Add example test, update example READMEs (#91130)
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Greg Spencer
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# Dartdoc Generation
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The Flutter API documentation contains code blocks that help provide context or
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a good starting point when learning to use any of Flutter's APIs.
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To generate these code blocks, Flutter uses dartdoc tools to turn documentation
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in the source code into API documentation, as seen on https://api.flutter.dev/.
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## Table of Contents
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- [Types of code blocks](#types-of-code-blocks)
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- [Snippet tool](#snippet-tool)
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- [Sample tool](#sample-tool)
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- [Skeletons](#skeletons)
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- [Test Doc Generation Workflow](#test-doc-generation-workflow)
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## Types of code blocks
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There are three kinds of code blocks.
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* A `snippet`, which is a more or less context-free code snippet that we
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magically determine how to analyze.
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* A `dartpad` sample, which gets placed into a full-fledged application, and can
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be executed inline in the documentation on the web page using
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DartPad.
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* A `sample`, which gets placed into a full-fledged application, but isn't
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placed into DartPad in the documentation because it doesn't make sense to do
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so.
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Ideally, every sample is a DartPad sample, but some samples don't have any visual
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representation and some just don't make sense that way (for example, sample
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code for setting the system UI's notification area color on Android won't do
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anything on the web).
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### Snippet Tool
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The code `snippet` tool generates a block containing a description and example
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code. Here is an example of the code `snippet` tool in use:
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```dart
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/// {@tool snippet}
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///
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/// If the avatar is to have an image, the image should be specified in the
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/// [backgroundImage] property:
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///
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/// ```dart
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/// CircleAvatar(
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/// backgroundImage: NetworkImage(userAvatarUrl),
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/// )
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/// ```
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/// {@end-tool}
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```
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This will generate sample code that can be copied to the clipboard and added to
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existing applications.
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This uses the skeleton for [snippet](config/skeletons/snippet.html) snippets
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when generating the HTML to put into the Dart docs.
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#### Analysis
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The `../bots/analyze_sample_code.dart` script finds code inside the `@tool
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snippet` sections and uses the Dart analyzer to check them.
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There are several kinds of sample code you can specify:
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* Constructor calls, typically showing what might exist in a build method. These
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will be inserted into an assignment expression assigning to a variable of type
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"dynamic" and followed by a semicolon, for analysis.
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* Class definitions. These start with "class", and are analyzed verbatim.
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* Other code. It gets included verbatim, though any line that says `// ...` is
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considered to separate the block into multiple blocks to be processed
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individually.
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The above means that it's tricky to include verbatim imperative code (e.g. a
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call to a method) since it won't be valid to have such code at the top level.
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Instead, wrap it in a function or even a whole class, or make it a valid
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variable declaration.
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You can declare code that should be included in the analysis but not shown in
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the API docs by adding a comment "// Examples can assume:" to the file (usually
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at the top of the file, after the imports), following by one or more
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commented-out lines of code. That code is included verbatim in the analysis. For
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example:
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```dart
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// Examples can assume:
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// final BuildContext context;
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// final String userAvatarUrl;
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```
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You can assume that the entire Flutter framework and most common
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`dart:*` packages are imported and in scope; `dart:math` as `math` and
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`dart:ui` as `ui`.
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### Sample Tool
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The code `sample` and `dartpad` tools can expand sample code into full Flutter
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applications. These sample applications can be directly copied and used to
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demonstrate the API's functionality in a sample application, or used with the
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`flutter create` command to create a local project with the sample code. The
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`dartpad` samples are embedded into the API docs web page and are live
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applications in the API documentation.
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```dart
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/// {@tool sample --template=stateless_widget_material}
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/// This example shows how to make a simple [FloatingActionButton] in a
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/// [Scaffold], with a pink [backgroundColor] and a thumbs up [Icon].
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///
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/// ```dart
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/// Widget build(BuildContext context) {
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/// return Scaffold(
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/// appBar: AppBar(
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/// title: Text('Floating Action Button Sample'),
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/// ),
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/// body: Center(
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/// child: Text('Press the button below!')
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/// ),
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/// floatingActionButton: FloatingActionButton(
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/// onPressed: () {
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/// // Add your onPressed code here!
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/// },
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/// child: Icon(Icons.thumb_up),
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/// backgroundColor: Colors.pink,
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/// ),
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/// );
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/// }
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/// ```
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/// {@end-tool}
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```
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This uses the skeleton for [application](config/skeletons/sample.html)
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snippets.
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The `sample` and `dartpad` tools also allow for quick Flutter app generation
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using the following command:
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```bash
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flutter create --sample=[directory.File.sampleNumber] [name_of_project_directory]
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```
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This command is displayed as part of the sample in the API docs.
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#### Templates
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To support showing an entire app when you click on the right tab of the
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code sample UI, we have to be able to insert the `sample` or `dartpad` block
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into the template and instantiate the right parts.
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To do this, there is a [config/templates](config/templates) directory that
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contains a list of templates. These templates represent an entire app that the
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`sample` or `dartpad` can be placed into, basically a replacement for
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`lib/main.dart` in a flutter app package.
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For more information about how to create, use, or update templates, see
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[config/templates/README.md](config/templates/README.md).
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#### Analysis
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The `../bots/analyze_sample_code.dart` script finds code inside the `@tool
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sample` sections and uses the Dart analyzer to check them after applying the
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specified template.
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## Skeletons
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A skeleton (concerning this tool) is an HTML template into which the Dart
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code blocks and descriptions are interpolated.
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There is currently one skeleton for
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[application](config/skeletons/sample.html) samples, one for
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[dartpad](config/skeletons/dartpad-sample.html), and one for
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[snippet](config/skeletons/snippet.html) code samples, but there could be more.
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Skeletons use mustache notation (e.g. `{{code}}`) to mark where components will
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be interpolated into the template. It doesn't use the mustache
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package since these are simple string substitutions, but it uses the same
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syntax.
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The code block generation tools that process the source input and emit HTML for
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output, which dartdoc places back into the documentation. Any options given to
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the `{@tool ...}` directive are passed on verbatim to the tool.
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The `snippets` tool renders these examples through a combination of markdown
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and HTML using the `{@inject-html}` dartdoc directive.
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## Test Doc Generation Workflow
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If you are making changes to an existing code block or are creating a new code
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block, follow these steps to generate a local copy of the API docs and verify
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that your code blocks are showing up correctly:
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1. Make an update to a code block or create a new code block.
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2. From the root directory, run `./dev/bots/docs.sh`. This should start
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generating a local copy of the API documentation.
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3. Once complete, check `./dev/docs/doc` to check your API documentation. The
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search bar will not work locally, so open `./dev/docs/doc/index.html` to
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navigate through the documentation, or search `./dev/docs/doc/flutter` for
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your page of interest.
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Note that generating the sample output will not allow you to run your code in
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DartPad, because DartPad pulls the code it runs from the appropriate docs server
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(master or stable).
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Copy the generated code and paste it into a regular DartPad instance to test if
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it runs in DartPad. To get the code that will be produced by your documentation
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changes, run sample analysis locally (see the next section) and paste the output
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into a DartPad at https://dartpad.dartlang.org.
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## Running sample analysis locally
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If all you want to do is analyze the sample code you have written locally, then
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generating the entire docs output takes a long time.
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Instead, you can run the analysis locally with this command from the Flutter root:
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```
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TMPDIR=/tmp bin/cache/dart-sdk/bin/dart dev/bots/analyze_sample_code.dart --temp=samples
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```
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This will analyze the samples, and leave the generated files in `/tmp/samples`
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You can find the sample you are working on in `/tmp/samples`. It is named using the
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path to the file it is in, and the line of the file that the `{@tool ...}` directive
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is on.
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For example, the file `sample.src.widgets.animated_list.52.dart` points to the sample
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in `packages/flutter/src/widgets/animated_list.dart` at line 52. You can then take the
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contents of that file, and paste it into [Dartpad](https://dartpad.dev) and see if it
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works. If the sample relies on new features that have just landed, it may not work
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until the features make it into the `dev` branch.
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