Effective Dart: Documentation

It's easy to think your code is obvious today without realizing how much you rely on context already in your head. People new to your code, and even your forgetful future self won't have that context. A concise, accurate comment only takes a few seconds to write but can save one of those people hours of time.

We all know code should be self-documenting and not all comments are helpful. But the reality is that most of us don't write as many comments as we should. It's like exercise: you technically can do too much, but it's a lot more likely that you're doing too little. Try to step it up.

The following tips apply to comments that you don't want included in the generated documentation.

// Not if anything comes before it.
if (_chunks.isNotEmpty) return false;

Capitalize the first word unless it's a case-sensitive identifier. End it with a period (or "!" or "?", I suppose). This is true for all comments: doc comments, inline stuff, even TODOs. Even if it's a sentence fragment.

void greet(String name) {
  // Assume we have a valid name.
  print('Hi, $name!');
}

void greet(String name) {
  /* Assume we have a valid name. */
  print('Hi, $name!');
}

You can use a block comment (/* ... */) to temporarily comment out a section of code, but all other comments should use //.

Doc comments are especially handy because dart doc parses them and generates beautiful doc pages from them. A doc comment is any comment that appears before a declaration and uses the special /// syntax that dart doc looks for.

Linter rule: slash_for_doc_comments

Using a doc comment instead of a regular comment enables dart doc to find it and generate documentation for it.

/// The number of characters in this chunk when unsplit.
int get length => ...

// The number of characters in this chunk when unsplit.
int get length => ...

For historical reasons, dart doc supports two syntaxes of doc comments: /// ("C# style") and /** ... */ ("JavaDoc style"). We prefer /// because it's more compact. /** and */ add two content-free lines to a multiline doc comment. The /// syntax is also easier to read in some situations, such as when a doc comment contains a bulleted list that uses * to mark list items.

If you stumble onto code that still uses the JavaDoc style, consider cleaning it up.

Linter rules: package_api_docs, public_member_api_docs

You don't have to document every single library, top-level variable, type, and member, but you should document most of them.

Unlike languages like Java where the class is the only unit of program organization, in Dart, a library is itself an entity that users work with directly, import, and think about. That makes the library directive a great place for documentation that introduces the reader to the main concepts and functionality provided within. Consider including:

• A single-sentence summary of what the library is for.
• Explanations of terminology used throughout the library.
• A couple of complete code samples that walk through using the API.
• Links to the most important or most commonly used classes and functions.
• Links to external references on the domain the library is concerned with.

To document a library, place a doc comment before the library directive and any annotations that might be attached at the start of the file.

/// A really great test library.
@TestOn('browser')
library;

Doc comments aren't just for external consumers of your library's public API. They can also be helpful for understanding private members that are called from other parts of the library.

Start your doc comment with a brief, user-centric description ending with a period. A sentence fragment is often sufficient. Provide just enough context for the reader to orient themselves and decide if they should keep reading or look elsewhere for the solution to their problem.

/// Deletes the file at [path] from the file system.
void delete(String path) {
  ...
}

/// Depending on the state of the file system and the user's permissions,
/// certain operations may or may not be possible. If there is no file at
/// [path] or it can't be accessed, this function throws either [IOError]
/// or [PermissionError], respectively. Otherwise, this deletes the file.
void delete(String path) {
  ...
}

Add a blank line after the first sentence to split it out into its own paragraph. If more than a single sentence of explanation is useful, put the rest in later paragraphs.

This helps you write a tight first sentence that summarizes the documentation. Also, tools like dart doc use the first paragraph as a short summary in places like lists of classes and members.

/// Deletes the file at [path].
///
/// Throws an [IOError] if the file could not be found. Throws a
/// [PermissionError] if the file is present but could not be deleted.
void delete(String path) {
  ...
}

/// Deletes the file at [path]. Throws an [IOError] if the file could not
/// be found. Throws a [PermissionError] if the file is present but could
/// not be deleted.
void delete(String path) {
  ...
}

The reader of a class's doc comment can clearly see the name of the class, what interfaces it implements, etc. When reading docs for a member, the signature is right there, and the enclosing class is obvious. None of that needs to be spelled out in the doc comment. Instead, focus on explaining what the reader doesn't already know.

class RadioButtonWidget extends Widget {
  /// Sets the tooltip to [lines], which should have been word wrapped using
  /// the current font.
  void tooltip(List<String> lines) {
    ...
  }
}

class RadioButtonWidget extends Widget {
  /// Sets the tooltip for this radio button widget to the list of strings in
  /// [lines].
  void tooltip(List<String> lines) {
    ...
  }
}

If you really don't have anything interesting to say that can't be inferred from the declaration itself, then omit the doc comment. It's better to say nothing than waste a reader's time telling them something they already know.

The doc comment should focus on what the code does.

/// Returns `true` if every element satisfies the [predicate].
bool all(bool predicate(T element)) => ...

/// Starts the stopwatch if not already running.
void start() {
  ...
}

The doc comment should stress what the property is. This is true even for getters which may do calculation or other work. What the caller cares about is the result of that work, not the work itself.

/// The current day of the week, where `0` is Sunday.
int weekday;

/// The number of checked buttons on the page.
int get checkedCount => ...

The doc comment should clarify the states this variable represents. This is true even for getters which may do calculation or other work. What the caller cares about is the result of that work, not the work itself.

/// Whether the modal is currently displayed to the user.
bool isVisible;

/// Whether the modal should confirm the user's intent on navigation.
bool get shouldConfirm => ...

/// Whether resizing the current browser window will also resize the modal.
bool get canResize => ...

If a property has both a getter and a setter, then create a doc comment for only one of them. dart doc treats the getter and setter like a single field, and if both the getter and the setter have doc comments, then dart doc discards the setter's doc comment.

/// The pH level of the water in the pool.
///
/// Ranges from 0-14, representing acidic to basic, with 7 being neutral.
int get phLevel => ...
set phLevel(int level) => ...

/// The depth of the water in the pool, in meters.
int get waterDepth => ...

/// Updates the water depth to a total of [meters] in height.
set waterDepth(int meters) => ...

Doc comments for classes are often the most important documentation in your program. They describe the type's invariants, establish the terminology it uses, and provide context to the other doc comments for the class's members. A little extra effort here can make all of the other members simpler to document.

/// A chunk of non-breaking output text terminated by a hard or soft newline.
///
/// ...
class Chunk { ... }

/// Returns the lesser of two numbers.
///
/// ```dart
/// min(5, 3) == 3
/// ```
num min(num a, num b) => ...

Humans are great at generalizing from examples, so even a single code sample makes an API easier to learn.

Linter rule: comment_references

If you surround things like variable, method, or type names in square brackets, then dart doc looks up the name and links to the relevant API docs. Parentheses are optional, but can make it clearer when you're referring to a method or constructor.

/// Throws a [StateError] if ...
/// similar to [anotherMethod()], but ...

To link to a member of a specific class, use the class name and member name, separated by a dot:

/// Similar to [Duration.inDays], but handles fractional days.

The dot syntax can also be used to refer to named constructors. For the unnamed constructor, use .new after the class name:

/// To create a point, call [Point.new] or use [Point.polar] to ...

Other languages use verbose tags and sections to describe what the parameters and returns of a method are.

/// Defines a flag with the given name and abbreviation.
///
/// @param name The name of the flag.
/// @param abbr The abbreviation for the flag.
/// @returns The new flag.
/// @throws ArgumentError If there is already an option with
///     the given name or abbreviation.
Flag addFlag(String name, String abbr) => ...

The convention in Dart is to integrate that into the description of the method and highlight parameters using square brackets.

/// Defines a flag.
///
/// Throws an [ArgumentError] if there is already an option named [name] or
/// there is already an option using abbreviation [abbr]. Returns the new flag.
Flag addFlag(String name, String abbr) => ...

/// A button that can be flipped on and off.
@Component(selector: 'toggle')
class ToggleComponent {}

@Component(selector: 'toggle')
/// A button that can be flipped on and off.
class ToggleComponent {}

You are allowed to use most markdown formatting in your doc comments and dart doc will process it accordingly using the markdown package.

There are tons of guides out there already to introduce you to Markdown. Its universal popularity is why we chose it. Here's just a quick example to give you a flavor of what's supported:

/// This is a paragraph of regular text.
///
/// This sentence has *two* _emphasized_ words (italics) and **two**
/// __strong__ ones (bold).
///
/// A blank line creates a separate paragraph. It has some `inline code`
/// delimited using backticks.
///
/// * Unordered lists.
/// * Look like ASCII bullet lists.
/// * You can also use `-` or `+`.
///
/// 1. Numbered lists.
/// 2. Are, well, numbered.
/// 1. But the values don't matter.
///
///     * You can nest lists too.
///     * They must be indented at least 4 spaces.
///     * (Well, 5 including the space after `///`.)
///
/// Code blocks are fenced in triple backticks:
///
/// ```dart
/// this.code
///     .will
///     .retain(its, formatting);
/// ```
///
/// The code language (for syntax highlighting) defaults to Dart. You can
/// specify it by putting the name of the language after the opening backticks:
///
/// ```html
/// <h1>HTML is magical!</h1>
/// ```
///
/// Links can be:
///
/// * https://www.just-a-bare-url.com
/// * [with the URL inline](https://google.com)
/// * [or separated out][ref link]
///
/// [ref link]: https://google.com
///
/// # A Header
///
/// ## A subheader
///
/// ### A subsubheader
///
/// #### If you need this many levels of headers, you're doing it wrong

When in doubt, format less. Formatting exists to illuminate your content, not replace it. Words are what matter.

It may be useful to use it in rare cases for things like tables, but in almost all cases, if it's too complex to express in Markdown, you're better off not expressing it.

Markdown has two ways to indicate a block of code: indenting the code four spaces on each line, or surrounding it in a pair of triple-backtick "fence" lines. The former syntax is brittle when used inside things like Markdown lists where indentation is already meaningful or when the code block itself contains indented code.

The backtick syntax avoids those indentation woes, lets you indicate the code's language, and is consistent with using backticks for inline code.

/// You can use [CodeBlockExample] like this:
///
/// ```dart
/// var example = CodeBlockExample();
/// print(example.isItGreat); // "Yes."
/// ```

/// You can use [CodeBlockExample] like this:
///
///     var example = CodeBlockExample();
///     print(example.isItGreat); // "Yes."

We think of ourselves as programmers, but most of the characters in a source file are intended primarily for humans to read. English is the language we code in to modify the brains of our coworkers. As for any programming language, it's worth putting effort into improving your proficiency.

This section lists a few guidelines for our docs. You can learn more about best practices for technical writing, in general, from articles such as Technical writing style.

Be clear and precise, but also terse.

Many people don't know what "i.e.", "e.g." and "et al." mean. That acronym that you're sure everyone in your field knows may not be as widely known as you think.

When documenting a member for a class, you often need to refer back to the object the member is being called on. Using "the" can be ambiguous.

class Box {
  /// The value this wraps.
  Object? _value;

  /// True if this box contains a value.
  bool get hasValue => _value != null;
}

Unless stated otherwise, the documentation on this site reflects Dart 3.5.3. Page last updated on 2024-05-06. View source or report an issue.