dart-async-programming（飞镖异步编程）

Writing Asynchronous Dart Code

Contents

• Core Guidelines
• Handling Futures
• Managing Streams
• Workflows
• Examples
• Related Skills

Core Guidelines

Write asynchronous Dart code using modern, declarative patterns. Avoid legacy callback-based approaches. Assume all network, file I/O, and database operations are asynchronous.

• Use async/await: Always prefer async and await over raw .then(), .catchError(), or .whenComplete() chains. This flattens the execution flow and improves readability.
• Handle Errors Gracefully: Wrap all await calls in try-catch blocks to handle exceptions. Do not rely on unhandled future rejections.
• Execute Concurrently: Use Future.wait to initiate and await multiple independent futures concurrently rather than awaiting them sequentially.
• Consume Streams Sequentially: Prefer await for over .forEach() or .listen() when consuming streams, unless you specifically need low-level subscription management (like pausing or resuming).
• Prevent Memory Leaks: Always call .close() on a StreamController when it is no longer needed or when the owning class is disposed.

Handling Futures

Sequential vs. Concurrent Execution

If operations depend on each other, await them sequentially. If operations are independent, initiate them concurrently to optimize execution time.

Sequential (Dependent):

final user = await fetchUser(id);
final profile = await fetchProfile(user.profileId);

Concurrent (Independent):

final results = await Future.wait([
  fetchUserData(id),
  fetchUserPreferences(id),
  fetchUserPermissions(id),
]);

Error Handling

Always use standard try-catch-finally blocks within async functions.

try {
  final data = await fetchData();
  process(data);
} on NetworkException catch (e) {
  handleNetworkError(e);
} catch (e) {
  handleGenericError(e);
} finally {
  cleanupResources();
}

Managing Streams

Consuming Streams

Use the asynchronous for-loop (await for) to process stream events sequentially. This automatically handles stream completion and integrates cleanly with try-catch for error handling.

try {
  await for (final chunk in fileStream) {
    processChunk(chunk);
  }
} catch (e) {
  logError('Stream processing failed: $e');
}

Creating and Managing Streams

When creating custom streams using StreamController, ensure you manage the lifecycle properly to avoid memory leaks.

class DataProvider {
  final _controller = StreamController<Data>.broadcast();

  Stream<Data> get dataStream => _controller.stream;

  void updateData(Data newData) {
    if (!_controller.isClosed) {
      _controller.add(newData);
    }
  }

  void dispose() {
    _controller.close(); // Mandatory cleanup
  }
}

Workflows

Task Progress: Implementing an Async Data Fetcher

Copy this checklist to track progress when implementing asynchronous data fetching logic:

• Identify if the required operations are sequential or independent.
• Mark the function with the async keyword and return a Future<T>.
• Wrap the asynchronous operations in a try-catch block.
• If independent, group futures using Future.wait.
• If sequential, use await for each step.
• Handle specific exceptions using on ExceptionType catch (e).
• Run validator -> review errors -> fix unhandled promise rejections.

Task Progress: Implementing a Stream Consumer

Copy this checklist to track progress when consuming a stream:

• Verify the stream source (Single subscription vs. Broadcast).
• Mark the consuming function as async.
• Wrap the consumption logic in a try-catch block.
• Implement an await for loop to iterate over the stream.
• Ensure the loop breaks or returns if a specific termination condition is met.
• If managing a StreamController, implement a dispose() or close() method.

Examples

Anti-Pattern vs. Best Practice: Futures

Anti-Pattern (Do Not Use):

Future<void> loadUserData() {
  return fetchUser()
    .then((user) {
      return fetchProfile(user.id)
        .then((profile) {
          print('User: ${user.name}, Profile: ${profile.status}');
        });
    })
    .catchError((error) {
      print('Error: $error');
    });
}

Best Practice:

Future<void> loadUserData() async {
  try {
    final user = await fetchUser();
    final profile = await fetchProfile(user.id);
    print('User: ${user.name}, Profile: ${profile.status}');
  } catch (error) {
    print('Error: $error');
  }
}

Anti-Pattern vs. Best Practice: Streams

Anti-Pattern (Do Not Use):

void processEvents(Stream<Event> eventStream) {
  eventStream.listen(
    (event) => handleEvent(event),
    onError: (error) => print('Error: $error'),
  );
}

Best Practice:

Future<void> processEvents(Stream<Event> eventStream) async {
  try {
    await for (final event in eventStream) {
      handleEvent(event);
    }
  } catch (error) {
    print('Error: $error');
  }
}

Related Skills

• dart-idiomatic-usage
• dart-concurrency-isolates