Dart Use Doc Examples

Using Examples in Dartdoc

Contents

• 1. The {@example} Directive
• 2. Using Regions
• 3. Hiding Setup Code
• 4. Marker Filtering Rules
• 5. Placement and Path Resolution
• 6. Verification

When writing documentation that requires multi-line code examples, you should generally extract those examples into standalone .dart files and inject them using the {@example} directive, rather than writing them inline inside /// comments. This ensures the examples can be analyzed, linted, and executed.

1. The {@example} Directive

The {@example} directive parses an external file and resolves it into a fenced Markdown code block in the generated documentation.

Syntax: {@example <path>[#<region>] [lang=LANGUAGE] [indent=keep|strip]}

• <path>: The path to the file. A leading / evaluates from the package root. Otherwise, it is relative to the current file.
• lang: The language for the markdown fence. Auto-detected from the file extension (e.g., dart), but can be explicit (e.g., lang=text).
• indent: strip (default) aggressively removes shared leading indentation from the code block.

Bad (Inline Markdown):

/// Makes a client service request to the backend.
///
/// ```dart
/// final client = Client();
/// client.send();
/// ```

Good (External File Injection):

/// Makes a client service request to the backend.
///
/// {@example /example/client_request.dart}

2. Using Regions

Often, an external example file contains imports, setup, or void main() wrappers that you don't want to show in the documentation. You can extract a specific block of code by appending #<region> to the {@example} directive path, and wrapping that code with #region and #endregion comments in the target file.

Dart Code (e.g., /example/client.dart):

import 'package:http/http.dart';

void main() {
  // #region request_snippet
  final client = Client();
  client.send();
  // #endregion request_snippet
}

Dartdoc Usage:

/// Connects the client to the server and sends a request.
///
/// {@example /example/client.dart#request_snippet}

3. Hiding Setup Code

If there is a specific line of code within your extracted region that is necessary for the compiler/analyzer to pass but irrelevant (or distracting) for the documentation reader, append #hide to that line.

Dart Code:

final mockServer = startServer(); // #hide
final data = await fetch(mockServer.url);

In the generated documentation, only final data = await fetch(mockServer.url); will be visible. The line with #hide is completely dropped.

4. Marker Filtering Rules

When working with #hide, #region, and #endregion markers, you must follow these two technical constraints:

• Region Required: The markers are only processed and stripped when you target a specific region suffix (e.g., {@example file.dart#region_name}). If you inject an entire file without a region suffix, the file is embedded exactly as it appears in the source, including any marker text like // #hide.
• Format Agnosticism: The marker system is completely format-agnostic. Dartdoc simply runs a regex to strip lines containing the marker strings, meaning it works identically in non-Dart files (e.g., inside YAML comments # #region or HTML comments <!-- #region -->).

5. Placement and Path Resolution

The {@example} directive is a block-level directive. It must appear on its own line prefixed with ///. Its internal <path> parser follows strict URI reference rules:

• Package-Root Paths (/): Paths starting with a leading slash automatically resolve directly to the root of the Dart package. Use this when the destination file is deep.
  • Example: {@example /test/data/sample.txt} exactly maps to <package_root>/test/data/sample.txt.
• Relative Paths: Paths without a leading slash resolve relative to the directory of the file containing the doc comment.
  • Example: {@example ../utils/demo.dart}
• Boundary Enforcement: Using .. segments to traverse upward is perfectly acceptable, but dartdoc natively stops directory traversal at the package root (it will never escape the package).
• No Network URLs: Absolute URIs (e.g., starting with https://) are strictly not supported. The example file must sit natively somewhere in the local filesystem.
• Separators & Encoding: Because dartdoc resolves the path as a URI, you must always use forward slashes (/) as folder separators (even on Windows). You can natively include URI-encoded characters (like %20 for spaces) as permitted by URI reference rules.

6. Verification

After injecting examples:

1. Run dart analyze on the example files to ensure the hidden setup code compiles.
2. (Optional) Run dart doc to verify that dartdoc successfully parsed the directive without throwing a "Failed to read file" or "missing region" warning.