zip(_:_:_:)
Combines elements from two other publishers and delivers a transformed output.
Declaration
func zip<P, Q, T>(_ publisher1: P, _ publisher2: Q, _ transform: @escaping (Self.Output, P.Output, Q.Output) -> T) -> Publishers.Map<Publishers.Zip3<Self, P, Q>, T> where P : Publisher, Q : Publisher, Self.Failure == P.Failure, P.Failure == Q.FailureParameters
- publisher1:
A second publisher.
- publisher2:
A third 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 three upstream publishers.
Discussion
Use zip(_:_:_:) to return a new publisher that combines the elements from two other publishers using a transformation you specify to publish a new value to the downstream subscriber. The returned publisher waits until all three 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, numbersPub, lettersPub and emojiPub are each a PassthroughSubject that emit values; zip(_:_:_:) 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.
let numbersPub = PassthroughSubject<Int, Never>()
let lettersPub = PassthroughSubject<String, Never>()
let emojiPub = PassthroughSubject<String, Never>()
cancellable = numbersPub
.zip(letters, emoji) { anInt, aLetter, anEmoji in
("\(String(repeating: anEmoji, count: anInt)) \(String(repeating: aLetter, count: anInt))")
}
.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>
lettersPub.send("A") // numbersPub: 1,2,3 lettersPub: "A" emojiPub: zip output: <none>
emojiPub.send("😀") // numbersPub: 2,3 lettersPub: "A" emojiPub:"😀" zip output: "😀 A"
lettersPub.send("B") // numbersPub: 2,3 lettersPub: "B" emojiPub: zip output: <none>
emojiPub.send("🥰") // numbersPub: 3 lettersPub: emojiPub:"😀", "🥰" zip output: "🥰🥰 BB"
// Prints:
// 😀 A
// 🥰🥰 BBIf any upstream publisher finishes successfully or fails with an error, so too does the zipped publisher.