whenComplete method Null safety

Registers a function to be called when this future completes.

The action function is called when this future completes, whether it does so with a value or with an error.

This is the asynchronous equivalent of a "finally" block.

The future returned by this call, f, will complete the same way as this future unless an error occurs in the action call, or in a Future returned by the action call. If the call to action does not return a future, its return value is ignored.

If the call to action throws, then f is completed with the thrown error.

If the call to action returns a Future, f2, then completion of f is delayed until f2 completes. If f2 completes with an error, that will be the result of f too. The value of f2 is always ignored.

This method is equivalent to:

Future<T> whenComplete(action() {
  return this.then((v) {
    var f2 = action();
    if (f2 is Future) return f2.then((_) => v);
    return v;
  }, onError: (e) {
    var f2 = action();
    if (f2 is Future) return f2.then((_) { throw e; });
    throw e;
  });
}

Example:

void main() async {
  var value =
      await waitTask().whenComplete(() => print('do something here'));
  // Prints "do something here" after waitTask() completed.
  print(value); // Prints "done"
}

Future<String> waitTask() {
  Future.delayed(const Duration(seconds: 5));
  return Future.value('done');
}
// Outputs: 'do some work here' after waitTask is completed.