Access last value from a SignalProducer when terminated - reactive-swift

Question

I have a signal producer, when it's terminated I would like to know if a value was sent, I only need the last one, seems so simple ...
let myProducer: SignalProducer<MyObject, MyError> = getMyProducer()
myProducer.on(terminated: {
// I need the last value here
// Or I need to know if value was never called
}).start()
I've tried to store the value in a local var :
let myProducer: SignalProducer<MyObject, MyError> = getMyProducer()
var myValue: MyObject?
myProducer.on(value: { value in
myValue = value
}, terminated: {
guard let value = myValue else {
// value was never called
return
}
// value was called
}).start()
But sometimes terminated is called while value has been called but myValue is still nil...

Answer 1

First, are you really sure that you want the terminated event?
Under normal conditions, an event stream ends with a completed event. Exceptions are failed when a failure has occurred and interrupted, when the observation was ended before the stream could complete normally (E.g. cancellation).
Second: Can your SignalProducer fail, and in the failure case, do you still want the last value sent before the failure?
If not, its as easy as using the take(last:) operator:
enum MyError: Error {
case testError
}
let (signal, input) = Signal<Int, MyError>.pipe()
let observer = Signal<Int, MyError>.Observer(
value: { print("value: \($0)") },
failed: { print("error: \($0)") },
completed: { print("completed") },
interrupted: { print("interrupted") }
)
signal
.take(last: 1)
.observe(observer)
input.send(value: 1) // Nothing printed
input.send(value: 2) // Nothing printed
input.send(value: 3) // Nothing printed
input.sendCompleted() // value 3 printed
I'm using a Signal here so I can manually send events to it just for demonstration, the same works for SignalProducer as well.
Note: If we send interrupted or a failed event, the last value 3 will not be sent because those to terminating events short circuit the normal flow.
If your SignalProducer can fail, and you still want to get the last value before the failure, you can use flatMapError to ignore the Error before the last operator:
signal
.flatMapError { _ in
return .empty
}
.take(last: 1)
.observe(observer)

Answer 2

my answer :
producer
.flatMapError { _ in SignalProducer<Value, NoError>.empty }
.collect()
startWithResult( { result in
case let .success(results):
done(with: results.last)
case let .failure(error):
() // should not happen as errors are flatmapped
})