then

Learn Future chaining with .then.

then

You specify what happens when a Future completes with .then. .then takes a callback; when the Future completes with a value the callback runs and receives the value. Future.value(5).then((v) => print(v));.

Multiple .then calls can be chained; each feeds the next. Future.value(5).then((v) => v * 2).then((v) => print(v)); prints 10. This is the idiomatic way to transform a value step by step. Even though async/await is more readable, .then is still common; especially for short chains.

Chaining then

When multiple .then calls are written side by side they form a chain. The first .then receives the Future's value; its return value is passed to the next .then. This is ideal for sequential transforms.

Example: Future.value(3).then((v) => v + 1).then((v) => v * 10).then((v) => print(v));. The output is 40. Each .then returns a new Future; so the chain is a sequence of Futures. If a step throws, the rest of the chain is skipped and control goes to .catchError. We will see error handling in the next lesson.

Exercise Your turn: then chain

Start with Future.value(5). Double the value with .then((v) => v * 2), then print it with .then((v) => print(v)). You build a two-step then chain.

Show example solution

void main() {
  Future.value(5).then((v) => v * 2).then((v) => print(v));
}

Test yourself

Match then

.then

runs on completion
(v) => v * 2

transforms the value
chaining

step by step
callback

the function passed

The then callback

Future.value(5).____((v) => print(v));

Show answer

Future.value(5).then((v) => print(v));

What does Future.value(3).then((v) => v + 1).then((v) => print(v)) print?
```
Future.value(3)
  .then((v) => v + 1)
  .then((v) => print(v));
```
- 1
- 3
- 4
- 7
Show answer

1 — The first then turns 3 into 4, the second prints 4; result is 4.

Next lesson: Future.error

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