Eliasz Sawicki

iOS developer from Gdansk

ReactiveCocoa 4 - Action

Today I’ll tell you about an Action type that is available in ReactiveCocoa 4.

Action is pretty straightforward, It takes input, does some work and returns output. Moreover, it can fail and provide us with an Error type. Let’s look at creation of a simple action that takes Int as an input and returns String or NSError if it fails.

let action = Action<Int, String, NSError>({ (number) -> SignalProducer<String, NSError> in
  return SignalProducer<Void, NSError> {observer, disposable in
    observer.sendNext("Number is \(number)")
    observer.sendCompleted()
  }
})

Execution

How do we execute it? In order to do that we have to perform two steps.

Create Producer

let producer = action.apply(5)

Apply method will return a SignalProducer with Action's output as value (String) and error of ActionError type. ActionError is an enum that can be either NotEnabled or ProducerError. ProducerError is a wrapper around error type that specify in our Action.

Execute

prodcuer.startWithSignal { (signal, disposable ) in
  signal.observeNext({ (value) in
    print("\(value)")
  })
  signal.observeFailed({ (actionError) in
    print("\(actionError)")
  })
}

Actual execution happens when we start a Signal from received SignalProducer. If we don’t do it, nothing will happen.

Repeating action

Remember the observer.sendCompleted() part in our action’s SignalProducer? It is really important, in fact we need to provide any terminating Event when we want to finish executing our Action. If we do not do that, any Signal started from SignalProducer will immediately receive NotEnabled error and we won’t receive any values.

let action = Action<Int, String, NSError>({ (number) -> SignalProducer<String, NSError> in
  return SignalProducer<String, NSError> {observer, disposable in
    observer.sendNext("Number is \(number)")
    let delayTime = dispatch_time(DISPATCH_TIME_NOW, Int64(1 * Double(NSEC_PER_SEC)))
    dispatch_after(delayTime, dispatch_get_main_queue()) {
      observer.sendCompleted()
    }
  }
})

let prodcuer = action.apply(5)
prodcuer.startWithSignal { (signal, _ ) in
  signal.observeNext({ (value) in
    print("1 - \(value)")
  })

  signal.observeFailed({ (actionError) in
    print("1 - \(actionError)")
  })
}

prodcuer.startWithSignal { (signal, _ ) in
  signal.observeNext({ (value) in
    print("2 - \(value)")
  })

  signal.observeFailed({ (actionError) in
    print("2 - \(actionError)")
  })
}

Reveived output: "1 - Number is 5" and "2 - NotEnabled".
We receive this output because of the fact that when we try to execute our action second time, our first execution hasn’t completed yet.

Observing Action

Beside observing single execution of an Action, you can also observe Action itself and you’ll receive values that come from each execution. Prepare for a fun part now. There are three ways to observe Action's values. You can observe Action's values, Action's errors and Action's events.

What’s the difference? Let’s have a look.

Action’s values

let action = Action<Int, String, NSError>({ (number) -> SignalProducer<String, NSError> in
    return SignalProducer<String, NSError> {observer, disposable in
        observer.sendNext("Number is \(number)")
        let delayTime = dispatch_time(DISPATCH_TIME_NOW, Int64(1 * Double(NSEC_PER_SEC)))
        dispatch_after(delayTime, dispatch_get_main_queue()) {
            observer.sendInterrupted()
        }
    }
})

action.values.observe { (event) in
  print("Value: \(event)")
}

action.apply(5).startWithSignal { (_ , _ ) in }

Output: "Value: Next Number is 5" and "Value: COMPLETED"
Our values signal is of type Signal<Output, NoError>, so you won’t get any errors here. If any terminating Event occurs during Action's execution, you’ll receive COMPLETED event.

Action’s errors

let action = Action<Int, String, NSError>({ (number) -> SignalProducer<String, NSError> in
    return SignalProducer<String, NSError> {observer, disposable in
        observer.sendNext("Number is \(number)")
        let delayTime = dispatch_time(DISPATCH_TIME_NOW, Int64(1 * Double(NSEC_PER_SEC)))
        dispatch_after(delayTime, dispatch_get_main_queue()) {
            observer.sendInterrupted()
        }
    }
})

action.errors.observe { (event) in
  print("Error: \(event)")
}

action.apply(5).startWithSignal { (_ , _ ) in }

Output: "Error: Next Error Domain=1 Code=1 "(null)"" and "Error: COMPLETED"
Errors’ signal is of type Signal<Error, NoError> so you can focus on observing errors that occur during Action's execution. Keep in mind, that you don’t use observeFailed method to observe errors, as they come with NEXT events. After Action's execution is finished, signal will receive COMPLETED event.

Action’s events

let action = Action<Int, String, NSError>({ (number) -> SignalProducer<String, NSError> in
    return SignalProducer<String, NSError> {observer, disposable in
        observer.sendNext("Number is \(number)")
        let delayTime = dispatch_time(DISPATCH_TIME_NOW, Int64(1 * Double(NSEC_PER_SEC)))
        dispatch_after(delayTime, dispatch_get_main_queue()) {
            observer.sendFailed(NSError(domain: "1", code: 1, userInfo: nil))
        }
    }
})

action.events.observe { (event) in
  print("Event: \(event)")
}

action.apply(5).startWithSignal { (_ , _ ) in }

Output: "Event: NEXT NEXT Number is 5", "Event: NEXT FAILED Error Domain=1 Code=1 ("null")" and "Event: COMPLETED"
Third option is to observe signal of ALL events. This signal is of type Signal<Event<Output, Error>, NoError>. It means, that you will receive ALL events as next values, even terminating.
I have to say that this output is a bit confusing. What is "Event: NEXT NEXT Number is 5"? Why do we get double NEXT here? Let’s go step by step here. Let’s look at Action's SignalProducer implementation. First thing that we send is observer.sendNext("Number is \(number)") It will send a NEXT event with a String value. Our event’s observer receives Next events with Next event from our action. That’s why we get "NEXT NEXT". Next event that is sent is "observer.sendFailed(NSError(domain: "1", code: 1, userInfo: nil))". As previously, our Failed event comes to event's observer as Next event. That is why we get “NEXT FAILED”. After this terminating event, a COMPLETED event is sent to our event's observer indicating that execution has finished.

I hope that this made Actions clearer to you! See you next time.

This article is cross-posted with my my company blog