switchToLatest operator

switchToLatest can be used with publishers that emit publishers.

When the value of the outer publisher changes, the subscription to the old inner publisher is cancelled and a subcription to the new inner publisher is created.

Example: let’s say we want to fetch a random number from an API every time a user taps a button.

The following function simulates an API call by adding a 1 second delay and returning a random number between 0 and 100.

func getRandomNumberFromApi() -> AnyPublisher<Int, Never> {
    Future() { promise in
        DispatchQueue.main.asyncAfter(deadline: .now() + 1, execute: {
            promise(.success(Int.random(in: 0...100)))
        })
    }
    .print("Random Number API")
    .eraseToAnyPublisher()
}

Let’s create a PassthroughSubject to publish an event whenever a user taps on a button.

let buttonTaps = PassthroughSubject<Void, Never>()

We can now use the map operator to fetch a random number whenever a user taps on the button.

buttonTaps.map { _ in getRandomNumberFromApi() }

The return type of this publisher is Publisher<Publisher<Int, Never>, Never>.

The complete code to print the random number when a user taps the button would be:

var cancellables = Set<AnyCancellable>();

buttonTaps
    .print("On Button Tap")
    .map { _ in getRandomNumberFromApi() }
    .switchToLatest()
    .sink { print($0) }
    .store(in: &cancellables)

Now let us assume the user taps the button. But before the API call is complete, the user taps the button again.

We can simulate this by calling send() on buttonTaps in quick succession:

buttonTaps.send()
buttonTaps.send()

We expect buttonTaps to emit another event before the API call completes and returns a random number.

Here’s what the output looks like:

On Button Tap: receive subscription: (PassthroughSubject)
On Button Tap: request unlimited
On Button Tap: receive value: (()) // 1
Random Number API: receive subscription: (Future)
Random Number API: request unlimited
On Button Tap: receive value: (()) // 2
Random Number API: receive cancel // 3
Random Number API: receive subscription: (Future)
Random Number API: request unlimited
Random Number API: receive value: (76) // 4
76
Random Number API: receive finished

1. The first button tap event is received
2. The second button tap event is received (before the API call has completed)
3. The previous API call (i.e. call to getRandomNumberFromApi()) is cancelled and a new subscription is created (next line of output)
4. The API call completes and a random number is returned.

Note: even when the button was tapped twice, only one random number was fetched. The first API call was cancelled as soon as the button was tapped a second time.