+  /// enables the use of the latter form.
+  ///
+  /// This is overridden by [FontFeature.oldstyleFigures].
+  ///
+  /// See also:
+  ///
+  ///  * <https://docs.microsoft.com/en-us/typography/opentype/spec/features_uz#zero>
+  const FontFeature.slashedZero() : feature = 'zero', value = 1;
+
+  /// Use oldstyle figures.
+  ///
+  /// Some fonts have variants of the figures (e.g. the digit 9) that, when
+  /// this feature is enabled, render with descenders under the baseline instead
+  /// of being entirely above the baseline.
+  ///
+  /// This overrides [FontFeature.slashedZero].
+  ///
+  /// See also:
+  ///
+  ///  * <https://docs.microsoft.com/en-us/typography/opentype/spec/features_ko#onum>
+  const FontFeature.oldstyleFigures() : feature = 'onum', value = 1;
+
+  /// Use proportional (varying width) figures.
+  ///
+  /// For fonts that have both proportional and tabular (monospace) figures,
+  /// this enables the proportional figures.
+  ///
+  /// This is mutually exclusive with [FontFeature.tabularFigures].
+  ///
+  /// The default behavior varies from font to font.
+  ///
+  /// See also:
+  ///
+  ///  * <https://docs.microsoft.com/en-us/typography/opentype/spec/features_pt#pnum>
+  const FontFeature.proportionalFigures() : feature = 'pnum', value = 1;
+
+  /// Use tabular (monospace) figures.
+  ///
+  /// For fonts that have both proportional (varying width) and tabular figures,
+  /// this enables the tabular figures.
+  ///
+  /// This is mutually exclusive with [FontFeature.proportionalFigures].
+  ///
+  /// The default behavior varies from font to font.
+  ///
+  /// See also:
+  ///
+  ///  * <https://docs.microsoft.com/en-us/typography/opentype/spec/features_pt#tnum>
+  const FontFeature.tabularFigures() : feature = 'tnum', value = 1;
+
+  /// The tag that identifies the effect of this feature.  Must consist of 4
+  /// ASCII characters (typically lowercase letters).
+  ///
+  /// See <https://docs.microsoft.com/en-us/typography/opentype/spec/featuretags>
+  final String feature;
+
+  /// The value assigned to this feature.
+  ///
+  /// Must be a positive integer.  Many features are Boolean values that accept
+  /// values of either 0 (feature is disabled) or 1 (feature is enabled).
+  final int value;
+
+  static const int _kEncodedSize = 8;
+
+  void _encode(ByteData byteData) {
+    assert(feature.codeUnits.every((int c) => c >= 0x20 && c <= 0x7F));
+    for (int i = 0; i < 4; i++) {
+      byteData.setUint8(i, feature.codeUnitAt(i));
+    }
+    byteData.setInt32(4, value, _kFakeHostEndian);
+  }
+
+  @override
+  bool operator ==(dynamic other) {
+    if (identical(this, other))
+      return true;
+    if (other.runtimeType != runtimeType)
+      return false;
+    final FontFeature typedOther = other;
+    return feature == typedOther.feature
+           && value == typedOther.value;
+  }
+
+  @override
+  int get hashCode => hashValues(feature, value);
+
+  @override
+  String toString() => 'FontFeature($feature, $value)';
+}
+
 /// Whether and how to align text horizontally.
 // The order of this enum must match the order of the values in RenderStyleConstants.h's ETextAlign.
 enum TextAlign {

lib/ui/natives.dart

@@ -15,6 +15,11 @@ class _Logger {
   static void _printString(String s) native 'Logger_PrintString';
 }
 
+// If we actually run on big endian machines, we'll need to do something smarter
+// here. We don't use [Endian.Host] because it's not a compile-time
+// constant and can't propagate into the set/get calls.
+const Endian _kFakeHostEndian = Endian.little;
+
 // A service protocol extension to schedule a frame to be rendered into the
 // window.
 Future<developer.ServiceExtensionResponse> _scheduleFrame(

lib/ui/painting.dart

@@ -1019,11 +1019,6 @@ enum Clip {
   antiAliasWithSaveLayer,
 }
 
-// If we actually run on big endian machines, we'll need to do something smarter
-// here. We don't use [Endian.Host] because it's not a compile-time
-// constant and can't propagate into the set/get calls.
-const Endian _kFakeHostEndian = Endian.little;
-
 // Indicates that the image should not be resized in this dimension.
 //
 // Used by [instantiateImageCodec] as a magical value to disable resizing