packages/flutter_markdown/lib/src/widget.dart - mirrors/packages - Git at Google

 // Copyright 2013 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.

 import 'dart:convert';

 import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:markdown/markdown.dart' as md;

 import '../flutter_markdown.dart';
 import '_functions_io.dart' if (dart.library.html) '_functions_web.dart';

 /// Signature for callbacks used by [MarkdownWidget] when the user taps a link.
 /// The callback will return the link text, destination, and title from the
 /// Markdown link tag in the document.
 ///
 /// Used by [MarkdownWidget.onTapLink].
 typedef MarkdownTapLinkCallback = void Function(
     String text, String? href, String title);

 /// Signature for custom image widget.
 ///
 /// Used by [MarkdownWidget.imageBuilder]
 typedef MarkdownImageBuilder = Widget Function(
     Uri uri, String? title, String? alt);

 /// Signature for custom checkbox widget.
 ///
 /// Used by [MarkdownWidget.checkboxBuilder]
 typedef MarkdownCheckboxBuilder = Widget Function(bool value);

 /// Signature for custom bullet widget.
 ///
 /// Used by [MarkdownWidget.bulletBuilder]
 typedef MarkdownBulletBuilder = Widget Function(int index, BulletStyle style);

 /// Enumeration sent to the user when calling [MarkdownBulletBuilder]
 ///
 /// Use this to differentiate the bullet styling when building your own.
 enum BulletStyle {
   /// An ordered list.
   orderedList,

   /// An unordered list.
   unorderedList,
 }

 /// Creates a format [TextSpan] given a string.
 ///
 /// Used by [MarkdownWidget] to highlight the contents of `pre` elements.
 abstract class SyntaxHighlighter {
   // ignore: one_member_abstracts
   /// Returns the formatted [TextSpan] for the given string.
   TextSpan format(String source);
 }

 /// An interface for an element builder.
 abstract class MarkdownElementBuilder {
   /// Called when an Element has been reached, before its children have been
   /// visited.
   void visitElementBefore(md.Element element) {}

   /// Called when a text node has been reached.
   ///
   /// If [MarkdownWidget.styleSheet] has a style of this tag, will passing
   /// to [preferredStyle].
   ///
   /// If you needn't build a widget, return null.
   Widget? visitText(md.Text text, TextStyle? preferredStyle) => null;

   /// Called when an Element has been reached, after its children have been
   /// visited.
   ///
   /// If [MarkdownWidget.styleSheet] has a style of this tag, will passing
   /// to [preferredStyle].
   ///
   /// If you needn't build a widget, return null.
   Widget? visitElementAfter(md.Element element, TextStyle? preferredStyle) =>
       null;
 }

 /// Enum to specify which theme being used when creating [MarkdownStyleSheet]
 ///
 /// [material] - create MarkdownStyleSheet based on MaterialTheme
 /// [cupertino] - create MarkdownStyleSheet based on CupertinoTheme
 /// [platform] - create MarkdownStyleSheet based on the Platform where the
 /// is running on. Material on Android and Cupertino on iOS
 enum MarkdownStyleSheetBaseTheme {
   /// Creates a MarkdownStyleSheet based on MaterialTheme.
   material,

   /// Creates a MarkdownStyleSheet based on CupertinoTheme.
   cupertino,

   /// Creates a MarkdownStyleSheet whose theme is based on the current platform.
   platform,
 }

 /// Enumeration of alignment strategies for the cross axis of list items.
 enum MarkdownListItemCrossAxisAlignment {
   /// Uses [CrossAxisAlignment.baseline] for the row the bullet and the list
   /// item are placed in.
   ///
   /// This alignment will ensure that the bullet always lines up with
   /// the list text on the baseline.
   ///
   /// However, note that this alignment does not support intrinsic height
   /// measurements because [RenderFlex] does not support it for
   /// [CrossAxisAlignment.baseline].
   /// See https://github.com/flutter/flutter_markdown/issues/311 for cases,
   /// where this might be a problem for you.
   ///
   /// See also:
   /// * [start], which allows for intrinsic height measurements.
   baseline,

   /// Uses [CrossAxisAlignment.start] for the row the bullet and the list item
   /// are placed in.
   ///
   /// This alignment will ensure that intrinsic height measurements work.
   ///
   /// However, note that this alignment might not line up the bullet with the
   /// list text in the way you would expect in certain scenarios.
   /// See https://github.com/flutter/flutter_markdown/issues/169 for example
   /// cases that do not produce expected results.
   ///
   /// See also:
   /// * [baseline], which will position the bullet and list item on the
   ///   baseline.
   start,
 }

 /// A base class for widgets that parse and display Markdown.
 ///
 /// Supports all standard Markdown from the original
 /// [Markdown specification](https://github.github.com/gfm/).
 ///
 /// See also:
 ///
 ///  * [Markdown], which is a scrolling container of Markdown.
 ///  * [MarkdownBody], which is a non-scrolling container of Markdown.
 ///  * <https://github.github.com/gfm/>
 abstract class MarkdownWidget extends StatefulWidget {
   /// Creates a widget that parses and displays Markdown.
   ///
   /// The [data] argument must not be null.
   const MarkdownWidget({
     Key? key,
     required this.data,
     this.selectable = false,
     this.styleSheet,
     this.styleSheetTheme = MarkdownStyleSheetBaseTheme.material,
     this.syntaxHighlighter,
     this.onTapLink,
     this.onTapText,
     this.imageDirectory,
     this.blockSyntaxes,
     this.inlineSyntaxes,
     this.extensionSet,
     this.imageBuilder,
     this.checkboxBuilder,
     this.bulletBuilder,
     this.builders = const <String, MarkdownElementBuilder>{},
     this.paddingBuilders = const <String, MarkdownPaddingBuilder>{},
     this.fitContent = false,
     this.listItemCrossAxisAlignment =
         MarkdownListItemCrossAxisAlignment.baseline,
     this.softLineBreak = false,
   }) : super(key: key);

   /// The Markdown to display.
   final String data;

   /// If true, the text is selectable.
   ///
   /// Defaults to false.
   final bool selectable;

   /// The styles to use when displaying the Markdown.
   ///
   /// If null, the styles are inferred from the current [Theme].
   final MarkdownStyleSheet? styleSheet;

   /// Setting to specify base theme for MarkdownStyleSheet
   ///
   /// Default to [MarkdownStyleSheetBaseTheme.material]
   final MarkdownStyleSheetBaseTheme? styleSheetTheme;

   /// The syntax highlighter used to color text in `pre` elements.
   ///
   /// If null, the [MarkdownStyleSheet.code] style is used for `pre` elements.
   final SyntaxHighlighter? syntaxHighlighter;

   /// Called when the user taps a link.
   final MarkdownTapLinkCallback? onTapLink;

   /// Default tap handler used when [selectable] is set to true
   final VoidCallback? onTapText;

   /// The base directory holding images referenced by Img tags with local or network file paths.
   final String? imageDirectory;

   /// Collection of custom block syntax types to be used parsing the Markdown data.
   final List<md.BlockSyntax>? blockSyntaxes;

   /// Collection of custom inline syntax types to be used parsing the Markdown data.
   final List<md.InlineSyntax>? inlineSyntaxes;

   /// Markdown syntax extension set
   ///
   /// Defaults to [md.ExtensionSet.gitHubFlavored]
   final md.ExtensionSet? extensionSet;

   /// Call when build an image widget.
   final MarkdownImageBuilder? imageBuilder;

   /// Call when build a checkbox widget.
   final MarkdownCheckboxBuilder? checkboxBuilder;

   /// Called when building a bullet
   final MarkdownBulletBuilder? bulletBuilder;

   /// Render certain tags, usually used with [extensionSet]
   ///
   /// For example, we will add support for `sub` tag:
   ///
   /// ```dart
   /// builders: {
   ///   'sub': SubscriptBuilder(),
   /// }
   /// ```
   ///
   /// The `SubscriptBuilder` is a subclass of [MarkdownElementBuilder].
   final Map<String, MarkdownElementBuilder> builders;

   /// Add padding for different tags (use only for block elements and img)
   ///
   /// For example, we will add padding for `img` tag:
   ///
   /// ```dart
   /// paddingBuilders: {
   ///   'img': ImgPaddingBuilder(),
   /// }
   /// ```
   ///
   /// The `ImgPaddingBuilder` is a subclass of [MarkdownPaddingBuilder].
   final Map<String, MarkdownPaddingBuilder> paddingBuilders;

   /// Whether to allow the widget to fit the child content.
   final bool fitContent;

   /// Controls the cross axis alignment for the bullet and list item content
   /// in lists.
   ///
   /// Defaults to [MarkdownListItemCrossAxisAlignment.baseline], which
   /// does not allow for intrinsic height measurements.
   final MarkdownListItemCrossAxisAlignment listItemCrossAxisAlignment;

   /// The soft line break is used to identify the spaces at the end of aline of
   /// text and the leading spaces in the immediately following the line of text.
   ///
   /// Default these spaces are removed in accordance with the Markdown
   /// specification on soft line breaks when lines of text are joined.
   final bool softLineBreak;

   /// Subclasses should override this function to display the given children,
   /// which are the parsed representation of [data].
   @protected
   Widget build(BuildContext context, List<Widget>? children);

   @override
   State<MarkdownWidget> createState() => _MarkdownWidgetState();
 }

 class _MarkdownWidgetState extends State<MarkdownWidget>
     implements MarkdownBuilderDelegate {
   List<Widget>? _children;
   final List<GestureRecognizer> _recognizers = <GestureRecognizer>[];

   @override
   void didChangeDependencies() {
     _parseMarkdown();
     super.didChangeDependencies();
   }

   @override
   void didUpdateWidget(MarkdownWidget oldWidget) {
     super.didUpdateWidget(oldWidget);
     if (widget.data != oldWidget.data ||
         widget.styleSheet != oldWidget.styleSheet) {
       _parseMarkdown();
     }
   }

   @override
   void dispose() {
     _disposeRecognizers();
     super.dispose();
   }

   void _parseMarkdown() {
     final MarkdownStyleSheet fallbackStyleSheet =
         kFallbackStyle(context, widget.styleSheetTheme);
     final MarkdownStyleSheet styleSheet =
         fallbackStyleSheet.merge(widget.styleSheet);

     _disposeRecognizers();

     final md.Document document = md.Document(
       blockSyntaxes: widget.blockSyntaxes,
       inlineSyntaxes: (widget.inlineSyntaxes ?? <md.InlineSyntax>[])
         ..add(TaskListSyntax()),
       extensionSet: widget.extensionSet ?? md.ExtensionSet.gitHubFlavored,
       encodeHtml: false,
     );

     // Parse the source Markdown data into nodes of an Abstract Syntax Tree.
     final List<String> lines = const LineSplitter().convert(widget.data);
     final List<md.Node> astNodes = document.parseLines(lines);

     // Configure a Markdown widget builder to traverse the AST nodes and
     // create a widget tree based on the elements.
     final MarkdownBuilder builder = MarkdownBuilder(
       delegate: this,
       selectable: widget.selectable,
       styleSheet: styleSheet,
       imageDirectory: widget.imageDirectory,
       imageBuilder: widget.imageBuilder,
       checkboxBuilder: widget.checkboxBuilder,
       bulletBuilder: widget.bulletBuilder,
       builders: widget.builders,
       paddingBuilders: widget.paddingBuilders,
       fitContent: widget.fitContent,
       listItemCrossAxisAlignment: widget.listItemCrossAxisAlignment,
       onTapText: widget.onTapText,
       softLineBreak: widget.softLineBreak,
     );

     _children = builder.build(astNodes);
   }

   void _disposeRecognizers() {
     if (_recognizers.isEmpty) {
       return;
     }
     final List<GestureRecognizer> localRecognizers =
         List<GestureRecognizer>.from(_recognizers);
     _recognizers.clear();
     for (final GestureRecognizer recognizer in localRecognizers) {
       recognizer.dispose();
     }
   }

   @override
   GestureRecognizer createLink(String text, String? href, String title) {
     final TapGestureRecognizer recognizer = TapGestureRecognizer()
       ..onTap = () {
         if (widget.onTapLink != null) {
           widget.onTapLink!(text, href, title);
         }
       };
     _recognizers.add(recognizer);
     return recognizer;
   }

   @override
   TextSpan formatText(MarkdownStyleSheet styleSheet, String code) {
     code = code.replaceAll(RegExp(r'\n$'), '');
     if (widget.syntaxHighlighter != null) {
       return widget.syntaxHighlighter!.format(code);
     }
     return TextSpan(style: styleSheet.code, text: code);
   }

   @override
   Widget build(BuildContext context) => widget.build(context, _children);
 }

 /// A non-scrolling widget that parses and displays Markdown.
 ///
 /// Supports all GitHub Flavored Markdown from the
 /// [specification](https://github.github.com/gfm/).
 ///
 /// See also:
 ///
 ///  * [Markdown], which is a scrolling container of Markdown.
 ///  * <https://github.github.com/gfm/>
 class MarkdownBody extends MarkdownWidget {
   /// Creates a non-scrolling widget that parses and displays Markdown.
   const MarkdownBody({
     Key? key,
     required String data,
     bool selectable = false,
     MarkdownStyleSheet? styleSheet,
     MarkdownStyleSheetBaseTheme? styleSheetTheme,
     SyntaxHighlighter? syntaxHighlighter,
     MarkdownTapLinkCallback? onTapLink,
     VoidCallback? onTapText,
     String? imageDirectory,
     List<md.BlockSyntax>? blockSyntaxes,
     List<md.InlineSyntax>? inlineSyntaxes,
     md.ExtensionSet? extensionSet,
     MarkdownImageBuilder? imageBuilder,
     MarkdownCheckboxBuilder? checkboxBuilder,
     MarkdownBulletBuilder? bulletBuilder,
     Map<String, MarkdownElementBuilder> builders =
         const <String, MarkdownElementBuilder>{},
     Map<String, MarkdownPaddingBuilder> paddingBuilders =
         const <String, MarkdownPaddingBuilder>{},
     MarkdownListItemCrossAxisAlignment listItemCrossAxisAlignment =
         MarkdownListItemCrossAxisAlignment.baseline,
     this.shrinkWrap = true,
     bool fitContent = true,
     bool softLineBreak = false,
   }) : super(
           key: key,
           data: data,
           selectable: selectable,
           styleSheet: styleSheet,
           styleSheetTheme: styleSheetTheme,
           syntaxHighlighter: syntaxHighlighter,
           onTapLink: onTapLink,
           onTapText: onTapText,
           imageDirectory: imageDirectory,
           blockSyntaxes: blockSyntaxes,
           inlineSyntaxes: inlineSyntaxes,
           extensionSet: extensionSet,
           imageBuilder: imageBuilder,
           checkboxBuilder: checkboxBuilder,
           builders: builders,
           paddingBuilders: paddingBuilders,
           listItemCrossAxisAlignment: listItemCrossAxisAlignment,
           bulletBuilder: bulletBuilder,
           fitContent: fitContent,
           softLineBreak: softLineBreak,
         );

   /// If [shrinkWrap] is `true`, [MarkdownBody] will take the minimum height
   /// that wraps its content. Otherwise, [MarkdownBody] will expand to the
   /// maximum allowed height.
   final bool shrinkWrap;

   @override
   Widget build(BuildContext context, List<Widget>? children) {
     if (children!.length == 1 && shrinkWrap) {
       return children.single;
     }
     return Column(
       mainAxisSize: shrinkWrap ? MainAxisSize.min : MainAxisSize.max,
       crossAxisAlignment:
           fitContent ? CrossAxisAlignment.start : CrossAxisAlignment.stretch,
       children: children,
     );
   }
 }

 /// A scrolling widget that parses and displays Markdown.
 ///
 /// Supports all GitHub Flavored Markdown from the
 /// [specification](https://github.github.com/gfm/).
 ///
 /// See also:
 ///
 ///  * [MarkdownBody], which is a non-scrolling container of Markdown.
 ///  * <https://github.github.com/gfm/>
 class Markdown extends MarkdownWidget {
   /// Creates a scrolling widget that parses and displays Markdown.
   const Markdown({
     Key? key,
     required String data,
     bool selectable = false,
     MarkdownStyleSheet? styleSheet,
     MarkdownStyleSheetBaseTheme? styleSheetTheme,
     SyntaxHighlighter? syntaxHighlighter,
     MarkdownTapLinkCallback? onTapLink,
     VoidCallback? onTapText,
     String? imageDirectory,
     List<md.BlockSyntax>? blockSyntaxes,
     List<md.InlineSyntax>? inlineSyntaxes,
     md.ExtensionSet? extensionSet,
     MarkdownImageBuilder? imageBuilder,
     MarkdownCheckboxBuilder? checkboxBuilder,
     MarkdownBulletBuilder? bulletBuilder,
     Map<String, MarkdownElementBuilder> builders =
         const <String, MarkdownElementBuilder>{},
     Map<String, MarkdownPaddingBuilder> paddingBuilders =
         const <String, MarkdownPaddingBuilder>{},
     MarkdownListItemCrossAxisAlignment listItemCrossAxisAlignment =
         MarkdownListItemCrossAxisAlignment.baseline,
     this.padding = const EdgeInsets.all(16.0),
     this.controller,
     this.physics,
     this.shrinkWrap = false,
     bool softLineBreak = false,
   }) : super(
           key: key,
           data: data,
           selectable: selectable,
           styleSheet: styleSheet,
           styleSheetTheme: styleSheetTheme,
           syntaxHighlighter: syntaxHighlighter,
           onTapLink: onTapLink,
           onTapText: onTapText,
           imageDirectory: imageDirectory,
           blockSyntaxes: blockSyntaxes,
           inlineSyntaxes: inlineSyntaxes,
           extensionSet: extensionSet,
           imageBuilder: imageBuilder,
           checkboxBuilder: checkboxBuilder,
           builders: builders,
           paddingBuilders: paddingBuilders,
           listItemCrossAxisAlignment: listItemCrossAxisAlignment,
           bulletBuilder: bulletBuilder,
           softLineBreak: softLineBreak,
         );

   /// The amount of space by which to inset the children.
   final EdgeInsets padding;

   /// An object that can be used to control the position to which this scroll view is scrolled.
   ///
   /// See also: [ScrollView.controller]
   final ScrollController? controller;

   /// How the scroll view should respond to user input.
   ///
   /// See also: [ScrollView.physics]
   final ScrollPhysics? physics;

   /// Whether the extent of the scroll view in the scroll direction should be
   /// determined by the contents being viewed.
   ///
   /// See also: [ScrollView.shrinkWrap]
   final bool shrinkWrap;

   @override
   Widget build(BuildContext context, List<Widget>? children) {
     return ListView(
       padding: padding,
       controller: controller,
       physics: physics,
       shrinkWrap: shrinkWrap,
       children: children!,
     );
   }
 }

 /// Parse [task list items](https://github.github.com/gfm/#task-list-items-extension-).
 class TaskListSyntax extends md.InlineSyntax {
   /// Cretaes a new instance.
   TaskListSyntax() : super(_pattern);

   // FIXME: Waiting for dart-lang/markdown#269 to land
   static const String _pattern = r'^ *\[([ xX])\] +';

   @override
   bool onMatch(md.InlineParser parser, Match match) {
     final md.Element el = md.Element.withTag('input');
     el.attributes['type'] = 'checkbox';
     el.attributes['disabled'] = 'true';
     el.attributes['checked'] = '${match[1]!.trim().isNotEmpty}';
     parser.addNode(el);
     return true;
   }
 }

 /// An interface for an padding builder for element.
 abstract class MarkdownPaddingBuilder {
   /// Called when an Element has been reached, before its children have been
   /// visited.
   void visitElementBefore(md.Element element) {}

   /// Called when a widget node has been rendering and need tag padding.
   EdgeInsets getPadding() => EdgeInsets.zero;
 }
	// Copyright 2013 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.

	import 'dart:convert';

	import 'package:flutter/gestures.dart';
	import 'package:flutter/material.dart';
	import 'package:flutter/rendering.dart';
	import 'package:markdown/markdown.dart' as md;

	import '../flutter_markdown.dart';
	import '_functions_io.dart' if (dart.library.html) '_functions_web.dart';

	/// Signature for callbacks used by [MarkdownWidget] when the user taps a link.
	/// The callback will return the link text, destination, and title from the
	/// Markdown link tag in the document.
	///
	/// Used by [MarkdownWidget.onTapLink].
	typedef MarkdownTapLinkCallback = void Function(
	String text, String? href, String title);

	/// Signature for custom image widget.
	///
	/// Used by [MarkdownWidget.imageBuilder]
	typedef MarkdownImageBuilder = Widget Function(
	Uri uri, String? title, String? alt);

	/// Signature for custom checkbox widget.
	///
	/// Used by [MarkdownWidget.checkboxBuilder]
	typedef MarkdownCheckboxBuilder = Widget Function(bool value);

	/// Signature for custom bullet widget.
	///
	/// Used by [MarkdownWidget.bulletBuilder]
	typedef MarkdownBulletBuilder = Widget Function(int index, BulletStyle style);

	/// Enumeration sent to the user when calling [MarkdownBulletBuilder]
	///
	/// Use this to differentiate the bullet styling when building your own.
	enum BulletStyle {
	/// An ordered list.
	orderedList,

	/// An unordered list.
	unorderedList,
	}

	/// Creates a format [TextSpan] given a string.
	///
	/// Used by [MarkdownWidget] to highlight the contents of `pre` elements.
	abstract class SyntaxHighlighter {
	// ignore: one_member_abstracts
	/// Returns the formatted [TextSpan] for the given string.
	TextSpan format(String source);
	}

	/// An interface for an element builder.
	abstract class MarkdownElementBuilder {
	/// Called when an Element has been reached, before its children have been
	/// visited.
	void visitElementBefore(md.Element element) {}

	/// Called when a text node has been reached.
	///
	/// If [MarkdownWidget.styleSheet] has a style of this tag, will passing
	/// to [preferredStyle].
	///
	/// If you needn't build a widget, return null.
	Widget? visitText(md.Text text, TextStyle? preferredStyle) => null;

	/// Called when an Element has been reached, after its children have been
	/// visited.
	///
	/// If [MarkdownWidget.styleSheet] has a style of this tag, will passing
	/// to [preferredStyle].
	///
	/// If you needn't build a widget, return null.
	Widget? visitElementAfter(md.Element element, TextStyle? preferredStyle) =>
	null;
	}

	/// Enum to specify which theme being used when creating [MarkdownStyleSheet]
	///
	/// [material] - create MarkdownStyleSheet based on MaterialTheme
	/// [cupertino] - create MarkdownStyleSheet based on CupertinoTheme
	/// [platform] - create MarkdownStyleSheet based on the Platform where the
	/// is running on. Material on Android and Cupertino on iOS
	enum MarkdownStyleSheetBaseTheme {
	/// Creates a MarkdownStyleSheet based on MaterialTheme.
	material,

	/// Creates a MarkdownStyleSheet based on CupertinoTheme.
	cupertino,

	/// Creates a MarkdownStyleSheet whose theme is based on the current platform.
	platform,
	}

	/// Enumeration of alignment strategies for the cross axis of list items.
	enum MarkdownListItemCrossAxisAlignment {
	/// Uses [CrossAxisAlignment.baseline] for the row the bullet and the list
	/// item are placed in.
	///
	/// This alignment will ensure that the bullet always lines up with
	/// the list text on the baseline.
	///
	/// However, note that this alignment does not support intrinsic height
	/// measurements because [RenderFlex] does not support it for
	/// [CrossAxisAlignment.baseline].
	/// See https://github.com/flutter/flutter_markdown/issues/311 for cases,
	/// where this might be a problem for you.
	///
	/// See also:
	/// * [start], which allows for intrinsic height measurements.
	baseline,

	/// Uses [CrossAxisAlignment.start] for the row the bullet and the list item
	/// are placed in.
	///
	/// This alignment will ensure that intrinsic height measurements work.
	///
	/// However, note that this alignment might not line up the bullet with the
	/// list text in the way you would expect in certain scenarios.
	/// See https://github.com/flutter/flutter_markdown/issues/169 for example
	/// cases that do not produce expected results.
	///
	/// See also:
	/// * [baseline], which will position the bullet and list item on the
	/// baseline.
	start,
	}

	/// A base class for widgets that parse and display Markdown.
	///
	/// Supports all standard Markdown from the original
	/// [Markdown specification](https://github.github.com/gfm/).
	///
	/// See also:
	///
	/// * [Markdown], which is a scrolling container of Markdown.
	/// * [MarkdownBody], which is a non-scrolling container of Markdown.
	/// * <https://github.github.com/gfm/>
	abstract class MarkdownWidget extends StatefulWidget {
	/// Creates a widget that parses and displays Markdown.
	///
	/// The [data] argument must not be null.
	const MarkdownWidget({
	Key? key,
	required this.data,
	this.selectable = false,
	this.styleSheet,
	this.styleSheetTheme = MarkdownStyleSheetBaseTheme.material,
	this.syntaxHighlighter,
	this.onTapLink,
	this.onTapText,
	this.imageDirectory,
	this.blockSyntaxes,
	this.inlineSyntaxes,
	this.extensionSet,
	this.imageBuilder,
	this.checkboxBuilder,
	this.bulletBuilder,
	this.builders = const <String, MarkdownElementBuilder>{},
	this.paddingBuilders = const <String, MarkdownPaddingBuilder>{},
	this.fitContent = false,
	this.listItemCrossAxisAlignment =
	MarkdownListItemCrossAxisAlignment.baseline,
	this.softLineBreak = false,
	}) : super(key: key);

	/// The Markdown to display.
	final String data;

	/// If true, the text is selectable.
	///
	/// Defaults to false.
	final bool selectable;

	/// The styles to use when displaying the Markdown.
	///
	/// If null, the styles are inferred from the current [Theme].
	final MarkdownStyleSheet? styleSheet;

	/// Setting to specify base theme for MarkdownStyleSheet
	///
	/// Default to [MarkdownStyleSheetBaseTheme.material]
	final MarkdownStyleSheetBaseTheme? styleSheetTheme;

	/// The syntax highlighter used to color text in `pre` elements.
	///
	/// If null, the [MarkdownStyleSheet.code] style is used for `pre` elements.
	final SyntaxHighlighter? syntaxHighlighter;

	/// Called when the user taps a link.
	final MarkdownTapLinkCallback? onTapLink;

	/// Default tap handler used when [selectable] is set to true
	final VoidCallback? onTapText;

	/// The base directory holding images referenced by Img tags with local or network file paths.
	final String? imageDirectory;

	/// Collection of custom block syntax types to be used parsing the Markdown data.
	final List<md.BlockSyntax>? blockSyntaxes;

	/// Collection of custom inline syntax types to be used parsing the Markdown data.
	final List<md.InlineSyntax>? inlineSyntaxes;

	/// Markdown syntax extension set
	///
	/// Defaults to [md.ExtensionSet.gitHubFlavored]
	final md.ExtensionSet? extensionSet;

	/// Call when build an image widget.
	final MarkdownImageBuilder? imageBuilder;

	/// Call when build a checkbox widget.
	final MarkdownCheckboxBuilder? checkboxBuilder;

	/// Called when building a bullet
	final MarkdownBulletBuilder? bulletBuilder;

	/// Render certain tags, usually used with [extensionSet]
	///
	/// For example, we will add support for `sub` tag:
	///
	/// ```dart
	/// builders: {
	/// 'sub': SubscriptBuilder(),
	/// }
	/// ```
	///
	/// The `SubscriptBuilder` is a subclass of [MarkdownElementBuilder].
	final Map<String, MarkdownElementBuilder> builders;

	/// Add padding for different tags (use only for block elements and img)
	///
	/// For example, we will add padding for `img` tag:
	///
	/// ```dart
	/// paddingBuilders: {
	/// 'img': ImgPaddingBuilder(),
	/// }
	/// ```
	///
	/// The `ImgPaddingBuilder` is a subclass of [MarkdownPaddingBuilder].
	final Map<String, MarkdownPaddingBuilder> paddingBuilders;

	/// Whether to allow the widget to fit the child content.
	final bool fitContent;

	/// Controls the cross axis alignment for the bullet and list item content
	/// in lists.
	///
	/// Defaults to [MarkdownListItemCrossAxisAlignment.baseline], which
	/// does not allow for intrinsic height measurements.
	final MarkdownListItemCrossAxisAlignment listItemCrossAxisAlignment;

	/// The soft line break is used to identify the spaces at the end of aline of
	/// text and the leading spaces in the immediately following the line of text.
	///
	/// Default these spaces are removed in accordance with the Markdown
	/// specification on soft line breaks when lines of text are joined.
	final bool softLineBreak;

	/// Subclasses should override this function to display the given children,
	/// which are the parsed representation of [data].
	@protected
	Widget build(BuildContext context, List<Widget>? children);

	@override
	State<MarkdownWidget> createState() => _MarkdownWidgetState();
	}

	class _MarkdownWidgetState extends State<MarkdownWidget>
	implements MarkdownBuilderDelegate {
	List<Widget>? _children;
	final List<GestureRecognizer> _recognizers = <GestureRecognizer>[];

	@override
	void didChangeDependencies() {
	_parseMarkdown();
	super.didChangeDependencies();
	}

	@override
	void didUpdateWidget(MarkdownWidget oldWidget) {
	super.didUpdateWidget(oldWidget);
	if (widget.data != oldWidget.data \|\|
	widget.styleSheet != oldWidget.styleSheet) {
	_parseMarkdown();
	}
	}

	@override
	void dispose() {
	_disposeRecognizers();
	super.dispose();
	}

	void _parseMarkdown() {
	final MarkdownStyleSheet fallbackStyleSheet =
	kFallbackStyle(context, widget.styleSheetTheme);
	final MarkdownStyleSheet styleSheet =
	fallbackStyleSheet.merge(widget.styleSheet);

	_disposeRecognizers();

	final md.Document document = md.Document(
	blockSyntaxes: widget.blockSyntaxes,
	inlineSyntaxes: (widget.inlineSyntaxes ?? <md.InlineSyntax>[])
	..add(TaskListSyntax()),
	extensionSet: widget.extensionSet ?? md.ExtensionSet.gitHubFlavored,
	encodeHtml: false,
	);

	// Parse the source Markdown data into nodes of an Abstract Syntax Tree.
	final List<String> lines = const LineSplitter().convert(widget.data);
	final List<md.Node> astNodes = document.parseLines(lines);

	// Configure a Markdown widget builder to traverse the AST nodes and
	// create a widget tree based on the elements.
	final MarkdownBuilder builder = MarkdownBuilder(
	delegate: this,
	selectable: widget.selectable,
	styleSheet: styleSheet,
	imageDirectory: widget.imageDirectory,
	imageBuilder: widget.imageBuilder,
	checkboxBuilder: widget.checkboxBuilder,
	bulletBuilder: widget.bulletBuilder,
	builders: widget.builders,
	paddingBuilders: widget.paddingBuilders,
	fitContent: widget.fitContent,
	listItemCrossAxisAlignment: widget.listItemCrossAxisAlignment,
	onTapText: widget.onTapText,
	softLineBreak: widget.softLineBreak,
	);

	_children = builder.build(astNodes);
	}

	void _disposeRecognizers() {
	if (_recognizers.isEmpty) {
	return;
	}
	final List<GestureRecognizer> localRecognizers =
	List<GestureRecognizer>.from(_recognizers);
	_recognizers.clear();
	for (final GestureRecognizer recognizer in localRecognizers) {
	recognizer.dispose();
	}
	}

	@override
	GestureRecognizer createLink(String text, String? href, String title) {
	final TapGestureRecognizer recognizer = TapGestureRecognizer()
	..onTap = () {
	if (widget.onTapLink != null) {
	widget.onTapLink!(text, href, title);
	}
	};
	_recognizers.add(recognizer);
	return recognizer;
	}

	@override
	TextSpan formatText(MarkdownStyleSheet styleSheet, String code) {
	code = code.replaceAll(RegExp(r'\n$'), '');
	if (widget.syntaxHighlighter != null) {
	return widget.syntaxHighlighter!.format(code);
	}
	return TextSpan(style: styleSheet.code, text: code);
	}

	@override
	Widget build(BuildContext context) => widget.build(context, _children);
	}

	/// A non-scrolling widget that parses and displays Markdown.
	///
	/// Supports all GitHub Flavored Markdown from the
	/// [specification](https://github.github.com/gfm/).
	///
	/// See also:
	///
	/// * [Markdown], which is a scrolling container of Markdown.
	/// * <https://github.github.com/gfm/>
	class MarkdownBody extends MarkdownWidget {
	/// Creates a non-scrolling widget that parses and displays Markdown.
	const MarkdownBody({
	Key? key,
	required String data,
	bool selectable = false,
	MarkdownStyleSheet? styleSheet,
	MarkdownStyleSheetBaseTheme? styleSheetTheme,
	SyntaxHighlighter? syntaxHighlighter,
	MarkdownTapLinkCallback? onTapLink,
	VoidCallback? onTapText,
	String? imageDirectory,
	List<md.BlockSyntax>? blockSyntaxes,
	List<md.InlineSyntax>? inlineSyntaxes,
	md.ExtensionSet? extensionSet,
	MarkdownImageBuilder? imageBuilder,
	MarkdownCheckboxBuilder? checkboxBuilder,
	MarkdownBulletBuilder? bulletBuilder,
	Map<String, MarkdownElementBuilder> builders =
	const <String, MarkdownElementBuilder>{},
	Map<String, MarkdownPaddingBuilder> paddingBuilders =
	const <String, MarkdownPaddingBuilder>{},
	MarkdownListItemCrossAxisAlignment listItemCrossAxisAlignment =
	MarkdownListItemCrossAxisAlignment.baseline,
	this.shrinkWrap = true,
	bool fitContent = true,
	bool softLineBreak = false,
	}) : super(
	key: key,
	data: data,
	selectable: selectable,
	styleSheet: styleSheet,
	styleSheetTheme: styleSheetTheme,
	syntaxHighlighter: syntaxHighlighter,
	onTapLink: onTapLink,
	onTapText: onTapText,
	imageDirectory: imageDirectory,
	blockSyntaxes: blockSyntaxes,
	inlineSyntaxes: inlineSyntaxes,
	extensionSet: extensionSet,
	imageBuilder: imageBuilder,
	checkboxBuilder: checkboxBuilder,
	builders: builders,
	paddingBuilders: paddingBuilders,
	listItemCrossAxisAlignment: listItemCrossAxisAlignment,
	bulletBuilder: bulletBuilder,
	fitContent: fitContent,
	softLineBreak: softLineBreak,
	);

	/// If [shrinkWrap] is `true`, [MarkdownBody] will take the minimum height
	/// that wraps its content. Otherwise, [MarkdownBody] will expand to the
	/// maximum allowed height.
	final bool shrinkWrap;

	@override
	Widget build(BuildContext context, List<Widget>? children) {
	if (children!.length == 1 && shrinkWrap) {
	return children.single;
	}
	return Column(
	mainAxisSize: shrinkWrap ? MainAxisSize.min : MainAxisSize.max,
	crossAxisAlignment:
	fitContent ? CrossAxisAlignment.start : CrossAxisAlignment.stretch,
	children: children,
	);
	}
	}

	/// A scrolling widget that parses and displays Markdown.
	///
	/// Supports all GitHub Flavored Markdown from the
	/// [specification](https://github.github.com/gfm/).
	///
	/// See also:
	///
	/// * [MarkdownBody], which is a non-scrolling container of Markdown.
	/// * <https://github.github.com/gfm/>
	class Markdown extends MarkdownWidget {
	/// Creates a scrolling widget that parses and displays Markdown.
	const Markdown({
	Key? key,
	required String data,
	bool selectable = false,
	MarkdownStyleSheet? styleSheet,
	MarkdownStyleSheetBaseTheme? styleSheetTheme,
	SyntaxHighlighter? syntaxHighlighter,
	MarkdownTapLinkCallback? onTapLink,
	VoidCallback? onTapText,
	String? imageDirectory,
	List<md.BlockSyntax>? blockSyntaxes,
	List<md.InlineSyntax>? inlineSyntaxes,
	md.ExtensionSet? extensionSet,
	MarkdownImageBuilder? imageBuilder,
	MarkdownCheckboxBuilder? checkboxBuilder,
	MarkdownBulletBuilder? bulletBuilder,
	Map<String, MarkdownElementBuilder> builders =
	const <String, MarkdownElementBuilder>{},
	Map<String, MarkdownPaddingBuilder> paddingBuilders =
	const <String, MarkdownPaddingBuilder>{},
	MarkdownListItemCrossAxisAlignment listItemCrossAxisAlignment =
	MarkdownListItemCrossAxisAlignment.baseline,
	this.padding = const EdgeInsets.all(16.0),
	this.controller,
	this.physics,
	this.shrinkWrap = false,
	bool softLineBreak = false,
	}) : super(
	key: key,
	data: data,
	selectable: selectable,
	styleSheet: styleSheet,
	styleSheetTheme: styleSheetTheme,
	syntaxHighlighter: syntaxHighlighter,
	onTapLink: onTapLink,
	onTapText: onTapText,
	imageDirectory: imageDirectory,
	blockSyntaxes: blockSyntaxes,
	inlineSyntaxes: inlineSyntaxes,
	extensionSet: extensionSet,
	imageBuilder: imageBuilder,
	checkboxBuilder: checkboxBuilder,
	builders: builders,
	paddingBuilders: paddingBuilders,
	listItemCrossAxisAlignment: listItemCrossAxisAlignment,
	bulletBuilder: bulletBuilder,
	softLineBreak: softLineBreak,
	);

	/// The amount of space by which to inset the children.
	final EdgeInsets padding;

	/// An object that can be used to control the position to which this scroll view is scrolled.
	///
	/// See also: [ScrollView.controller]
	final ScrollController? controller;

	/// How the scroll view should respond to user input.
	///
	/// See also: [ScrollView.physics]
	final ScrollPhysics? physics;

	/// Whether the extent of the scroll view in the scroll direction should be
	/// determined by the contents being viewed.
	///
	/// See also: [ScrollView.shrinkWrap]
	final bool shrinkWrap;

	@override
	Widget build(BuildContext context, List<Widget>? children) {
	return ListView(
	padding: padding,
	controller: controller,
	physics: physics,
	shrinkWrap: shrinkWrap,
	children: children!,
	);
	}
	}

	/// Parse [task list items](https://github.github.com/gfm/#task-list-items-extension-).
	class TaskListSyntax extends md.InlineSyntax {
	/// Cretaes a new instance.
	TaskListSyntax() : super(_pattern);

	// FIXME: Waiting for dart-lang/markdown#269 to land
	static const String _pattern = r'^ *\[([ xX])\] +';

	@override
	bool onMatch(md.InlineParser parser, Match match) {
	final md.Element el = md.Element.withTag('input');
	el.attributes['type'] = 'checkbox';
	el.attributes['disabled'] = 'true';
	el.attributes['checked'] = '${match[1]!.trim().isNotEmpty}';
	parser.addNode(el);
	return true;
	}
	}

	/// An interface for an padding builder for element.
	abstract class MarkdownPaddingBuilder {
	/// Called when an Element has been reached, before its children have been
	/// visited.
	void visitElementBefore(md.Element element) {}

	/// Called when a widget node has been rendering and need tag padding.
	EdgeInsets getPadding() => EdgeInsets.zero;
	}