Future In Dart

Future In Dart

In dart, the Future represents a value or error that is not yet available. It is used to represent a potential value, or error, that will be available at some time in the future.

How To Create Future In Dart

You can create a future in dart by using Future class. Here the function will return Future<String> after 5 seconds.

// function that returns a future
Future<String> getUserName() async {
  return Future.delayed(Duration(seconds: 2), () => 'Mark');
}

You can also create a future by using Future.value() method. Here the function will return Future<String> immediately.

// function that returns a future
Future<String> getUserName() {
  return Future.value('Mark');
}

How To Use Future In Dart

You can use future in dart by using then() method. Here the function will return Future<String> after 5 seconds.

// function that returns a future
Future<String> getUserName() async {
  return Future.delayed(Duration(seconds: 2), () => 'Mark');
}

// main function
void main() {
  print("Start");
  getUserName().then((value) => print(value));
  print("End");
}

Start
End
Mark

More About Future

Future represents the result of an asynchronous operation and can have 2 states.

State Of Future

• Uncompleted
• Completed

Uncompleted

When you call an asynchronous function, it returns to an uncompleted future. It means the future is waiting for the function asynchronous operation to finish or to throw an error.

Completed

It can be completed with value or completed with error. Future<int> produces an int value, and Future<String> produces a String value. If the future doesn’t produce any value, then the type of future is Future<void>.

Example 2: Future In Dart

In this example below, we are creating a function middleFunction() that returns a future. The function will return Future<String> after 5 seconds.

void main() {
  print("Start");
  getData();
  print("End");
}

void getData() async{
  String data = await middleFunction();
  print(data);
}

Future<String> middleFunction(){
  return Future.delayed(Duration(seconds:5), ()=> "Hello");
}

Start
End
Hello