This is Understanding Combine, written by Matt Neuburg. It is a work in progress. Corrections and suggestions are greatly appreciated (you can comment here). So are donations; please consider keeping me going by funding this work at http://www.paypal.me/mattneub. Or buy my books: the current editions are iOS 13 Programming Fundamentals with Swift and Programming iOS 13. Thank you!


Share

.share() (Publishers.Share) effectively wraps its upstream in a class object; Publishers.Share is a class, not a struct. In general, when your goal is to let your pipeline be subscribed to by different subscribers at different times simultaneously, that’s is all you need to do. Your pipeline now has “reference semantics”; what the subscribers are subscribing to are different references to one and the same pipeline.

I’ll illustrate with a Timer-based pipeline:

let t = Timer.publish(every: 1, on: .main, in: .common).autoconnect()
    .scan(0) {i,_ in i+1}
    .share()
t.sink {print("ONE", $0)}
    .store(in: &self.storage)
delay(3) {
    t.sink {print("TWO", $0)}
        .store(in: &self.storage)
}

The output is:

ONE 1
ONE 2
ONE 3
ONE 4
TWO 4
ONE 5
TWO 5
...

What that demonstrates is that the second subscriber comes along later and joins the pipeline, and receives the same values that the first pipeline is receiving.

Often your use of .share will be in conjunction with an instance property. I’ll rewrite the example to illustrate that:

let myTimer = Timer.publish(every: 1, on: .main, in: .common).autoconnect()
    .scan(0) {i,_ in i+1}
    .share()
    .eraseToAnyPublisher()
override func viewDidLoad() {
    super.viewDidLoad()
    self.myTimer.sink {print("ONE", $0)}
        .store(in: &self.storage)
    delay(3) {
        self.myTimer.sink {print("TWO", $0)}
            .store(in: &self.storage)
    }
}

What we have now is an AnyPublisher instance property myTimer that any method, even in another class if this is a public property, can subscribe to; when it does, it will receive the values that the timer pipeline is currently emitting. One imagines various objects subscribing and unsubscribing to participate as needed in the ongoing flow of numbers. The timer doesn’t start counting until the first subscriber appears, but we can “tickle” the pipeline to get it going at a moment of our own choosing. For example, suppose we want this to be a timer that starts running when our view controller’s view first appears:

let myTimer = Timer.publish(every: 1, on: .main, in: .common).autoconnect()
    .scan(0) {i,_ in i+1}
    .share()
    .eraseToAnyPublisher()
var timerStarted = false
override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)
    if !timerStarted {
        timerStarted = true
        self.myTimer.sink {_ in}
            .store(in:&self.storage)
    }
}

Our viewDidAppear uses a kind of dummy subscriber to get the pipeline going. Now when a real subscriber comes along later, it will find the stream of numbers already underway, counting off the seconds since viewDidAppear was called for the first time.

What happens if our shared publisher has multiple downstream pipelines subscribed to it and an operator fails in one of those pipelines? As you know, that causes a cancel message to percolate up the pipeline. But it stops when it reaches the .share operator. So that downstream pipeline is terminated, but the publisher itself keeps on publishing, and any other subscribed downstream pipelines keep receiving values.

On the other hand, if the last remaining subscriber fails, its cancel does percolate all the way up to the Timer publisher, and the whole pipeline terminates.


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