Impara Chaining with then and catchError | Chaining and Coordination

When working with asynchronous code in Dart, you often need to perform a series of operations where each step depends on the previous one. The then method allows you to chain these operations together, ensuring that each step starts only after the previous Future completes. By chaining multiple then calls, you can build a sequence of asynchronous actions that run one after another.

Sometimes, things go wrong in an asynchronous chain. To handle errors gracefully, you use the catchError method. This method catches any error that occurs in the chain up to that point, letting you respond to failures without crashing your program. You can place catchError after any set of then calls to handle errors from any earlier step in the chain.

Chaining with then and handling errors with catchError is especially useful when you want to avoid using async and await, or when you want to keep your asynchronous flow more explicit and readable.

main.dart


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Future<String> fetchUser() {
  return Future.delayed(Duration(seconds: 1), () => "Alice");
}

Future<String> fetchUserProfile(String user) {
  return Future.delayed(Duration(seconds: 1), () => "Profile of $user");
}

Future<String> fetchUserPosts(String profile) {
  // Simulate an error for demonstration
  return Future.delayed(Duration(seconds: 1), () => throw Exception("Failed to fetch posts"));
}

void main() {
  fetchUser()
    .then((user) {
      print("User: $user");
      return fetchUserProfile(user);
    })
    .then((profile) {
      print("Profile: $profile");
      return fetchUserPosts(profile);
    })
    .then((posts) {
      print("Posts: $posts");
    })
    .catchError((error) {
      print("An error occurred: $error");
    });
}

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Sezione 2. Capitolo 1

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Suggested prompts:

Can you show an example of chaining `then` and `catchError` in Dart?

When should I use `then` and `catchError` instead of `async`/`await`?

How does error propagation work in a chain of `then` calls?

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main.dart


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Future<String> fetchUser() {
  return Future.delayed(Duration(seconds: 1), () => "Alice");
}

Future<String> fetchUserProfile(String user) {
  return Future.delayed(Duration(seconds: 1), () => "Profile of $user");
}

Future<String> fetchUserPosts(String profile) {
  // Simulate an error for demonstration
  return Future.delayed(Duration(seconds: 1), () => throw Exception("Failed to fetch posts"));
}

void main() {
  fetchUser()
    .then((user) {
      print("User: $user");
      return fetchUserProfile(user);
    })
    .then((profile) {
      print("Profile: $profile");
      return fetchUserPosts(profile);
    })
    .then((posts) {
      print("Posts: $posts");
    })
    .catchError((error) {
      print("An error occurred: $error");
    });
}

Tutto è chiaro?

Grazie per i tuoi commenti!

Sezione 2. Capitolo 1