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!


SwitchToLatest

.switchToLatest (Publishers.SwitchToLatest) isn’t really a transformational operator, but this is a good place to talk about it because of its close affinity to .flatMap: they both take a publisher and start it publishing. But there are some important differences:

  • Whereas .flatMap makes a publisher, .switchToLatest expects a publisher as the value that it receives from upstream.

  • Whereas .flatMap retains the publishers that it makes, .switchToLatest throws away all but the most recent publisher that it receives.

So what .switchToLatest does is this: It waits for a publisher to come to it as a value from upstream. When it does, it retains that publisher and starts it publishing — and what the downstream sees are the values produced by that publisher. At the same time, if this operator was already retaining a publisher that it received from upstream, it throws away that publisher, replacing it with the new one. The publishers coming from upstream must all have the same generic Output and Failure types, so that the types expected downstream of the .switchToLatest operator will be consistent.

Now, you may be thinking: a stream of publishers? What on earth kind of stream is that? But keep in mind that we don’t need publishers coming all the way down the pipeline from above. We only need this operator’s immediate upstream object to produce a publisher. In fact, the most likely scenario is that the immediate upstream operator is .map; it will take in some value from upstream and create a publisher and send it downstream. Then .switchToLatest will do its thing. If you imagine .map and .switchToLatest working together, you can see that they form a pair that is very like .flatMap! The difference is in how they handle the publishers that are produced.

So this is an operator that allows us to swap one publisher for another. The new publisher takes the previous publisher’s place and interrupts it. That can be a valuable thing to do.

Here’s a case in point: a resettable timer. In one of my apps, which is a game, the user is penalized for every 10 seconds that goes by without the user’s making a valid move. So if 10 seconds goes by and the user hasn’t made a valid move, I want to be signalled so that I can subtract points from the user’s score. But if the user does make a valid move, I don’t want to be signalled; instead, I want to reset the timer so that it starts counting from zero once again. How would you do that with Combine and a Timer publisher? You’d use .switchToLatest!

Presume that I’ve got an instance variable that’s a PassthroughSubject, called userMoved. It doesn’t really matter what kind of thing it emits; it happens it’s a Bool, but that fact won’t even emerge in the example. The important thing is that when the user makes a valid move, I tell the PassthroughSubject to send. The pipeline starts with that PassthroughSubject, so every time the user moves, a signal comes down the pipeline. I use the .map operator to respond to that signal by producing a Timer publisher, and I use .switchToLatest to throw out the previous Timer publisher (if there is one) and replace it with the new one:

self.userMoved
    .map { _ -> AnyPublisher<Date,Never> in
        let t = Timer.publish(every: 10, on: .main, in: .common)
        _ = t.connect()
        return AnyPublisher(t)
    }
    .switchToLatest()

The result is just what I’m after. If the user fails to move within ten seconds, that pipeline produces a signal (it happens to be a Date, but again that’s not very important). If the user does move, the current timer is thrown away and a new one takes its place, so there’s no signal and the march towards ten seconds starts all over again.

WARNING: Due to a bug, .switchToLatest is unusable before iOS 13.4.


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