zip(_:_:_:_:)
Combines elements from three other publishers and delivers a transformed output.
Declaration
func zip<P, Q, R, T>(_ publisher1: P, _ publisher2: Q, _ publisher3: R, _ transform: @escaping (Self.Output, P.Output, Q.Output, R.Output) -> T) -> Publishers.Map<Publishers.Zip4<Self, P, Q, R>, T> where P : Publisher, Q : Publisher, R : Publisher, Self.Failure == P.Failure, P.Failure == Q.Failure, Q.Failure == R.FailureParameters
- publisher1:
A second publisher.
- publisher2:
A third publisher.
- publisher3:
A fourth publisher.
- transform:
A closure that receives the most-recent value from each publisher and returns a new value to publish.
Return Value
A publisher that uses the transform closure to emit new elements, produced by combining the most recent value from four upstream publishers.
Discussion
Use zip(_:_:_:_:) to return a new publisher that combines the elements from three other publishers using a transformation you specify to publish a new value to the downstream subscriber. The returned publisher waits until all four publishers have emitted an event, then delivers the oldest unconsumed event from each publisher together that the operator uses in the transformation.
In this example, the PassthroughSubject publishers, numbersPub, fractionsPub, lettersPub, and emojiPub emit values. The zip(_:_:_:_:) operator receives the oldest value from each publisher and uses the Int from numbersPub and publishes a string that repeats the String from lettersPub and emojiPub that many times and prints out the value in fractionsPub.
let numbersPub = PassthroughSubject<Int, Never>() // first publisher
let lettersPub = PassthroughSubject<String, Never>() // second
let emojiPub = PassthroughSubject<String, Never>() // third
let fractionsPub = PassthroughSubject<Double, Never>()// fourth
cancellable = numbersPub
.zip(lettersPub, emojiPub, fractionsPub) { anInt, aLetter, anEmoji, aFraction in
("\(String(repeating: anEmoji, count: anInt)) \(String(repeating: aLetter, count: anInt)) \(aFraction)")
}
.sink { print("\($0)") }
numbersPub.send(1) // numbersPub: 1 lettersPub: emojiPub: zip output: <none>
numbersPub.send(2) // numbersPub: 1,2 lettersPub: emojiPub: zip output: <none>
numbersPub.send(3) // numbersPub: 1,2,3 lettersPub: emojiPub: zip output: <none>
fractionsPub.send(0.1) // numbersPub: 1,2,3 lettersPub: "A" emojiPub: zip output: <none>
lettersPub.send("A") // numbersPub: 1,2,3 lettersPub: "A" emojiPub: zip output: <none>
emojiPub.send("😀") // numbersPub: 1,2,3 lettersPub: "A" emojiPub:"😀" zip output: "😀 A"
lettersPub.send("B") // numbersPub: 2,3 lettersPub: "B" emojiPub: zip output: <none>
fractionsPub.send(0.8) // numbersPub: 2,3 lettersPub: "A" emojiPub: zip output: <none>
emojiPub.send("🥰") // numbersPub: 3 lettersPub: "B" emojiPub: zip output: "🥰🥰 BB"
// Prints:
//1 😀 A 0.1
//2 🥰🥰 BB 0.8If any upstream publisher finishes successfully or fails with an error, so too does the zipped publisher.