text.dart

// Copyright 2016 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 'package:flutter/foundation.dart';
import 'package:flutter/painting.dart';

import 'basic.dart';
import 'framework.dart';
import 'inherited_theme.dart';
import 'media_query.dart';

// Examples can assume:
// String _name;

/// The text style to apply to descendant [Text] widgets without explicit style.
///
/// See also:
///
///  * [AnimatedDefaultTextStyle], which animates changes in the text style
///    smoothly over a given duration.
///  * [DefaultTextStyleTransition], which takes a provided [Animation] to
///    animate changes in text style smoothly over time.
class DefaultTextStyle extends InheritedTheme {
  /// Creates a default text style for the given subtree.
  ///
  /// Consider using [DefaultTextStyle.merge] to inherit styling information
  /// from the current default text style for a given [BuildContext].
  ///
  /// The [style] and [child] arguments are required and must not be null.
  ///
  /// The [softWrap] and [overflow] arguments must not be null (though they do
  /// have default values).
  ///
  /// The [maxLines] property may be null (and indeed defaults to null), but if
  /// it is not null, it must be greater than zero.
  const DefaultTextStyle({
    Key key,
    @required this.style,
    this.textAlign,
    this.softWrap = true,
    this.overflow = TextOverflow.clip,
    this.maxLines,
    this.textWidthBasis = TextWidthBasis.parent,
    @required Widget child,
  }) : assert(style != null),
       assert(softWrap != null),
       assert(overflow != null),
       assert(maxLines == null || maxLines > 0),
       assert(child != null),
       assert(textWidthBasis != null),
       super(key: key, child: child);

  /// A const-constructible default text style that provides fallback values.
  ///
  /// Returned from [of] when the given [BuildContext] doesn't have an enclosing default text style.
  ///
  /// This constructor creates a [DefaultTextStyle] that lacks a [child], which
  /// means the constructed value cannot be incorporated into the tree.
  const DefaultTextStyle.fallback()
    : style = const TextStyle(),
      textAlign = null,
      softWrap = true,
      maxLines = null,
      overflow = TextOverflow.clip,
      textWidthBasis = TextWidthBasis.parent;

  /// Creates a default text style that overrides the text styles in scope at
  /// this point in the widget tree.
  ///
  /// The given [style] is merged with the [style] from the default text style
  /// for the [BuildContext] where the widget is inserted, and any of the other
  /// arguments that are not null replace the corresponding properties on that
  /// same default text style.
  ///
  /// This constructor cannot be used to override the [maxLines] property of the
  /// ancestor with the value null, since null here is used to mean "defer to
  /// ancestor". To replace a non-null [maxLines] from an ancestor with the null
  /// value (to remove the restriction on number of lines), manually obtain the
  /// ambient [DefaultTextStyle] using [DefaultTextStyle.of], then create a new
  /// [DefaultTextStyle] using the [new DefaultTextStyle] constructor directly.
  /// See the source below for an example of how to do this (since that's
  /// essentially what this constructor does).
  static Widget merge({
    Key key,
    TextStyle style,
    TextAlign textAlign,
    bool softWrap,
    TextOverflow overflow,
    int maxLines,
    TextWidthBasis textWidthBasis,
    @required Widget child,
  }) {
    assert(child != null);
    return Builder(
      builder: (BuildContext context) {
        final DefaultTextStyle parent = DefaultTextStyle.of(context);
        return DefaultTextStyle(
          key: key,
          style: parent.style.merge(style),
          textAlign: textAlign ?? parent.textAlign,
          softWrap: softWrap ?? parent.softWrap,
          overflow: overflow ?? parent.overflow,
          maxLines: maxLines ?? parent.maxLines,
          textWidthBasis: textWidthBasis ?? parent.textWidthBasis,
          child: child,
        );
      },
    );
  }

  /// The text style to apply.
  final TextStyle style;

  /// How the text should be aligned horizontally.
  final TextAlign textAlign;

  /// Whether the text should break at soft line breaks.
  ///
  /// If false, the glyphs in the text will be positioned as if there was unlimited horizontal space.
  final bool softWrap;

  /// How visual overflow should be handled.
  final TextOverflow overflow;

  /// An optional maximum number of lines for the text to span, wrapping if necessary.
  /// If the text exceeds the given number of lines, it will be truncated according
  /// to [overflow].
  ///
  /// If this is 1, text will not wrap. Otherwise, text will be wrapped at the
  /// edge of the box.
  ///
  /// If this is non-null, it will override even explicit null values of
  /// [Text.maxLines].
  final int maxLines;

  /// The strategy to use when calculating the width of the Text.
  /// See [TextWidthBasis] for possible values and their implications.
  final TextWidthBasis textWidthBasis;

  /// The closest instance of this class that encloses the given context.
  ///
  /// If no such instance exists, returns an instance created by
  /// [DefaultTextStyle.fallback], which contains fallback values.
  ///
  /// Typical usage is as follows:
  ///
  /// ```dart
  /// DefaultTextStyle style = DefaultTextStyle.of(context);
  /// ```
  static DefaultTextStyle of(BuildContext context) {
    return context.inheritFromWidgetOfExactType(DefaultTextStyle) ?? const DefaultTextStyle.fallback();
  }

  @override
  bool updateShouldNotify(DefaultTextStyle oldWidget) {
    return style != oldWidget.style ||
        textAlign != oldWidget.textAlign ||
        softWrap != oldWidget.softWrap ||
        overflow != oldWidget.overflow ||
        maxLines != oldWidget.maxLines ||
        textWidthBasis != oldWidget.textWidthBasis;
  }

  @override
  Widget wrap(BuildContext context, Widget child) {
    final DefaultTextStyle defaultTextStyle = context.ancestorWidgetOfExactType(DefaultTextStyle);
    return identical(this, defaultTextStyle) ? child : DefaultTextStyle(
      style: style,
      textAlign: textAlign,
      softWrap: softWrap,
      overflow: overflow,
      maxLines: maxLines,
      textWidthBasis: textWidthBasis,
      child: child,
    );
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    style?.debugFillProperties(properties);
    properties.add(EnumProperty<TextAlign>('textAlign', textAlign, defaultValue: null));
    properties.add(FlagProperty('softWrap', value: softWrap, ifTrue: 'wrapping at box width', ifFalse: 'no wrapping except at line break characters', showName: true));
    properties.add(EnumProperty<TextOverflow>('overflow', overflow, defaultValue: null));
    properties.add(IntProperty('maxLines', maxLines, defaultValue: null));
    properties.add(EnumProperty<TextWidthBasis>('textWidthBasis', textWidthBasis, defaultValue: TextWidthBasis.parent));
  }
}

/// A run of text with a single style.
///
/// The [Text] widget displays a string of text with single style. The string
/// might break across multiple lines or might all be displayed on the same line
/// depending on the layout constraints.
///
/// The [style] argument is optional. When omitted, the text will use the style
/// from the closest enclosing [DefaultTextStyle]. If the given style's
/// [TextStyle.inherit] property is true (the default), the given style will
/// be merged with the closest enclosing [DefaultTextStyle]. This merging
/// behavior is useful, for example, to make the text bold while using the
/// default font family and size.
///
/// {@tool sample}
///
/// This example shows how to display text using the [Text] widget. If the text
/// overflows, it truncates the text with an ellipsis.
///
/// ![A screenshot of the Text widget](https://flutter.github.io/assets-for-api-docs/assets/widgets/text.png)
///
/// ![A screenshot of the Text widget displaying an ellipsis to trim the overflowing text](https://flutter.github.io/assets-for-api-docs/assets/widgets/text_ellipsis.png)
///
/// ```dart
/// Text(
///   'Hello, $_name! How are you?',
///   textAlign: TextAlign.center,
///   overflow: TextOverflow.ellipsis,
///   style: TextStyle(fontWeight: FontWeight.bold),
/// )
/// ```
/// {@end-tool}
///
/// Using the [Text.rich] constructor, the [Text] widget can
/// display a paragraph with differently styled [TextSpan]s. The sample
/// that follows displays "Hello beautiful world" with different styles
/// for each word.
///
/// {@tool sample}
///
/// ![A screenshot of the following rich text example](https://flutter.github.io/assets-for-api-docs/assets/widgets/text_rich.png)
///
/// ```dart
/// const Text.rich(
///   TextSpan(
///     text: 'Hello', // default text style
///     children: <TextSpan>[
///       TextSpan(text: ' beautiful ', style: TextStyle(fontStyle: FontStyle.italic)),
///       TextSpan(text: 'world', style: TextStyle(fontWeight: FontWeight.bold)),
///     ],
///   ),
/// )
/// ```
/// {@end-tool}
///
/// ## Interactivity
///
/// To make [Text] react to touch events, wrap it in a [GestureDetector] widget
/// with a [GestureDetector.onTap] handler.
///
/// In a material design application, consider using a [FlatButton] instead, or
/// if that isn't appropriate, at least using an [InkWell] instead of
/// [GestureDetector].
///
/// To make sections of the text interactive, use [RichText] and specify a
/// [TapGestureRecognizer] as the [TextSpan.recognizer] of the relevant part of
/// the text.
///
/// See also:
///
///  * [RichText], which gives you more control over the text styles.
///  * [DefaultTextStyle], which sets default styles for [Text] widgets.
class Text extends StatelessWidget {
  /// Creates a text widget.
  ///
  /// If the [style] argument is null, the text will use the style from the
  /// closest enclosing [DefaultTextStyle].
  ///
  /// The [data] parameter must not be null.
  const Text(
    this.data, {
    Key key,
    this.style,
    this.strutStyle,
    this.textAlign,
    this.textDirection,
    this.locale,
    this.softWrap,
    this.overflow,
    this.textScaleFactor,
    this.maxLines,
    this.semanticsLabel,
    this.textWidthBasis,
  }) : assert(
         data != null,
         'A non-null String must be provided to a Text widget.',
       ),
       textSpan = null,
       super(key: key);

  /// Creates a text widget with a [InlineSpan].
  ///
  /// The following subclasses of [InlineSpan] may be used to build rich text:
  ///
  /// * [TextSpan]s define text and children [InlineSpan]s.
  /// * [WidgetSpan]s define embedded inline widgets.
  ///
  /// The [textSpan] parameter must not be null.
  ///
  /// See [RichText] which provides a lower-level way to draw text.
  const Text.rich(
    this.textSpan, {
    Key key,
    this.style,
    this.strutStyle,
    this.textAlign,
    this.textDirection,
    this.locale,
    this.softWrap,
    this.overflow,
    this.textScaleFactor,
    this.maxLines,
    this.semanticsLabel,
    this.textWidthBasis,
  }) : assert(
         textSpan != null,
         'A non-null TextSpan must be provided to a Text.rich widget.',
       ),
       data = null,
       super(key: key);

  /// The text to display.
  ///
  /// This will be null if a [textSpan] is provided instead.
  final String data;

  /// The text to display as a [InlineSpan].
  ///
  /// This will be null if [data] is provided instead.
  final InlineSpan textSpan;

  /// If non-null, the style to use for this text.
  ///
  /// If the style's "inherit" property is true, the style will be merged with
  /// the closest enclosing [DefaultTextStyle]. Otherwise, the style will
  /// replace the closest enclosing [DefaultTextStyle].
  final TextStyle style;

  /// {@macro flutter.painting.textPainter.strutStyle}
  final StrutStyle strutStyle;

  /// How the text should be aligned horizontally.
  final TextAlign textAlign;

  /// The directionality of the text.
  ///
  /// This decides how [textAlign] values like [TextAlign.start] and
  /// [TextAlign.end] are interpreted.
  ///
  /// This is also used to disambiguate how to render bidirectional text. For
  /// example, if the [data] is an English phrase followed by a Hebrew phrase,
  /// in a [TextDirection.ltr] context the English phrase will be on the left
  /// and the Hebrew phrase to its right, while in a [TextDirection.rtl]
  /// context, the English phrase will be on the right and the Hebrew phrase on
  /// its left.
  ///
  /// Defaults to the ambient [Directionality], if any.
  final TextDirection textDirection;

  /// Used to select a font when the same Unicode character can
  /// be rendered differently, depending on the locale.
  ///
  /// It's rarely necessary to set this property. By default its value
  /// is inherited from the enclosing app with `Localizations.localeOf(context)`.
  ///
  /// See [RenderParagraph.locale] for more information.
  final Locale locale;

  /// Whether the text should break at soft line breaks.
  ///
  /// If false, the glyphs in the text will be positioned as if there was unlimited horizontal space.
  final bool softWrap;

  /// How visual overflow should be handled.
  final TextOverflow overflow;

  /// The number of font pixels for each logical pixel.
  ///
  /// For example, if the text scale factor is 1.5, text will be 50% larger than
  /// the specified font size.
  ///
  /// The value given to the constructor as textScaleFactor. If null, will
  /// use the [MediaQueryData.textScaleFactor] obtained from the ambient
  /// [MediaQuery], or 1.0 if there is no [MediaQuery] in scope.
  final double textScaleFactor;

  /// An optional maximum number of lines for the text to span, wrapping if necessary.
  /// If the text exceeds the given number of lines, it will be truncated according
  /// to [overflow].
  ///
  /// If this is 1, text will not wrap. Otherwise, text will be wrapped at the
  /// edge of the box.
  ///
  /// If this is null, but there is an ambient [DefaultTextStyle] that specifies
  /// an explicit number for its [DefaultTextStyle.maxLines], then the
  /// [DefaultTextStyle] value will take precedence. You can use a [RichText]
  /// widget directly to entirely override the [DefaultTextStyle].
  final int maxLines;

  /// An alternative semantics label for this text.
  ///
  /// If present, the semantics of this widget will contain this value instead
  /// of the actual text. This will overwrite any of the semantics labels applied
  /// directly to the [TextSpan]s.
  ///
  /// This is useful for replacing abbreviations or shorthands with the full
  /// text value:
  ///
  /// ```dart
  /// Text(r'$$', semanticsLabel: 'Double dollars')
  /// ```
  final String semanticsLabel;

  /// {@macro flutter.dart:ui.text.TextWidthBasis}
  final TextWidthBasis textWidthBasis;

  @override
  Widget build(BuildContext context) {
    final DefaultTextStyle defaultTextStyle = DefaultTextStyle.of(context);
    TextStyle effectiveTextStyle = style;
    if (style == null || style.inherit)
      effectiveTextStyle = defaultTextStyle.style.merge(style);
    if (MediaQuery.boldTextOverride(context))
      effectiveTextStyle = effectiveTextStyle.merge(const TextStyle(fontWeight: FontWeight.bold));
    Widget result = RichText(
      textAlign: textAlign ?? defaultTextStyle.textAlign ?? TextAlign.start,
      textDirection: textDirection, // RichText uses Directionality.of to obtain a default if this is null.
      locale: locale, // RichText uses Localizations.localeOf to obtain a default if this is null
      softWrap: softWrap ?? defaultTextStyle.softWrap,
      overflow: overflow ?? defaultTextStyle.overflow,
      textScaleFactor: textScaleFactor ?? MediaQuery.textScaleFactorOf(context),
      maxLines: maxLines ?? defaultTextStyle.maxLines,
      strutStyle: strutStyle,
      textWidthBasis: textWidthBasis ?? defaultTextStyle.textWidthBasis,
      text: TextSpan(
        style: effectiveTextStyle,
        text: data,
        children: textSpan != null ? <TextSpan>[textSpan] : null,
      ),
    );
    if (semanticsLabel != null) {
      result = Semantics(
        textDirection: textDirection,
        label: semanticsLabel,
        child: ExcludeSemantics(
          child: result,
        ),
      );
    }
    return result;
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(StringProperty('data', data, showName: false));
    if (textSpan != null) {
      properties.add(textSpan.toDiagnosticsNode(name: 'textSpan', style: DiagnosticsTreeStyle.transition));
    }
    style?.debugFillProperties(properties);
    properties.add(EnumProperty<TextAlign>('textAlign', textAlign, defaultValue: null));
    properties.add(EnumProperty<TextDirection>('textDirection', textDirection, defaultValue: null));
    properties.add(DiagnosticsProperty<Locale>('locale', locale, defaultValue: null));
    properties.add(FlagProperty('softWrap', value: softWrap, ifTrue: 'wrapping at box width', ifFalse: 'no wrapping except at line break characters', showName: true));
    properties.add(EnumProperty<TextOverflow>('overflow', overflow, defaultValue: null));
    properties.add(DoubleProperty('textScaleFactor', textScaleFactor, defaultValue: null));
    properties.add(IntProperty('maxLines', maxLines, defaultValue: null));
    if (semanticsLabel != null) {
      properties.add(StringProperty('semanticsLabel', semanticsLabel));
    }
  }
}