then
- Dart course
- Beginner
- Lesson 108/116
- 5 min
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.
Practice 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.
Starter code
void main() {
// Start with a value Future
// .then to double
// .then to print
}Sample solution
void main() {
Future.value(5).then((v) => v * 2).then((v) => print(v));
}Expected output
10
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match then
| Term | Meaning |
|---|---|
.then |
runs on completion |
(v) => v * 2 |
transforms the value |
chaining |
step by step |
callback |
the function passed |
2 The then callback
Future.value(5).____((v) => print(v));Future.value(5).then((v) => print(v));Explanation: then provides the completion callback.
3 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 Correct answer
- 3
- 4
- 7
Explanation: The first then turns 3 into 4, the second prints 4; result is 4.