Future chaining

Learn Future chaining with .then and .catchError.

then and catchError

With .then and .catchError you catch the result or the error of a Future. .then runs when the Future completes successfully; .catchError when it throws.

Example: Future.value(10).then((v) => print(v)).catchError((e) => print(e));. Here .then receives the value 10 and prints it. If there were an error .catchError would run.

These methods are chainable; each .then feeds the next. They are an alternative to await; sometimes more convenient.

Chaining

.then and .catchError are used chained. Each .then returns a new Future; the next .then receives its result. This way the data flow works like a pipeline.

Example: Future.value(10).then((v) => v * 2).then((v) => print(v));. The first .then turns 10 into 20; the second .then prints 20. Output: 20.

If any step in the chain errors, the next .catchError catches it. This centralizes error handling.

Exercise Print the result with then

void main() async is already written. Create Future.value(10). Add .then((v) => print('Result: $v')). Add .catchError((e) => print('Error: $e')). Wait for the whole chain with await.

Show example solution

void main() async {
  await Future.value(10)
    .then((v) => print('Result: $v'))
    .catchError((e) => print('Error: $e'));
}

Test yourself

Match the then/catchError parts

.then((v) => ...)

Success handler
.catchError((e) => ...)

Error handler
v

Result value
e

Error object
When does catchError run?
```
Future.value(10)
  .then((v) => print(v))
  .catchError((e) => print(e));
```
- When the Future completes successfully
- Never
- Always
- When the Future throws an error
Show answer

When the Future throws an error — catchError runs only when the Future throws an error.

The then call

Future.value(10)
  ____((v) => print('Result: $v'))
  .catchError((e) => print('Error: $e'));

Show answer

Future.value(10)
  then((v) => print('Result: $v'))
  .catchError((e) => print('Error: $e'));

Next lesson: Future.wait

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