description

# whenComplete method Null safety

Future<T> whenComplete (

1. FutureOr<void> action(
   )

)

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;
      });
    }

## Implementation

    Future<T> whenComplete(FutureOr<void> action());