Waiting for a certain condition

The waitCondition method lets you create futures that complete when the application state meets a specific condition. It's available both on the Store and on the ReduxAction classes.

For example:

class SaveAppointment extends ReduxAction<AppState> {  
  final Appointment appointment;  
  SaveAppointment(this.appointment);    

  Future<AppState> reduce() {    
    dispatch(CreateCalendarIfNecessaryAction());    
    await waitCondition((state) => state.calendar != null);
    return state.copy(calendar: state.calendar.copyAdding(appointment));
  }
}

The above action needs to add an appointment to a calendar, but it can only do that if the calendar already exists. Maybe the calendar already exists, but if not, creating a calendar is a complex async process, which may take some time to complete.

To handle this, the action dispatches another action to create the calendar if needed, and then use await waitCondition() to wait until a calendar becomes present in the state. Only then it will continue, and add the appointment to the calendar.

Details

This is the signature of the waitCondition method:

Future<ReduxAction<St>?> waitCondition(
    bool Function(St) condition, {
    bool completeImmediately = true,
    int? timeoutMillis,
  });

See the API documentation for details on completeImmediately and timeoutMillis.

In tests

Suppose you have a test that dispatches some actions, and you want to check that it results in a specific state. You can use store.waitCondition() to wait until the state matches what you expect.

If the state never matches, waitCondition will eventually time out, throw an exception, and fail the test.

Note: waitCondition returns the action that caused the condition to complete. This is helpful in tests when you need to verify that the right action was dispatched or check the action's status.