The Flutter API documentation contains code blocks that help provide context or a good starting point when learning to use any of Flutter's APIs.
To generate these code blocks, Flutter uses dartdoc tools to turn documentation in the source code into API documentation, as seen on https://api.flutter.dev/.
There's two kinds of code blocks.
samples, which are more or less context-free snippets that we magically determine how to analyze, and
snippets, which get placed into a full-fledged application, and can be actually executed inline in the documentation using DartPad.
The code sample tool generates a block containing a description and example code. Here is an example of the code sample tool in use:
/// {@tool sample} /// /// If the avatar is to have an image, the image should be specified in the /// [backgroundImage] property: /// /// ```dart /// CircleAvatar( /// backgroundImage: NetworkImage(userAvatarUrl), /// ) /// ``` /// {@end-tool}
This will generate sample code that can be copied to the clipboard and added to existing applications.
This uses the skeleton for sample snippets when generating the HTML to put into the Dart docs.
The ../bots/analyze-sample-code.dart script finds code inside the @tool sample sections and uses the Dart analyzer to check them.
There are several kinds of sample code you can specify:
Constructor calls, typically showing what might exist in a build method. These will be inserted into an assignment expression assigning to a variable of type “dynamic” and followed by a semicolon, for the purposes of analysis.
Class definitions. These start with “class”, and are analyzed verbatim.
Other code. It gets included verbatim, though any line that says // ... is considered to separate the block into multiple blocks to be processed individually.
The above means that it‘s tricky to include verbatim imperative code (e.g. a call to a method), since it won’t be valid to have such code at the top level. Instead, wrap it in a function or even a whole class, or make it a valid variable declaration.
You can declare code that should be included in the analysis but not shown in the API docs by adding a comment “// Examples can assume:” to the file (usually at the top of the file, after the imports), following by one or more commented-out lines of code. That code is included verbatim in the analysis. For example:
// Examples can assume: // final BuildContext context; // final String userAvatarUrl;
You can assume that the entire Flutter framework and most common dart:* packages are imported and in scope; dart:math as math and dart:ui as ui.
The code snippet tool can expand sample code into full Flutter applications. These sample applications can be directly copied and used to demonstrate the API's functionality in a sample application:
/// {@tool snippet --template=stateless_widget_material} /// This example shows how to make a simple [FloatingActionButton] in a /// [Scaffold], with a pink [backgroundColor] and a thumbs up [Icon]. /// /// ```dart /// Widget build(BuildContext context) { /// return Scaffold( /// appBar: AppBar( /// title: Text('Floating Action Button Sample'), /// ), /// body: Center( /// child: Text('Press the button below!') /// ), /// floatingActionButton: FloatingActionButton( /// onPressed: () { /// // Add your onPressed code here! /// }, /// child: Icon(Icons.thumb_up), /// backgroundColor: Colors.pink, /// ), /// ); /// } /// ``` /// {@end-tool}
This uses the skeleton for application snippets.
Code snippets also allow for quick Flutter app generation using the following command:
flutter create --sample=[directory.File.sampleNumber] [name_of_project_directory]
In order to support showing an entire app when you click on the right tab of the code snippet UI, we have to be able to insert the snippet into the template and instantiate the right parts.
To do this, there is a config/templates directory that contains a list of templates. These templates represent an entire app that the snippet can be placed into, basically a replacement for lib/main.dart in a flutter app package.
For more information about how to create, use, or update templates, see config/templates/README.md.
The ../bots/analyze-sample-code.dart script finds code inside the @tool snippet sections and uses the Dart analyzer to check them after applying the specified template.
A skeleton (in relation to this tool) is an HTML template into which the Dart code blocks and descriptions are interpolated.
There is currently one skeleton for application snippets and one for sample snippets, but there could be more.
Skeletons use mustache notation (e.g. {{code}}) to mark where components will be interpolated into the template. It doesn't actually use the mustache package, since these are simple string substitutions, but it uses the same syntax.
The code block generation tools process the source input and emit HTML for output, which dartdoc places back into the documentation. Any options given to the {@tool ...} directive are passed on verbatim to the tool.
The snippets tool renders these examples through a combination of markdown and HTML using the {@inject-html} dartdoc directive.
If you are making changes to an existing code block or are creating a new code block, follow these steps to generate a local copy of the API docs and verify that your code blocks are showing up correctly:
./dev/bots/docs.sh. This should start generating a local copy of the API documentation../dev/docs/doc to check your API documentation. The search bar will not work locally, so open ./dev/docs/doc/index.html to navigate through the documentation, or search ./dev/docs/doc/flutter for your page of interest.