packages/flutter/lib/src/painting/text_style.dart - external/github.com/flutter/flutter.git - Git at Google

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.

 import 'dart:ui' as ui show ParagraphStyle, TextStyle, lerpDouble;

 import 'package:flutter/foundation.dart';

 import 'basic_types.dart';

 /// An immutable style in which paint text.
 ///
 /// ## Sample code
 ///
 /// ### Bold
 ///
 /// Here, a single line of text in a [Text] widget is given a specific style
 /// override. The style is mixed with the ambient [DefaultTextStyle] by the
 /// [Text] widget.
 ///
 /// ```dart
 /// new Text(
 ///   'No, we need bold strokes. We need this plan.',
 ///   style: new TextStyle(fontWeight: FontWeight.bold),
 /// )
 /// ```
 ///
 /// ### Italics
 ///
 /// As in the previous example, the [Text] widget is given a specific style
 /// override which is implicitly mixed with the ambient [DefaultTextStyle].
 ///
 /// ```dart
 /// new Text(
 ///   'Welcome to the present, we\'re running a real nation.',
 ///   style: new TextStyle(fontStyle: FontStyle.italic),
 /// )
 /// ```
 ///
 /// ### Opacity
 ///
 /// Each line here is progressively more opaque. The base color is
 /// [material.Colors.black], and [Color.withOpacity] is used to create a
 /// derivative color with the desired opacity. The root [TextSpan] for this
 /// [RichText] widget is explicitly given the ambient [DefaultTextStyle], since
 /// [RichText] does not do that automatically. The inner [TextStyle] objects are
 /// implicitly mixed with the parent [TextSpan]'s [TextSpan.style].
 ///
 /// ```dart
 /// new RichText(
 ///   text: new TextSpan(
 ///     style: DefaultTextStyle.of(context).style,
 ///     children: <TextSpan>[
 ///       new TextSpan(
 ///         text: 'You don\'t have the votes.\n',
 ///         style: new TextStyle(color: Colors.black.withOpacity(0.6)),
 ///       ),
 ///       new TextSpan(
 ///         text: 'You don\'t have the votes!\n',
 ///         style: new TextStyle(color: Colors.black.withOpacity(0.8)),
 ///       ),
 ///       new TextSpan(
 ///         text: 'You\'re gonna need congressional approval and you don\'t have the votes!\n',
 ///         style: new TextStyle(color: Colors.black.withOpacity(1.0)),
 ///       ),
 ///     ],
 ///   ),
 /// )
 /// ```
 ///
 /// ### Size
 ///
 /// In this example, the ambient [DefaultTextStyle] is explicitly manipulated to
 /// obtain a [TextStyle] that doubles the default font size.
 ///
 /// ```dart
 /// new Text(
 ///   'These are wise words, enterprising men quote \'em.',
 ///   style: DefaultTextStyle.of(context).style.apply(fontSizeFactor: 2.0),
 /// )
 /// ```
 ///
 /// ### Line height
 ///
 /// The [height] property can be used to change the line height. Here, the line
 /// height is set to 100 logical pixels, so that the text is very spaced out.
 ///
 /// ```dart
 /// new Text(
 ///   'Don\'t act surprised, you guys, cuz I wrote \'em!',
 ///   style: new TextStyle(height: 100.0),
 /// )
 /// ```
 ///
 /// ### Wavy red underline with black text
 ///
 /// Styles can be combined. In this example, the misspelt word is drawn in black
 /// text and underlined with a wavy red line to indicate a spelling error. (The
 /// remainder is styled according to the Flutter default text styles, not the
 /// ambient [DefaultTextStyle], since no explicit style is given and [RichText]
 /// does not automatically use the ambient [DefaultTextStyle].)
 ///
 /// ```dart
 /// new RichText(
 ///   text: new TextSpan(
 ///     text: 'Don\'t tax the South ',
 ///     children: <TextSpan>[
 ///       new TextSpan(
 ///         text: 'cuz',
 ///         style: new TextStyle(
 ///           color: Colors.black,
 ///           decoration: TextDecoration.underline,
 ///           decorationColor: Colors.red,
 ///           decorationStyle: TextDecorationStyle.wavy,
 ///         ),
 ///       ),
 ///       new TextSpan(
 ///         text: ' we got it made in the shade',
 ///       ),
 ///     ],
 ///   ),
 /// )
 /// ```
 ///
 /// See also:
 ///
 ///  * [Text], the widget for showing text in a single style.
 ///  * [DefaultTextStyle], the widget that specifies the default text styles for
 ///    [Text] widgets, configured using a [TextStyle].
 ///  * [RichText], the widget for showing a paragraph of mix-style text.
 ///  * [TextSpan], the class that wraps a [TextStyle] for the purposes of
 ///    passing it to a [RichText].
 @immutable
 class TextStyle extends Diagnosticable {
   /// Creates a text style.
   const TextStyle({
     this.inherit: true,
     this.color,
     this.fontFamily,
     this.fontSize,
     this.fontWeight,
     this.fontStyle,
     this.letterSpacing,
     this.wordSpacing,
     this.textBaseline,
     this.height,
     this.decoration,
     this.decorationColor,
     this.decorationStyle,
   }) : assert(inherit != null);

   /// Whether null values are replaced with their value in an ancestor text
   /// style (e.g., in a [TextSpan] tree).
   ///
   /// If this is false, properties that don't have explicit values will revert
   /// to the defaults: white in color, a font size of 10 pixels, in a sans-serif
   /// font face.
   final bool inherit;

   /// The color to use when painting the text.
   final Color color;

   /// The name of the font to use when painting the text (e.g., Roboto).
   final String fontFamily;

   /// The size of glyphs (in logical pixels) to use when painting the text.
   ///
   /// During painting, the [fontSize] is multiplied by the current
   /// `textScaleFactor` to let users make it easier to read text by increasing
   /// its size.
   final double fontSize;

   /// The typeface thickness to use when painting the text (e.g., bold).
   final FontWeight fontWeight;

   /// The typeface variant to use when drawing the letters (e.g., italics).
   final FontStyle fontStyle;

   /// The amount of space (in logical pixels) to add between each letter.
   /// A negative value can be used to bring the letters closer.
   final double letterSpacing;

   /// The amount of space (in logical pixels) to add at each sequence of
   /// white-space (i.e. between each word). A negative value can be used to
   /// bring the words closer.
   final double wordSpacing;

   /// The common baseline that should be aligned between this text span and its
   /// parent text span, or, for the root text spans, with the line box.
   final TextBaseline textBaseline;

   /// The height of this text span, as a multiple of the font size.
   ///
   /// If applied to the root [TextSpan], this value sets the line height, which
   /// is the minimum distance between subsequent text baselines, as multiple of
   /// the font size.
   final double height;

   /// The decorations to paint near the text (e.g., an underline).
   final TextDecoration decoration;

   /// The color in which to paint the text decorations.
   final Color decorationColor;

   /// The style in which to paint the text decorations (e.g., dashed).
   final TextDecorationStyle decorationStyle;

   /// Creates a copy of this text style but with the given fields replaced with
   /// the new values.
   TextStyle copyWith({
     Color color,
     String fontFamily,
     double fontSize,
     FontWeight fontWeight,
     FontStyle fontStyle,
     double letterSpacing,
     double wordSpacing,
     TextBaseline textBaseline,
     double height,
     TextDecoration decoration,
     Color decorationColor,
     TextDecorationStyle decorationStyle,
   }) {
     return new TextStyle(
       inherit: inherit,
       color: color ?? this.color,
       fontFamily: fontFamily ?? this.fontFamily,
       fontSize: fontSize ?? this.fontSize,
       fontWeight: fontWeight ?? this.fontWeight,
       fontStyle: fontStyle ?? this.fontStyle,
       letterSpacing: letterSpacing ?? this.letterSpacing,
       wordSpacing: wordSpacing ?? this.wordSpacing,
       textBaseline: textBaseline ?? this.textBaseline,
       height: height ?? this.height,
       decoration: decoration ?? this.decoration,
       decorationColor: decorationColor ?? this.decorationColor,
       decorationStyle: decorationStyle ?? this.decorationStyle,
     );
   }

   /// Creates a copy of this text style but with the numeric fields multiplied
   /// by the given factors and then incremented by the given deltas.
   ///
   /// For example, `style.apply(fontSizeFactor: 2.0, fontSizeDelta: 1.0)` would
   /// return a [TextStyle] whose [fontSize] is `style.fontSize * 2.0 + 1.0`.
   ///
   /// For the [fontWeight], the delta is applied to the [FontWeight] enum index
   /// values, so that for instance `style.apply(fontWeightDelta: -2)` when
   /// applied to a `style` whose [fontWeight] is [FontWeight.w500] will return a
   /// [TextStyle] with a [FontWeight.w300].
   ///
   /// The arguments must not be null.
   ///
   /// If the underlying values are null, then the corresponding factors and/or
   /// deltas must not be specified.
   ///
   /// The non-numeric fields can be controlled using the corresponding arguments.
   TextStyle apply({
     Color color,
     String fontFamily,
     double fontSizeFactor: 1.0,
     double fontSizeDelta: 0.0,
     int fontWeightDelta: 0,
     double letterSpacingFactor: 1.0,
     double letterSpacingDelta: 0.0,
     double wordSpacingFactor: 1.0,
     double wordSpacingDelta: 0.0,
     double heightFactor: 1.0,
     double heightDelta: 0.0,
   }) {
     assert(fontSizeFactor != null);
     assert(fontSizeDelta != null);
     assert(fontSize != null || (fontSizeFactor == 1.0 && fontSizeDelta == 0.0));
     assert(fontWeightDelta != null);
     assert(fontWeight != null || fontWeightDelta == 0.0);
     assert(letterSpacingFactor != null);
     assert(letterSpacingDelta != null);
     assert(letterSpacing != null || (letterSpacingFactor == 1.0 && letterSpacingDelta == 0.0));
     assert(wordSpacingFactor != null);
     assert(wordSpacingDelta != null);
     assert(wordSpacing != null || (wordSpacingFactor == 1.0 && wordSpacingDelta == 0.0));
     assert(heightFactor != null);
     assert(heightDelta != null);
     assert(heightFactor != null || (heightFactor == 1.0 && heightDelta == 0.0));
     return new TextStyle(
       inherit: inherit,
       color: color ?? this.color,
       fontFamily: fontFamily ?? this.fontFamily,
       fontSize: fontSize == null ? null : fontSize * fontSizeFactor + fontSizeDelta,
       fontWeight: fontWeight == null ? null : FontWeight.values[(fontWeight.index + fontWeightDelta).clamp(0, FontWeight.values.length - 1)],
       fontStyle: fontStyle,
       letterSpacing: letterSpacing == null ? null : letterSpacing * letterSpacingFactor + letterSpacingDelta,
       wordSpacing: wordSpacing == null ? null : wordSpacing * wordSpacingFactor + wordSpacingDelta,
       textBaseline: textBaseline,
       height: height == null ? null : height * heightFactor + heightDelta,
       decoration: decoration,
       decorationColor: decorationColor,
       decorationStyle: decorationStyle,
     );
   }

   /// Returns a new text style that matches this text style but with some values
   /// replaced by the non-null parameters of the given text style. If the given
   /// text style is null, simply returns this text style.
   TextStyle merge(TextStyle other) {
     if (other == null)
       return this;
     assert(other.inherit);
     return copyWith(
       color: other.color,
       fontFamily: other.fontFamily,
       fontSize: other.fontSize,
       fontWeight: other.fontWeight,
       fontStyle: other.fontStyle,
       letterSpacing: other.letterSpacing,
       wordSpacing: other.wordSpacing,
       textBaseline: other.textBaseline,
       height: other.height,
       decoration: other.decoration,
       decorationColor: other.decorationColor,
       decorationStyle: other.decorationStyle
     );
   }

   /// Interpolate between two text styles.
   ///
   /// This will not work well if the styles don't set the same fields.
   static TextStyle lerp(TextStyle begin, TextStyle end, double t) {
     assert(begin.inherit == end.inherit);
     return new TextStyle(
       inherit: end.inherit,
       color: Color.lerp(begin.color, end.color, t),
       fontFamily: t < 0.5 ? begin.fontFamily : end.fontFamily,
       fontSize: ui.lerpDouble(begin.fontSize ?? end.fontSize, end.fontSize ?? begin.fontSize, t),
       fontWeight: FontWeight.lerp(begin.fontWeight, end.fontWeight, t),
       fontStyle: t < 0.5 ? begin.fontStyle : end.fontStyle,
       letterSpacing: ui.lerpDouble(begin.letterSpacing ?? end.letterSpacing, end.letterSpacing ?? begin.letterSpacing, t),
       wordSpacing: ui.lerpDouble(begin.wordSpacing ?? end.wordSpacing, end.wordSpacing ?? begin.wordSpacing, t),
       textBaseline: t < 0.5 ? begin.textBaseline : end.textBaseline,
       height: ui.lerpDouble(begin.height ?? end.height, end.height ?? begin.height, t),
       decoration: t < 0.5 ? begin.decoration : end.decoration,
       decorationColor: Color.lerp(begin.decorationColor, end.decorationColor, t),
       decorationStyle: t < 0.5 ? begin.decorationStyle : end.decorationStyle
     );
   }

   /// The style information for text runs, encoded for use by `dart:ui`.
   ui.TextStyle getTextStyle({ double textScaleFactor: 1.0 }) {
     return new ui.TextStyle(
       color: color,
       decoration: decoration,
       decorationColor: decorationColor,
       decorationStyle: decorationStyle,
       fontWeight: fontWeight,
       fontStyle: fontStyle,
       textBaseline: textBaseline,
       fontFamily: fontFamily,
       fontSize: fontSize == null ? null : fontSize * textScaleFactor,
       letterSpacing: letterSpacing,
       wordSpacing: wordSpacing,
       height: height
     );
   }

   /// The style information for paragraphs, encoded for use by `dart:ui`.
   ///
   /// The `textScaleFactor` argument must not be null. If omitted, it defaults
   /// to 1.0. The other arguments may be null. The `maxLines` argument, if
   /// specified and non-null, must be greater than zero.
   ui.ParagraphStyle getParagraphStyle({
       TextAlign textAlign,
       TextDirection textDirection,
       double textScaleFactor: 1.0,
       String ellipsis,
       int maxLines,
   }) {
     assert(textScaleFactor != null);
     assert(maxLines == null || maxLines > 0);
     return new ui.ParagraphStyle(
       textAlign: textAlign,
       textDirection: textDirection,
       fontWeight: fontWeight,
       fontStyle: fontStyle,
       fontFamily: fontFamily,
       fontSize: fontSize == null ? null : fontSize * textScaleFactor,
       lineHeight: height,
       maxLines: maxLines,
       ellipsis: ellipsis,
     );
   }

   /// Describe the difference between this style and another, in terms of how
   /// much damage it will make to the rendering.
   ///
   /// See also:
   ///
   ///  * [TextSpan.compareTo], which does the same thing for entire [TextSpan]s.
   RenderComparison compareTo(TextStyle other) {
     if (identical(this, other))
       return RenderComparison.identical;
     if (inherit != other.inherit ||
         fontFamily != other.fontFamily ||
         fontSize != other.fontSize ||
         fontWeight != other.fontWeight ||
         fontStyle != other.fontStyle ||
         letterSpacing != other.letterSpacing ||
         wordSpacing != other.wordSpacing ||
         textBaseline != other.textBaseline ||
         height != other.height)
       return RenderComparison.layout;
     if (color != other.color ||
         decoration != other.decoration ||
         decorationColor != other.decorationColor ||
         decorationStyle != other.decorationStyle)
       return RenderComparison.paint;
     return RenderComparison.identical;
   }

   @override
   bool operator ==(dynamic other) {
     if (identical(this, other))
       return true;
     if (other.runtimeType != runtimeType)
       return false;
     final TextStyle typedOther = other;
     return inherit == typedOther.inherit &&
            color == typedOther.color &&
            fontFamily == typedOther.fontFamily &&
            fontSize == typedOther.fontSize &&
            fontWeight == typedOther.fontWeight &&
            fontStyle == typedOther.fontStyle &&
            letterSpacing == typedOther.letterSpacing &&
            wordSpacing == typedOther.wordSpacing &&
            textBaseline == typedOther.textBaseline &&
            height == typedOther.height &&
            decoration == typedOther.decoration &&
            decorationColor == typedOther.decorationColor &&
            decorationStyle == typedOther.decorationStyle;
   }

   @override
   int get hashCode {
     return hashValues(
       inherit,
       color,
       fontFamily,
       fontSize,
       fontWeight,
       fontStyle,
       letterSpacing,
       wordSpacing,
       textBaseline,
       height,
       decoration,
       decorationColor,
       decorationStyle
     );
   }

   @override
   String toStringShort() => '$runtimeType';

   /// Adds all properties prefixing property names with the optional `prefix`.
   @override
   void debugFillProperties(DiagnosticPropertiesBuilder properties, { String prefix: '' }) {
     super.debugFillProperties(properties);
     final List<DiagnosticsNode> styles = <DiagnosticsNode>[];
     styles.add(new DiagnosticsProperty<Color>('${prefix}color', color, defaultValue: null));
     styles.add(new StringProperty('${prefix}family', fontFamily, defaultValue: null, quoted: false));
     styles.add(new DoubleProperty('${prefix}size', fontSize, defaultValue: null));
     String weightDescription;
     if (fontWeight != null) {
       switch (fontWeight) {
         case FontWeight.w100:
           weightDescription = '100';
           break;
         case FontWeight.w200:
           weightDescription = '200';
           break;
         case FontWeight.w300:
           weightDescription = '300';
           break;
         case FontWeight.w400:
           weightDescription = '400';
           break;
         case FontWeight.w500:
           weightDescription = '500';
           break;
         case FontWeight.w600:
           weightDescription = '600';
           break;
         case FontWeight.w700:
           weightDescription = '700';
           break;
         case FontWeight.w800:
           weightDescription = '800';
           break;
         case FontWeight.w900:
           weightDescription = '900';
           break;
       }
     }
     // TODO(jacobr): switch this to use enumProperty which will either cause the
     // weight description to change to w600 from 600 or require existing
     // enumProperty to handle this special case.
     styles.add(new DiagnosticsProperty<FontWeight>(
       '${prefix}weight',
       fontWeight,
       description: weightDescription,
       defaultValue: null,
     ));
     styles.add(new EnumProperty<FontStyle>('${prefix}style', fontStyle, defaultValue: null));
     styles.add(new DoubleProperty('${prefix}letterSpacing', letterSpacing, unit: 'x', defaultValue: null));
     styles.add(new DoubleProperty('${prefix}wordSpacing', wordSpacing, unit: 'x', defaultValue: null));
     styles.add(new EnumProperty<TextBaseline>('${prefix}baseline', textBaseline, defaultValue: null));
     styles.add(new DoubleProperty('${prefix}height', height, unit: 'x', defaultValue: null));
     if (decoration != null || decorationColor != null || decorationStyle != null) {
       final List<String> decorationDescription = <String>[];
       if (decorationStyle != null)
         decorationDescription.add(describeEnum((decorationStyle)));

       // Hide decorationColor from the default text view as it is shown in the
       // terse decoration summary as well.
       styles.add(new DiagnosticsProperty<Color>('${prefix}decorationColor', decorationColor, defaultValue: null, hidden: true));

       if (decorationColor != null)
         decorationDescription.add('$decorationColor');

       // Intentionally collide with the property 'decoration' added below.
       // Tools that show hidden properties could choose the first property
       // matching the name to disambiguate.
       styles.add(new DiagnosticsProperty<TextDecoration>('${prefix}decoration', decoration, defaultValue: null, hidden: true));
       if (decoration != null)
         decorationDescription.add('$decoration');
       assert(decorationDescription.isNotEmpty);
       styles.add(new MessageProperty('${prefix}decoration', decorationDescription.join(' ')));
     }

     final bool styleSpecified = styles.any((DiagnosticsNode n) => !n.hidden);
     properties.add(new DiagnosticsProperty<bool>('${prefix}inherit', inherit, hidden: !styleSpecified && inherit));
     for (DiagnosticsNode style in styles)
       properties.add(style);

     if (!styleSpecified)
       properties.add(new FlagProperty('inherit', value: inherit, ifTrue: '$prefix<all styles inherited>', ifFalse: '$prefix<no style specified>'));
   }
 }
	// Copyright 2015 The Chromium Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style license that can be
	// found in the LICENSE file.

	import 'dart:ui' as ui show ParagraphStyle, TextStyle, lerpDouble;

	import 'package:flutter/foundation.dart';

	import 'basic_types.dart';

	/// An immutable style in which paint text.
	///
	/// ## Sample code
	///
	/// ### Bold
	///
	/// Here, a single line of text in a [Text] widget is given a specific style
	/// override. The style is mixed with the ambient [DefaultTextStyle] by the
	/// [Text] widget.
	///
	/// ```dart
	/// new Text(
	/// 'No, we need bold strokes. We need this plan.',
	/// style: new TextStyle(fontWeight: FontWeight.bold),
	/// )
	/// ```
	///
	/// ### Italics
	///
	/// As in the previous example, the [Text] widget is given a specific style
	/// override which is implicitly mixed with the ambient [DefaultTextStyle].
	///
	/// ```dart
	/// new Text(
	/// 'Welcome to the present, we\'re running a real nation.',
	/// style: new TextStyle(fontStyle: FontStyle.italic),
	/// )
	/// ```
	///
	/// ### Opacity
	///
	/// Each line here is progressively more opaque. The base color is
	/// [material.Colors.black], and [Color.withOpacity] is used to create a
	/// derivative color with the desired opacity. The root [TextSpan] for this
	/// [RichText] widget is explicitly given the ambient [DefaultTextStyle], since
	/// [RichText] does not do that automatically. The inner [TextStyle] objects are
	/// implicitly mixed with the parent [TextSpan]'s [TextSpan.style].
	///
	/// ```dart
	/// new RichText(
	/// text: new TextSpan(
	/// style: DefaultTextStyle.of(context).style,
	/// children: <TextSpan>[
	/// new TextSpan(
	/// text: 'You don\'t have the votes.\n',
	/// style: new TextStyle(color: Colors.black.withOpacity(0.6)),
	/// ),
	/// new TextSpan(
	/// text: 'You don\'t have the votes!\n',
	/// style: new TextStyle(color: Colors.black.withOpacity(0.8)),
	/// ),
	/// new TextSpan(
	/// text: 'You\'re gonna need congressional approval and you don\'t have the votes!\n',
	/// style: new TextStyle(color: Colors.black.withOpacity(1.0)),
	/// ),
	/// ],
	/// ),
	/// )
	/// ```
	///
	/// ### Size
	///
	/// In this example, the ambient [DefaultTextStyle] is explicitly manipulated to
	/// obtain a [TextStyle] that doubles the default font size.
	///
	/// ```dart
	/// new Text(
	/// 'These are wise words, enterprising men quote \'em.',
	/// style: DefaultTextStyle.of(context).style.apply(fontSizeFactor: 2.0),
	/// )
	/// ```
	///
	/// ### Line height
	///
	/// The [height] property can be used to change the line height. Here, the line
	/// height is set to 100 logical pixels, so that the text is very spaced out.
	///
	/// ```dart
	/// new Text(
	/// 'Don\'t act surprised, you guys, cuz I wrote \'em!',
	/// style: new TextStyle(height: 100.0),
	/// )
	/// ```
	///
	/// ### Wavy red underline with black text
	///
	/// Styles can be combined. In this example, the misspelt word is drawn in black
	/// text and underlined with a wavy red line to indicate a spelling error. (The
	/// remainder is styled according to the Flutter default text styles, not the
	/// ambient [DefaultTextStyle], since no explicit style is given and [RichText]
	/// does not automatically use the ambient [DefaultTextStyle].)
	///
	/// ```dart
	/// new RichText(
	/// text: new TextSpan(
	/// text: 'Don\'t tax the South ',
	/// children: <TextSpan>[
	/// new TextSpan(
	/// text: 'cuz',
	/// style: new TextStyle(
	/// color: Colors.black,
	/// decoration: TextDecoration.underline,
	/// decorationColor: Colors.red,
	/// decorationStyle: TextDecorationStyle.wavy,
	/// ),
	/// ),
	/// new TextSpan(
	/// text: ' we got it made in the shade',
	/// ),
	/// ],
	/// ),
	/// )
	/// ```
	///
	/// See also:
	///
	/// * [Text], the widget for showing text in a single style.
	/// * [DefaultTextStyle], the widget that specifies the default text styles for
	/// [Text] widgets, configured using a [TextStyle].
	/// * [RichText], the widget for showing a paragraph of mix-style text.
	/// * [TextSpan], the class that wraps a [TextStyle] for the purposes of
	/// passing it to a [RichText].
	@immutable
	class TextStyle extends Diagnosticable {
	/// Creates a text style.
	const TextStyle({
	this.inherit: true,
	this.color,
	this.fontFamily,
	this.fontSize,
	this.fontWeight,
	this.fontStyle,
	this.letterSpacing,
	this.wordSpacing,
	this.textBaseline,
	this.height,
	this.decoration,
	this.decorationColor,
	this.decorationStyle,
	}) : assert(inherit != null);

	/// Whether null values are replaced with their value in an ancestor text
	/// style (e.g., in a [TextSpan] tree).
	///
	/// If this is false, properties that don't have explicit values will revert
	/// to the defaults: white in color, a font size of 10 pixels, in a sans-serif
	/// font face.
	final bool inherit;

	/// The color to use when painting the text.
	final Color color;

	/// The name of the font to use when painting the text (e.g., Roboto).
	final String fontFamily;

	/// The size of glyphs (in logical pixels) to use when painting the text.
	///
	/// During painting, the [fontSize] is multiplied by the current
	/// `textScaleFactor` to let users make it easier to read text by increasing
	/// its size.
	final double fontSize;

	/// The typeface thickness to use when painting the text (e.g., bold).
	final FontWeight fontWeight;

	/// The typeface variant to use when drawing the letters (e.g., italics).
	final FontStyle fontStyle;

	/// The amount of space (in logical pixels) to add between each letter.
	/// A negative value can be used to bring the letters closer.
	final double letterSpacing;

	/// The amount of space (in logical pixels) to add at each sequence of
	/// white-space (i.e. between each word). A negative value can be used to
	/// bring the words closer.
	final double wordSpacing;

	/// The common baseline that should be aligned between this text span and its
	/// parent text span, or, for the root text spans, with the line box.
	final TextBaseline textBaseline;

	/// The height of this text span, as a multiple of the font size.
	///
	/// If applied to the root [TextSpan], this value sets the line height, which
	/// is the minimum distance between subsequent text baselines, as multiple of
	/// the font size.
	final double height;

	/// The decorations to paint near the text (e.g., an underline).
	final TextDecoration decoration;

	/// The color in which to paint the text decorations.
	final Color decorationColor;

	/// The style in which to paint the text decorations (e.g., dashed).
	final TextDecorationStyle decorationStyle;

	/// Creates a copy of this text style but with the given fields replaced with
	/// the new values.
	TextStyle copyWith({
	Color color,
	String fontFamily,
	double fontSize,
	FontWeight fontWeight,
	FontStyle fontStyle,
	double letterSpacing,
	double wordSpacing,
	TextBaseline textBaseline,
	double height,
	TextDecoration decoration,
	Color decorationColor,
	TextDecorationStyle decorationStyle,
	}) {
	return new TextStyle(
	inherit: inherit,
	color: color ?? this.color,
	fontFamily: fontFamily ?? this.fontFamily,
	fontSize: fontSize ?? this.fontSize,
	fontWeight: fontWeight ?? this.fontWeight,
	fontStyle: fontStyle ?? this.fontStyle,
	letterSpacing: letterSpacing ?? this.letterSpacing,
	wordSpacing: wordSpacing ?? this.wordSpacing,
	textBaseline: textBaseline ?? this.textBaseline,
	height: height ?? this.height,
	decoration: decoration ?? this.decoration,
	decorationColor: decorationColor ?? this.decorationColor,
	decorationStyle: decorationStyle ?? this.decorationStyle,
	);
	}

	/// Creates a copy of this text style but with the numeric fields multiplied
	/// by the given factors and then incremented by the given deltas.
	///
	/// For example, `style.apply(fontSizeFactor: 2.0, fontSizeDelta: 1.0)` would
	/// return a [TextStyle] whose [fontSize] is `style.fontSize * 2.0 + 1.0`.
	///
	/// For the [fontWeight], the delta is applied to the [FontWeight] enum index
	/// values, so that for instance `style.apply(fontWeightDelta: -2)` when
	/// applied to a `style` whose [fontWeight] is [FontWeight.w500] will return a
	/// [TextStyle] with a [FontWeight.w300].
	///
	/// The arguments must not be null.
	///
	/// If the underlying values are null, then the corresponding factors and/or
	/// deltas must not be specified.
	///
	/// The non-numeric fields can be controlled using the corresponding arguments.
	TextStyle apply({
	Color color,
	String fontFamily,
	double fontSizeFactor: 1.0,
	double fontSizeDelta: 0.0,
	int fontWeightDelta: 0,
	double letterSpacingFactor: 1.0,
	double letterSpacingDelta: 0.0,
	double wordSpacingFactor: 1.0,
	double wordSpacingDelta: 0.0,
	double heightFactor: 1.0,
	double heightDelta: 0.0,
	}) {
	assert(fontSizeFactor != null);
	assert(fontSizeDelta != null);
	assert(fontSize != null \|\| (fontSizeFactor == 1.0 && fontSizeDelta == 0.0));
	assert(fontWeightDelta != null);
	assert(fontWeight != null \|\| fontWeightDelta == 0.0);
	assert(letterSpacingFactor != null);
	assert(letterSpacingDelta != null);
	assert(letterSpacing != null \|\| (letterSpacingFactor == 1.0 && letterSpacingDelta == 0.0));
	assert(wordSpacingFactor != null);
	assert(wordSpacingDelta != null);
	assert(wordSpacing != null \|\| (wordSpacingFactor == 1.0 && wordSpacingDelta == 0.0));
	assert(heightFactor != null);
	assert(heightDelta != null);
	assert(heightFactor != null \|\| (heightFactor == 1.0 && heightDelta == 0.0));
	return new TextStyle(
	inherit: inherit,
	color: color ?? this.color,
	fontFamily: fontFamily ?? this.fontFamily,
	fontSize: fontSize == null ? null : fontSize * fontSizeFactor + fontSizeDelta,
	fontWeight: fontWeight == null ? null : FontWeight.values[(fontWeight.index + fontWeightDelta).clamp(0, FontWeight.values.length - 1)],
	fontStyle: fontStyle,
	letterSpacing: letterSpacing == null ? null : letterSpacing * letterSpacingFactor + letterSpacingDelta,
	wordSpacing: wordSpacing == null ? null : wordSpacing * wordSpacingFactor + wordSpacingDelta,
	textBaseline: textBaseline,
	height: height == null ? null : height * heightFactor + heightDelta,
	decoration: decoration,
	decorationColor: decorationColor,
	decorationStyle: decorationStyle,
	);
	}

	/// Returns a new text style that matches this text style but with some values
	/// replaced by the non-null parameters of the given text style. If the given
	/// text style is null, simply returns this text style.
	TextStyle merge(TextStyle other) {
	if (other == null)
	return this;
	assert(other.inherit);
	return copyWith(
	color: other.color,
	fontFamily: other.fontFamily,
	fontSize: other.fontSize,
	fontWeight: other.fontWeight,
	fontStyle: other.fontStyle,
	letterSpacing: other.letterSpacing,
	wordSpacing: other.wordSpacing,
	textBaseline: other.textBaseline,
	height: other.height,
	decoration: other.decoration,
	decorationColor: other.decorationColor,
	decorationStyle: other.decorationStyle
	);
	}

	/// Interpolate between two text styles.
	///
	/// This will not work well if the styles don't set the same fields.
	static TextStyle lerp(TextStyle begin, TextStyle end, double t) {
	assert(begin.inherit == end.inherit);
	return new TextStyle(
	inherit: end.inherit,
	color: Color.lerp(begin.color, end.color, t),
	fontFamily: t < 0.5 ? begin.fontFamily : end.fontFamily,
	fontSize: ui.lerpDouble(begin.fontSize ?? end.fontSize, end.fontSize ?? begin.fontSize, t),
	fontWeight: FontWeight.lerp(begin.fontWeight, end.fontWeight, t),
	fontStyle: t < 0.5 ? begin.fontStyle : end.fontStyle,
	letterSpacing: ui.lerpDouble(begin.letterSpacing ?? end.letterSpacing, end.letterSpacing ?? begin.letterSpacing, t),
	wordSpacing: ui.lerpDouble(begin.wordSpacing ?? end.wordSpacing, end.wordSpacing ?? begin.wordSpacing, t),
	textBaseline: t < 0.5 ? begin.textBaseline : end.textBaseline,
	height: ui.lerpDouble(begin.height ?? end.height, end.height ?? begin.height, t),
	decoration: t < 0.5 ? begin.decoration : end.decoration,
	decorationColor: Color.lerp(begin.decorationColor, end.decorationColor, t),
	decorationStyle: t < 0.5 ? begin.decorationStyle : end.decorationStyle
	);
	}

	/// The style information for text runs, encoded for use by `dart:ui`.
	ui.TextStyle getTextStyle({ double textScaleFactor: 1.0 }) {
	return new ui.TextStyle(
	color: color,
	decoration: decoration,
	decorationColor: decorationColor,
	decorationStyle: decorationStyle,
	fontWeight: fontWeight,
	fontStyle: fontStyle,
	textBaseline: textBaseline,
	fontFamily: fontFamily,
	fontSize: fontSize == null ? null : fontSize * textScaleFactor,
	letterSpacing: letterSpacing,
	wordSpacing: wordSpacing,
	height: height
	);
	}

	/// The style information for paragraphs, encoded for use by `dart:ui`.
	///
	/// The `textScaleFactor` argument must not be null. If omitted, it defaults
	/// to 1.0. The other arguments may be null. The `maxLines` argument, if
	/// specified and non-null, must be greater than zero.
	ui.ParagraphStyle getParagraphStyle({
	TextAlign textAlign,
	TextDirection textDirection,
	double textScaleFactor: 1.0,
	String ellipsis,
	int maxLines,
	}) {
	assert(textScaleFactor != null);
	assert(maxLines == null \|\| maxLines > 0);
	return new ui.ParagraphStyle(
	textAlign: textAlign,
	textDirection: textDirection,
	fontWeight: fontWeight,
	fontStyle: fontStyle,
	fontFamily: fontFamily,
	fontSize: fontSize == null ? null : fontSize * textScaleFactor,
	lineHeight: height,
	maxLines: maxLines,
	ellipsis: ellipsis,
	);
	}

	/// Describe the difference between this style and another, in terms of how
	/// much damage it will make to the rendering.
	///
	/// See also:
	///
	/// * [TextSpan.compareTo], which does the same thing for entire [TextSpan]s.
	RenderComparison compareTo(TextStyle other) {
	if (identical(this, other))
	return RenderComparison.identical;
	if (inherit != other.inherit \|\|
	fontFamily != other.fontFamily \|\|
	fontSize != other.fontSize \|\|
	fontWeight != other.fontWeight \|\|
	fontStyle != other.fontStyle \|\|
	letterSpacing != other.letterSpacing \|\|
	wordSpacing != other.wordSpacing \|\|
	textBaseline != other.textBaseline \|\|
	height != other.height)
	return RenderComparison.layout;
	if (color != other.color \|\|
	decoration != other.decoration \|\|
	decorationColor != other.decorationColor \|\|
	decorationStyle != other.decorationStyle)
	return RenderComparison.paint;
	return RenderComparison.identical;
	}

	@override
	bool operator ==(dynamic other) {
	if (identical(this, other))
	return true;
	if (other.runtimeType != runtimeType)
	return false;
	final TextStyle typedOther = other;
	return inherit == typedOther.inherit &&
	color == typedOther.color &&
	fontFamily == typedOther.fontFamily &&
	fontSize == typedOther.fontSize &&
	fontWeight == typedOther.fontWeight &&
	fontStyle == typedOther.fontStyle &&
	letterSpacing == typedOther.letterSpacing &&
	wordSpacing == typedOther.wordSpacing &&
	textBaseline == typedOther.textBaseline &&
	height == typedOther.height &&
	decoration == typedOther.decoration &&
	decorationColor == typedOther.decorationColor &&
	decorationStyle == typedOther.decorationStyle;
	}

	@override
	int get hashCode {
	return hashValues(
	inherit,
	color,
	fontFamily,
	fontSize,
	fontWeight,
	fontStyle,
	letterSpacing,
	wordSpacing,
	textBaseline,
	height,
	decoration,
	decorationColor,
	decorationStyle
	);
	}

	@override
	String toStringShort() => '$runtimeType';

	/// Adds all properties prefixing property names with the optional `prefix`.
	@override
	void debugFillProperties(DiagnosticPropertiesBuilder properties, { String prefix: '' }) {
	super.debugFillProperties(properties);
	final List<DiagnosticsNode> styles = <DiagnosticsNode>[];
	styles.add(new DiagnosticsProperty<Color>('${prefix}color', color, defaultValue: null));
	styles.add(new StringProperty('${prefix}family', fontFamily, defaultValue: null, quoted: false));
	styles.add(new DoubleProperty('${prefix}size', fontSize, defaultValue: null));
	String weightDescription;
	if (fontWeight != null) {
	switch (fontWeight) {
	case FontWeight.w100:
	weightDescription = '100';
	break;
	case FontWeight.w200:
	weightDescription = '200';
	break;
	case FontWeight.w300:
	weightDescription = '300';
	break;
	case FontWeight.w400:
	weightDescription = '400';
	break;
	case FontWeight.w500:
	weightDescription = '500';
	break;
	case FontWeight.w600:
	weightDescription = '600';
	break;
	case FontWeight.w700:
	weightDescription = '700';
	break;
	case FontWeight.w800:
	weightDescription = '800';
	break;
	case FontWeight.w900:
	weightDescription = '900';
	break;
	}
	}
	// TODO(jacobr): switch this to use enumProperty which will either cause the
	// weight description to change to w600 from 600 or require existing
	// enumProperty to handle this special case.
	styles.add(new DiagnosticsProperty<FontWeight>(
	'${prefix}weight',
	fontWeight,
	description: weightDescription,
	defaultValue: null,
	));
	styles.add(new EnumProperty<FontStyle>('${prefix}style', fontStyle, defaultValue: null));
	styles.add(new DoubleProperty('${prefix}letterSpacing', letterSpacing, unit: 'x', defaultValue: null));
	styles.add(new DoubleProperty('${prefix}wordSpacing', wordSpacing, unit: 'x', defaultValue: null));
	styles.add(new EnumProperty<TextBaseline>('${prefix}baseline', textBaseline, defaultValue: null));
	styles.add(new DoubleProperty('${prefix}height', height, unit: 'x', defaultValue: null));
	if (decoration != null \|\| decorationColor != null \|\| decorationStyle != null) {
	final List<String> decorationDescription = <String>[];
	if (decorationStyle != null)
	decorationDescription.add(describeEnum((decorationStyle)));

	// Hide decorationColor from the default text view as it is shown in the
	// terse decoration summary as well.
	styles.add(new DiagnosticsProperty<Color>('${prefix}decorationColor', decorationColor, defaultValue: null, hidden: true));

	if (decorationColor != null)
	decorationDescription.add('$decorationColor');

	// Intentionally collide with the property 'decoration' added below.
	// Tools that show hidden properties could choose the first property
	// matching the name to disambiguate.
	styles.add(new DiagnosticsProperty<TextDecoration>('${prefix}decoration', decoration, defaultValue: null, hidden: true));
	if (decoration != null)
	decorationDescription.add('$decoration');
	assert(decorationDescription.isNotEmpty);
	styles.add(new MessageProperty('${prefix}decoration', decorationDescription.join(' ')));
	}

	final bool styleSpecified = styles.any((DiagnosticsNode n) => !n.hidden);
	properties.add(new DiagnosticsProperty<bool>('${prefix}inherit', inherit, hidden: !styleSpecified && inherit));
	for (DiagnosticsNode style in styles)
	properties.add(style);

	if (!styleSpecified)
	properties.add(new FlagProperty('inherit', value: inherit, ifTrue: '$prefix<all styles inherited>', ifFalse: '$prefix<no style specified>'));
	}
	}