// Copyright 2014 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// @dart = 2.8

import 'package:flutter/foundation.dart';
import 'package:flutter/painting.dart';

import 'typography.dart';

/// Material design text theme.
///
/// Definitions for the various typographical styles found in Material Design
/// (e.g., button, caption). Rather than creating a [TextTheme] directly,
/// you can obtain an instance as [Typography.black] or [Typography.white].
///
/// To obtain the current text theme, call [Theme.of] with the current
/// [BuildContext] and read the [ThemeData.textTheme] property.
///
/// The names of the TextTheme properties match this table from the
/// [Material Design spec](https://material.io/design/typography/the-type-system.html#type-scale)
/// with two exceptions: the styles called H1-H6 in the spec are
/// headline1-headline6 in the API, and body1,body2 are called
/// bodyText1 and bodyText2.
///
/// ![](https://storage.googleapis.com/spec-host-backup/mio-design%2Fassets%2F1W8kyGVruuG_O8psvyiOaCf1lLFIMzB-N%2Ftypesystem-typescale.png)
///
/// ## Migrating from the 2014 names
///
/// The Material Design typography scheme was significantly changed in the
/// current (2018) version of the specification
/// ([https://material.io/design/typography](https://material.io/design/typography)).
///
/// The 2018 spec has thirteen text styles:
/// ```
/// NAME         SIZE  WEIGHT  SPACING
/// headline1    96.0  light   -1.5
/// headline2    60.0  light   -0.5
/// headline3    48.0  regular  0.0
/// headline4    34.0  regular  0.25
/// headline5    24.0  regular  0.0
/// headline6    20.0  medium   0.15
/// subtitle1    16.0  regular  0.15
/// subtitle2    14.0  medium   0.1
/// body1        16.0  regular  0.5   (bodyText1)
/// body2        14.0  regular  0.25  (bodyText2)
/// button       14.0  medium   1.25
/// caption      12.0  regular  0.4
/// overline     10.0  regular  1.5
/// ```
///
/// ...where "light" is `FontWeight.w300`, "regular" is `FontWeight.w400` and
/// "medium" is `FontWeight.w500`.
///
/// The [TextTheme] API was originally based on the original material (2014)
/// design spec, which used different text style names. For backwards
/// compatibility's sake, this API continues to expose the old names. The table
/// below should help with understanding the mapping of the API's old names and
/// the new names (those in terms of the 2018 material specification).
///
/// Each of the [TextTheme] text styles corresponds to one of the
/// styles from 2018 spec. By default, the font sizes, font weights
/// and letter spacings have not changed from their original,
/// 2014, values.
///
/// ```
/// NAME       SIZE   WEIGHT   SPACING  2018 NAME
/// display4   112.0  thin     0.0      headline1
/// display3   56.0   normal   0.0      headline2
/// display2   45.0   normal   0.0      headline3
/// display1   34.0   normal   0.0      headline4
/// headline   24.0   normal   0.0      headline5
/// title      20.0   medium   0.0      headline6
/// subhead    16.0   normal   0.0      subtitle1
/// body2      14.0   medium   0.0      body1 (bodyText1)
/// body1      14.0   normal   0.0      body2 (bodyText2)
/// caption    12.0   normal   0.0      caption
/// button     14.0   medium   0.0      button
/// subtitle   14.0   medium   0.0      subtitle2
/// overline   10.0   normal   0.0      overline
/// ```
///
/// Where "thin" is `FontWeight.w100`, "normal" is `FontWeight.w400` and
/// "medium" is `FontWeight.w500`. Letter spacing for all of the original
/// text styles was 0.0.
///
/// The old names are deprecated in this API.
///
/// Since the names `body1` and `body2` are used in both specifications but with
/// different meanings, the API uses the terms `bodyText1` and `bodyText2` for
/// the new API.
///
/// To configure a [Theme] for the new sizes, weights, and letter spacings,
/// initialize its [ThemeData.typography] value using [Typography.material2018].
///
/// See also:
///
///  * [Typography], the class that generates [TextTheme]s appropriate for a platform.
///  * [Theme], for other aspects of a material design application that can be
///    globally adjusted, such as the color scheme.
///  * <https://material.io/design/typography/>
@immutable
class TextTheme with Diagnosticable {
  /// Creates a text theme that uses the given values.
  ///
  /// Rather than creating a new text theme, consider using [Typography.black]
  /// or [Typography.white], which implement the typography styles in the
  /// material design specification:
  ///
  /// <https://material.io/design/typography/#type-scale>
  ///
  /// If you do decide to create your own text theme, consider using one of
  /// those predefined themes as a starting point for [copyWith] or [apply].
  const TextTheme({
    TextStyle headline1,
    TextStyle headline2,
    TextStyle headline3,
    TextStyle headline4,
    TextStyle headline5,
    TextStyle headline6,
    TextStyle subtitle1,
    TextStyle subtitle2,
    TextStyle bodyText1,
    TextStyle bodyText2,
    this.caption,
    this.button,
    this.overline,
    @Deprecated(
      'This is the term used in the 2014 version of material design. The modern term is headline1. '
      'This feature was deprecated after v1.13.8.'
    )
    TextStyle display4,
    @Deprecated(
      'This is the term used in the 2014 version of material design. The modern term is headline2. '
      'This feature was deprecated after v1.13.8.'
    )
    TextStyle display3,
    @Deprecated(
      'This is the term used in the 2014 version of material design. The modern term is headline3. '
      'This feature was deprecated after v1.13.8.'
    )
    TextStyle display2,
    @Deprecated(
      'This is the term used in the 2014 version of material design. The modern term is headline4. '
      'This feature was deprecated after v1.13.8.'
    )
    TextStyle display1,
    @Deprecated(
      'This is the term used in the 2014 version of material design. The modern term is headline5. '
      'This feature was deprecated after v1.13.8.'
    )
    TextStyle headline,
    @Deprecated(
      'This is the term used in the 2014 version of material design. The modern term is headline6. '
      'This feature was deprecated after v1.13.8.'
    )
    TextStyle title,
    @Deprecated(
      'This is the term used in the 2014 version of material design. The modern term is subtitle1. '
      'This feature was deprecated after v1.13.8.'
    )
    TextStyle subhead,
    @Deprecated(
      'This is the term used in the 2014 version of material design. The modern term is subtitle2. '
      'This feature was deprecated after v1.13.8.'
    )
    TextStyle subtitle,
    @Deprecated(
      'This is the term used in the 2014 version of material design. The modern term is bodyText1. '
      'This feature was deprecated after v1.13.8.'
    )
    TextStyle body2,
    @Deprecated(
      'This is the term used in the 2014 version of material design. The modern term is bodyText2. '
      'This feature was deprecated after v1.13.8.'
    )
    TextStyle body1,
  }) : assert(
         (headline1 == null && headline2 == null && headline3 == null && headline4 == null && headline5 == null && headline6 == null &&
          subtitle1 == null && subtitle2 == null &&
          bodyText1 == null && bodyText2 == null) ||
         (display4 == null && display3 == null && display2 == null && display1 == null && headline == null && title == null &&
          subhead == null && subtitle == null &&
          body2 == null && body1 == null), 'Cannot mix 2014 and 2018 terms in call to TextTheme() constructor.'),
       headline1 = headline1 ?? display4,
       headline2 = headline2 ?? display3,
       headline3 = headline3 ?? display2,
       headline4 = headline4 ?? display1,
       headline5 = headline5 ?? headline,
       headline6 = headline6 ?? title,
       subtitle1 = subtitle1 ?? subhead,
       subtitle2 = subtitle2 ?? subtitle,
       bodyText1 = bodyText1 ?? body2,
       bodyText2 = bodyText2 ?? body1;

  /// Extremely large text.
  final TextStyle headline1;

  /// Very, very large text.
  ///
  /// Used for the date in the dialog shown by [showDatePicker].
  final TextStyle headline2;

  /// Very large text.
  final TextStyle headline3;

  /// Large text.
  final TextStyle headline4;

  /// Used for large text in dialogs (e.g., the month and year in the dialog
  /// shown by [showDatePicker]).
  final TextStyle headline5;

  /// Used for the primary text in app bars and dialogs (e.g., [AppBar.title]
  /// and [AlertDialog.title]).
  final TextStyle headline6;

  /// Used for the primary text in lists (e.g., [ListTile.title]).
  final TextStyle subtitle1;

  /// For medium emphasis text that's a little smaller than [subtitle1].
  final TextStyle subtitle2;

  /// Used for emphasizing text that would otherwise be [bodyText2].
  final TextStyle bodyText1;

  /// The default text style for [Material].
  final TextStyle bodyText2;

  /// Used for auxiliary text associated with images.
  final TextStyle caption;

  /// Used for text on [RaisedButton] and [FlatButton].
  final TextStyle button;

  /// The smallest style.
  ///
  /// Typically used for captions or to introduce a (larger) headline.
  final TextStyle overline;

  /// Extremely large text.
  ///
  /// This was the name used in the material design 2014 specification. The new
  /// specification calls this [headline1].
  @Deprecated(
    'This is the term used in the 2014 version of material design. The modern term is headline1. '
    'This feature was deprecated after v1.13.8.'
  )
  TextStyle get display4 => headline1;

  /// Very, very large text.
  ///
  /// This was the name used in the material design 2014 specification. The new
  /// specification calls this [headline2].
  @Deprecated(
    'This is the term used in the 2014 version of material design. The modern term is headline2. '
    'This feature was deprecated after v1.13.8.'
  )
  TextStyle get display3 => headline2;

  /// Very large text.
  ///
  /// This was the name used in the material design 2014 specification. The new
  /// specification calls this [headline3].
  @Deprecated(
    'This is the term used in the 2014 version of material design. The modern term is headline3. '
    'This feature was deprecated after v1.13.8.'
  )
  TextStyle get display2 => headline3;

  /// Large text.
  ///
  /// This was the name used in the material design 2014 specification. The new
  /// specification calls this [headline4].
  @Deprecated(
    'This is the term used in the 2014 version of material design. The modern term is headline4. '
    'This feature was deprecated after v1.13.8.'
  )
  TextStyle get display1 => headline4;

  /// Used for large text in dialogs.
  ///
  /// This was the name used in the material design 2014 specification. The new
  /// specification calls this [headline5].
  @Deprecated(
    'This is the term used in the 2014 version of material design. The modern term is headline5. '
    'This feature was deprecated after v1.13.8.'
  )
  TextStyle get headline => headline5;

  /// Used for the primary text in app bars and dialogs.
  ///
  /// This was the name used in the material design 2014 specification. The new
  /// specification calls this [headline6].
  @Deprecated(
    'This is the term used in the 2014 version of material design. The modern term is headline6. '
    'This feature was deprecated after v1.13.8.'
  )
  TextStyle get title => headline6;

  /// Used for the primary text in lists (e.g., [ListTile.title]).
  ///
  /// This was the name used in the material design 2014 specification. The new
  /// specification calls this [subtitle1].
  @Deprecated(
    'This is the term used in the 2014 version of material design. The modern term is subtitle1. '
    'This feature was deprecated after v1.13.8.'
  )
  TextStyle get subhead => subtitle1;

  /// For medium emphasis text that's a little smaller than [subhead].
  ///
  /// This was the name used in the material design 2014 specification. The new
  /// specification calls this [subtitle2].
  @Deprecated(
    'This is the term used in the 2014 version of material design. The modern term is subtitle2. '
    'This feature was deprecated after v1.13.8.'
  )
  TextStyle get subtitle => subtitle2;

  /// Used for emphasizing text that would otherwise be [body1].
  ///
  /// This was the name used in the material design 2014 specification. The new
  /// specification calls this `body1`, and it is exposed in this API as
  /// [bodyText1].
  @Deprecated(
    'This is the term used in the 2014 version of material design. The modern term is bodyText1. '
    'This feature was deprecated after v1.13.8.'
  )
  TextStyle get body2 => bodyText1;

  /// Used for the default text style for [Material].
  ///
  /// This was the name used in the material design 2014 specification. The new
  /// specification calls this `body2`, and it is exposed in this API as
  /// [bodyText2].
  @Deprecated(
    'This is the term used in the 2014 version of material design. The modern term is bodyText2. '
    'This feature was deprecated after v1.13.8.'
  )
  TextStyle get body1 => bodyText2;


  /// Creates a copy of this text theme but with the given fields replaced with
  /// the new values.
  ///
  /// Consider using [Typography.black] or [Typography.white], which implement
  /// the typography styles in the material design specification, as a starting
  /// point.
  ///
  /// {@tool snippet}
  ///
  /// ```dart
  /// /// A Widget that sets the ambient theme's title text color for its
  /// /// descendants, while leaving other ambient theme attributes alone.
  /// class TitleColorThemeCopy extends StatelessWidget {
  ///   TitleColorThemeCopy({Key key, this.child, this.titleColor}) : super(key: key);
  ///
  ///   final Color titleColor;
  ///   final Widget child;
  ///
  ///   @override
  ///   Widget build(BuildContext context) {
  ///     final ThemeData theme = Theme.of(context);
  ///     return Theme(
  ///       data: theme.copyWith(
  ///         textTheme: theme.textTheme.copyWith(
  ///           headline6: theme.textTheme.headline6.copyWith(
  ///             color: titleColor,
  ///           ),
  ///         ),
  ///       ),
  ///       child: child,
  ///     );
  ///   }
  /// }
  /// ```
  /// {@end-tool}
  ///
  /// See also:
  ///
  ///  * [merge] is used instead of [copyWith] when you want to merge all
  ///    of the fields of a TextTheme instead of individual fields.
  TextTheme copyWith({
    TextStyle headline1,
    TextStyle headline2,
    TextStyle headline3,
    TextStyle headline4,
    TextStyle headline5,
    TextStyle headline6,
    TextStyle subtitle1,
    TextStyle subtitle2,
    TextStyle bodyText1,
    TextStyle bodyText2,
    TextStyle caption,
    TextStyle button,
    TextStyle overline,
    @Deprecated(
      'This is the term used in the 2014 version of material design. The modern term is headline1. '
      'This feature was deprecated after v1.13.8.'
    )
    TextStyle display4,
    @Deprecated(
      'This is the term used in the 2014 version of material design. The modern term is headline2. '
      'This feature was deprecated after v1.13.8.'
    )
    TextStyle display3,
    @Deprecated(
      'This is the term used in the 2014 version of material design. The modern term is headline3. '
      'This feature was deprecated after v1.13.8.'
    )
    TextStyle display2,
    @Deprecated(
      'This is the term used in the 2014 version of material design. The modern term is headline4. '
      'This feature was deprecated after v1.13.8.'
    )
    TextStyle display1,
    @Deprecated(
      'This is the term used in the 2014 version of material design. The modern term is headline5. '
      'This feature was deprecated after v1.13.8.'
    )
    TextStyle headline,
    @Deprecated(
      'This is the term used in the 2014 version of material design. The modern term is headline6. '
      'This feature was deprecated after v1.13.8.'
    )
    TextStyle title,
    @Deprecated(
      'This is the term used in the 2014 version of material design. The modern term is subtitle1. '
      'This feature was deprecated after v1.13.8.'
    )
    TextStyle subhead,
    @Deprecated(
      'This is the term used in the 2014 version of material design. The modern term is subtitle2. '
      'This feature was deprecated after v1.13.8.'
    )
    TextStyle subtitle,
    @Deprecated(
      'This is the term used in the 2014 version of material design. The modern term is bodyText1. '
      'This feature was deprecated after v1.13.8.'
    )
    TextStyle body2,
    @Deprecated(
      'This is the term used in the 2014 version of material design. The modern term is bodyText2. '
      'This feature was deprecated after v1.13.8.'
    )
    TextStyle body1,
  }) {
    assert(
      (headline1 == null && headline2 == null && headline3 == null && headline4 == null && headline5 == null && headline6 == null &&
       subtitle1 == null && subtitle2 == null &&
       bodyText1 == null && bodyText2 == null) ||
      (display4 == null && display3 == null && display2 == null && display1 == null && headline == null && title == null &&
       subhead == null && subtitle == null &&
       body2 == null && body1 == null), 'Cannot mix 2014 and 2018 terms in call to TextTheme.copyWith().');
    return TextTheme(
      headline1: headline1 ?? display4 ?? this.headline1,
      headline2: headline2 ?? display3 ?? this.headline2,
      headline3: headline3 ?? display2 ?? this.headline3,
      headline4: headline4 ?? display1 ?? this.headline4,
      headline5: headline5 ?? headline ?? this.headline5,
      headline6: headline6 ?? title ?? this.headline6,
      subtitle1: subtitle1 ?? subhead ?? this.subtitle1,
      subtitle2: subtitle2 ?? subtitle ?? this.subtitle2,
      bodyText1: bodyText1 ?? body2 ?? this.bodyText1,
      bodyText2: bodyText2 ?? body1 ?? this.bodyText2,
      caption: caption ?? this.caption,
      button: button ?? this.button,
      overline: overline ?? this.overline,
    );
  }

  /// Creates a new [TextTheme] where each text style from this object has been
  /// merged with the matching text style from the `other` object.
  ///
  /// The merging is done by calling [TextStyle.merge] on each respective pair
  /// of text styles from this and the [other] text themes and is subject to
  /// the value of [TextStyle.inherit] flag. For more details, see the
  /// documentation on [TextStyle.merge] and [TextStyle.inherit].
  ///
  /// If this theme, or the `other` theme has members that are null, then the
  /// non-null one (if any) is used. If the `other` theme is itself null, then
  /// this [TextTheme] is returned unchanged. If values in both are set, then
  /// the values are merged using [TextStyle.merge].
  ///
  /// This is particularly useful if one [TextTheme] defines one set of
  /// properties and another defines a different set, e.g. having colors
  /// defined in one text theme and font sizes in another, or when one
  /// [TextTheme] has only some fields defined, and you want to define the rest
  /// by merging it with a default theme.
  ///
  /// {@tool snippet}
  ///
  /// ```dart
  /// /// A Widget that sets the ambient theme's title text color for its
  /// /// descendants, while leaving other ambient theme attributes alone.
  /// class TitleColorTheme extends StatelessWidget {
  ///   TitleColorTheme({Key key, this.child, this.titleColor}) : super(key: key);
  ///
  ///   final Color titleColor;
  ///   final Widget child;
  ///
  ///   @override
  ///   Widget build(BuildContext context) {
  ///     ThemeData theme = Theme.of(context);
  ///     // This partialTheme is incomplete: it only has the title style
  ///     // defined. Just replacing theme.textTheme with partialTheme would
  ///     // set the title, but everything else would be null. This isn't very
  ///     // useful, so merge it with the existing theme to keep all of the
  ///     // preexisting definitions for the other styles.
  ///     TextTheme partialTheme = TextTheme(headline6: TextStyle(color: titleColor));
  ///     theme = theme.copyWith(textTheme: theme.textTheme.merge(partialTheme));
  ///     return Theme(data: theme, child: child);
  ///   }
  /// }
  /// ```
  /// {@end-tool}
  ///
  /// See also:
  ///
  ///  * [copyWith] is used instead of [merge] when you wish to override
  ///    individual fields in the [TextTheme] instead of merging all of the
  ///    fields of two [TextTheme]s.
  TextTheme merge(TextTheme other) {
    if (other == null)
      return this;
    return copyWith(
      headline1: headline1?.merge(other.headline1) ?? other.headline1,
      headline2: headline2?.merge(other.headline2) ?? other.headline2,
      headline3: headline3?.merge(other.headline3) ?? other.headline3,
      headline4: headline4?.merge(other.headline4) ?? other.headline4,
      headline5: headline5?.merge(other.headline5) ?? other.headline5,
      headline6: headline6?.merge(other.headline6) ?? other.headline6,
      subtitle1: subtitle1?.merge(other.subtitle1) ?? other.subtitle1,
      subtitle2: subtitle2?.merge(other.subtitle2) ?? other.subtitle2,
      bodyText1: bodyText1?.merge(other.bodyText1) ?? other.bodyText1,
      bodyText2: bodyText2?.merge(other.bodyText2) ?? other.bodyText2,
      caption: caption?.merge(other.caption) ?? other.caption,
      button: button?.merge(other.button) ?? other.button,
      overline: overline?.merge(other.overline) ?? other.overline,
    );
  }

  /// Creates a copy of this text theme but with the given field replaced in
  /// each of the individual text styles.
  ///
  /// The `displayColor` is applied to [headline4], [headline3], [headline2],
  /// [headline1], and [caption]. The `bodyColor` is applied to the remaining
  /// text styles.
  ///
  /// Consider using [Typography.black] or [Typography.white], which implement
  /// the typography styles in the material design specification, as a starting
  /// point.
  TextTheme apply({
    String fontFamily,
    double fontSizeFactor = 1.0,
    double fontSizeDelta = 0.0,
    Color displayColor,
    Color bodyColor,
    TextDecoration decoration,
    Color decorationColor,
    TextDecorationStyle decorationStyle,
  }) {
    return TextTheme(
      headline1: headline1?.apply(
        color: displayColor,
        decoration: decoration,
        decorationColor: decorationColor,
        decorationStyle: decorationStyle,
        fontFamily: fontFamily,
        fontSizeFactor: fontSizeFactor,
        fontSizeDelta: fontSizeDelta,
      ),
      headline2: headline2?.apply(
        color: displayColor,
        decoration: decoration,
        decorationColor: decorationColor,
        decorationStyle: decorationStyle,
        fontFamily: fontFamily,
        fontSizeFactor: fontSizeFactor,
        fontSizeDelta: fontSizeDelta,
      ),
      headline3: headline3?.apply(
        color: displayColor,
        decoration: decoration,
        decorationColor: decorationColor,
        decorationStyle: decorationStyle,
        fontFamily: fontFamily,
        fontSizeFactor: fontSizeFactor,
        fontSizeDelta: fontSizeDelta,
      ),
      headline4: headline4?.apply(
        color: displayColor,
        decoration: decoration,
        decorationColor: decorationColor,
        decorationStyle: decorationStyle,
        fontFamily: fontFamily,
        fontSizeFactor: fontSizeFactor,
        fontSizeDelta: fontSizeDelta,
      ),
      headline5: headline5?.apply(
        color: bodyColor,
        decoration: decoration,
        decorationColor: decorationColor,
        decorationStyle: decorationStyle,
        fontFamily: fontFamily,
        fontSizeFactor: fontSizeFactor,
        fontSizeDelta: fontSizeDelta,
      ),
      headline6: headline6?.apply(
        color: bodyColor,
        decoration: decoration,
        decorationColor: decorationColor,
        decorationStyle: decorationStyle,
        fontFamily: fontFamily,
        fontSizeFactor: fontSizeFactor,
        fontSizeDelta: fontSizeDelta,
      ),
      subtitle1: subtitle1?.apply(
        color: bodyColor,
        decoration: decoration,
        decorationColor: decorationColor,
        decorationStyle: decorationStyle,
        fontFamily: fontFamily,
        fontSizeFactor: fontSizeFactor,
        fontSizeDelta: fontSizeDelta,
      ),
      subtitle2: subtitle2?.apply(
        color: bodyColor,
        decoration: decoration,
        decorationColor: decorationColor,
        decorationStyle: decorationStyle,
        fontFamily: fontFamily,
        fontSizeFactor: fontSizeFactor,
        fontSizeDelta: fontSizeDelta,
      ),
      bodyText1: bodyText1?.apply(
        color: bodyColor,
        decoration: decoration,
        decorationColor: decorationColor,
        decorationStyle: decorationStyle,
        fontFamily: fontFamily,
        fontSizeFactor: fontSizeFactor,
        fontSizeDelta: fontSizeDelta,
      ),
      bodyText2: bodyText2?.apply(
        color: bodyColor,
        decoration: decoration,
        decorationColor: decorationColor,
        decorationStyle: decorationStyle,
        fontFamily: fontFamily,
        fontSizeFactor: fontSizeFactor,
        fontSizeDelta: fontSizeDelta,
      ),
      caption: caption?.apply(
        color: displayColor,
        decoration: decoration,
        decorationColor: decorationColor,
        decorationStyle: decorationStyle,
        fontFamily: fontFamily,
        fontSizeFactor: fontSizeFactor,
        fontSizeDelta: fontSizeDelta,
      ),
      button: button?.apply(
        color: bodyColor,
        decoration: decoration,
        decorationColor: decorationColor,
        decorationStyle: decorationStyle,
        fontFamily: fontFamily,
        fontSizeFactor: fontSizeFactor,
        fontSizeDelta: fontSizeDelta,
      ),
      overline: overline?.apply(
        color: bodyColor,
        decoration: decoration,
        decorationColor: decorationColor,
        decorationStyle: decorationStyle,
        fontFamily: fontFamily,
        fontSizeFactor: fontSizeFactor,
        fontSizeDelta: fontSizeDelta,
      ),
    );
  }

  /// Linearly interpolate between two text themes.
  ///
  /// {@macro dart.ui.shadow.lerp}
  static TextTheme lerp(TextTheme a, TextTheme b, double t) {
    assert(t != null);
    return TextTheme(
      headline1: TextStyle.lerp(a?.headline1, b?.headline1, t),
      headline2: TextStyle.lerp(a?.headline2, b?.headline2, t),
      headline3: TextStyle.lerp(a?.headline3, b?.headline3, t),
      headline4: TextStyle.lerp(a?.headline4, b?.headline4, t),
      headline5: TextStyle.lerp(a?.headline5, b?.headline5, t),
      headline6: TextStyle.lerp(a?.headline6, b?.headline6, t),
      subtitle1: TextStyle.lerp(a?.subtitle1, b?.subtitle1, t),
      subtitle2: TextStyle.lerp(a?.subtitle2, b?.subtitle2, t),
      bodyText1: TextStyle.lerp(a?.bodyText1, b?.bodyText1, t),
      bodyText2: TextStyle.lerp(a?.bodyText2, b?.bodyText2, t),
      caption: TextStyle.lerp(a?.caption, b?.caption, t),
      button: TextStyle.lerp(a?.button, b?.button, t),
      overline: TextStyle.lerp(a?.overline, b?.overline, t),
    );
  }

  @override
  bool operator ==(Object other) {
    if (identical(this, other))
      return true;
    if (other.runtimeType != runtimeType)
      return false;
    return other is TextTheme
      && headline1 == other.headline1
      && headline2 == other.headline2
      && headline3 == other.headline3
      && headline4 == other.headline4
      && headline5 == other.headline5
      && headline6 == other.headline6
      && subtitle1 == other.subtitle1
      && subtitle2 == other.subtitle2
      && bodyText1 == other.bodyText1
      && bodyText2 == other.bodyText2
      && caption == other.caption
      && button == other.button
      && overline == other.overline;
  }

  @override
  int get hashCode {
    // The hashValues() function supports up to 20 arguments.
    return hashValues(
      headline1,
      headline2,
      headline3,
      headline4,
      headline5,
      headline6,
      subtitle1,
      subtitle2,
      bodyText1,
      bodyText2,
      caption,
      button,
      overline,
    );
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    final TextTheme defaultTheme = Typography.material2018(platform: defaultTargetPlatform).black;
    properties.add(DiagnosticsProperty<TextStyle>('headline1', headline1, defaultValue: defaultTheme.headline1));
    properties.add(DiagnosticsProperty<TextStyle>('headline2', headline2, defaultValue: defaultTheme.headline2));
    properties.add(DiagnosticsProperty<TextStyle>('headline3', headline3, defaultValue: defaultTheme.headline3));
    properties.add(DiagnosticsProperty<TextStyle>('headline4', headline4, defaultValue: defaultTheme.headline4));
    properties.add(DiagnosticsProperty<TextStyle>('headline5', headline5, defaultValue: defaultTheme.headline5));
    properties.add(DiagnosticsProperty<TextStyle>('headline6', headline6, defaultValue: defaultTheme.headline6));
    properties.add(DiagnosticsProperty<TextStyle>('subtitle1', subtitle1, defaultValue: defaultTheme.subtitle1));
    properties.add(DiagnosticsProperty<TextStyle>('subtitle2', subtitle2, defaultValue: defaultTheme.subtitle2));
    properties.add(DiagnosticsProperty<TextStyle>('bodyText1', bodyText1, defaultValue: defaultTheme.bodyText1));
    properties.add(DiagnosticsProperty<TextStyle>('bodyText2', bodyText2, defaultValue: defaultTheme.bodyText2));
    properties.add(DiagnosticsProperty<TextStyle>('caption', caption, defaultValue: defaultTheme.caption));
    properties.add(DiagnosticsProperty<TextStyle>('button', button, defaultValue: defaultTheme.button));
    properties.add(DiagnosticsProperty<TextStyle>('overline', overline, defaultValue: defaultTheme.overline));
  }
}
