Waiting for a certain condition

The waitCondition method lets you create futures that complete when the store state meets a certain condition.

For example:

class SaveAppointmentAction extends ReduxAction<AppState> {  
  final Appointment appointment;  
  SaveAppointmentAction(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 that end, the action dispatches an action to create the calendar if necessary, and then use the store.waitCondition() method to wait until a calendar is present in the state. Only then it will 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,
  });

Check the API documentation for details on completeImmediately and timeoutMillis.

In tests

Suppose you have a test that dispatches some actions, and you want to test that it results in some specific state. You can use waitCondition to wait for the state to be as expected.

If the state is not as expected, the waitCondition method will time out, throwing an exception, and failing the test.

Also note, the method returns the action responsible for the state change that completed the condition. This can be useful in case your test needs to verify that the correct action was dispatched, or check the action.status.