Make the Brightness estimator public (#10508)

packages/flutter/lib/src/material/theme_data.dart

@@ -43,32 +43,6 @@ const Color _kLightThemeSplashColor = const Color(0x66C8C8C8);
 const Color _kDarkThemeHighlightColor = const Color(0x40CCCCCC);
 const Color _kDarkThemeSplashColor = const Color(0x40CCCCCC);
 
-// See <https://www.w3.org/TR/WCAG20/#relativeluminancedef>
-double _linearizeColorComponent(double component) {
-  if (component <= 0.03928)
-    return component / 12.92;
-  return math.pow((component + 0.055) / 1.055, 2.4);
-}
-
-Brightness _estimateBrightnessForColor(Color color) {
-  // See <https://www.w3.org/TR/WCAG20/#relativeluminancedef>
-  final double R = _linearizeColorComponent(color.red / 0xFF);
-  final double G = _linearizeColorComponent(color.green / 0xFF);
-  final double B = _linearizeColorComponent(color.blue / 0xFF);
-  final double L = 0.2126 * R + 0.7152 * G + 0.0722 * B;
-
-  // See <https://www.w3.org/TR/WCAG20/#contrast-ratiodef>
-  // The spec says to use kThreshold=0.0525, but Material Design appears to bias
-  // more towards using light text than WCAG20 recommends. Material Design spec
-  // doesn't say what value to use, but 0.15 seemed close to what the Material
-  // Design spec shows for its color palette on
-  // <https://material.io/guidelines/style/color.html#color-color-palette>.
-  const double kThreshold = 0.15;
-  if ((L + 0.05) * (L + 0.05) > kThreshold )
-    return Brightness.light;
-  return Brightness.dark;
-}
-
 /// Holds the color and typography values for a material design theme.
 ///
 /// Use this class to configure a [Theme] widget.
@@ -134,10 +108,10 @@ class ThemeData {
     final bool isDark = brightness == Brightness.dark;
     primarySwatch ??= Colors.blue;
     primaryColor ??= isDark ? Colors.grey[900] : primarySwatch[500];
-    primaryColorBrightness ??= _estimateBrightnessForColor(primaryColor);
+    primaryColorBrightness ??= estimateBrightnessForColor(primaryColor);
     final bool primaryIsDark = primaryColorBrightness == Brightness.dark;
     accentColor ??= isDark ? Colors.tealAccent[200] : primarySwatch[500];
-    accentColorBrightness ??= _estimateBrightnessForColor(accentColor);
+    accentColorBrightness ??= estimateBrightnessForColor(accentColor);
     final bool accentIsDark = accentColorBrightness == Brightness.dark;
     canvasColor ??= isDark ? Colors.grey[850] : Colors.grey[50];
     scaffoldBackgroundColor ??= canvasColor;
@@ -467,6 +441,37 @@ class ThemeData {
     );
   }
 
+  // See <https://www.w3.org/TR/WCAG20/#relativeluminancedef>
+  static double _linearizeColorComponent(double component) {
+    if (component <= 0.03928)
+      return component / 12.92;
+    return math.pow((component + 0.055) / 1.055, 2.4);
+  }
+
+  /// Determines whether the given [Color] is [Brightness.light] or
+  /// [Brightness.dark].
+  ///
+  /// This compares the luminosity of the given color to a threshold value that
+  /// matches the material design specification.
+  static Brightness estimateBrightnessForColor(Color color) {
+    // See <https://www.w3.org/TR/WCAG20/#relativeluminancedef>
+    final double R = _linearizeColorComponent(color.red / 0xFF);
+    final double G = _linearizeColorComponent(color.green / 0xFF);
+    final double B = _linearizeColorComponent(color.blue / 0xFF);
+    final double L = 0.2126 * R + 0.7152 * G + 0.0722 * B;
+
+    // See <https://www.w3.org/TR/WCAG20/#contrast-ratiodef>
+    // The spec says to use kThreshold=0.0525, but Material Design appears to bias
+    // more towards using light text than WCAG20 recommends. Material Design spec
+    // doesn't say what value to use, but 0.15 seemed close to what the Material
+    // Design spec shows for its color palette on
+    // <https://material.io/guidelines/style/color.html#color-color-palette>.
+    const double kThreshold = 0.15;
+    if ((L + 0.05) * (L + 0.05) > kThreshold )
+      return Brightness.light;
+    return Brightness.dark;
+  }
+
   /// Linearly interpolate between two themes.
   ///
   /// The arguments must not be null.

packages/flutter/test/material/theme_data_test.dart

@@ -90,7 +90,20 @@ void main() {
     expect(themeData.accentTextTheme.display4.fontFamily, equals('Ahem'));
   });
 
-  test('Can estimate brightness', () {
+  test('Can estimate brightness - directly', () {
+    expect(ThemeData.estimateBrightnessForColor(Colors.white), equals(Brightness.light));
+    expect(ThemeData.estimateBrightnessForColor(Colors.black), equals(Brightness.dark));
+    expect(ThemeData.estimateBrightnessForColor(Colors.blue), equals(Brightness.dark));
+    expect(ThemeData.estimateBrightnessForColor(Colors.yellow), equals(Brightness.light));
+    expect(ThemeData.estimateBrightnessForColor(Colors.deepOrange), equals(Brightness.dark));
+    expect(ThemeData.estimateBrightnessForColor(Colors.orange), equals(Brightness.light));
+    expect(ThemeData.estimateBrightnessForColor(Colors.lime), equals(Brightness.light));
+    expect(ThemeData.estimateBrightnessForColor(Colors.grey), equals(Brightness.light));
+    expect(ThemeData.estimateBrightnessForColor(Colors.teal), equals(Brightness.dark));
+    expect(ThemeData.estimateBrightnessForColor(Colors.indigo), equals(Brightness.dark));
+  });
+
+  test('Can estimate brightness - indirectly', () {
     expect(new ThemeData(primaryColor: Colors.white).primaryColorBrightness, equals(Brightness.light));
     expect(new ThemeData(primaryColor: Colors.black).primaryColorBrightness, equals(Brightness.dark));
     expect(new ThemeData(primaryColor: Colors.blue).primaryColorBrightness, equals(Brightness.dark));