zip(_:_:)
Combines elements from another publisher and delivers a transformed output.
Declaration
func zip<P, T>(_ other: P, _ transform: @escaping (Self.Output, P.Output) -> T) -> Publishers.Map<Publishers.Zip<Self, P>, T> where P : Publisher, Self.Failure == P.FailureParameters
- other:
Another 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 two upstream publishers.
Discussion
Use zip(_:_:) to return a new publisher that combines the elements from two publishers using a transformation you specify to publish a new value to the downstream. The returned publisher waits until both 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, PassthroughSubject instances numbersPub and lettersPub emit values; zip(_:_:) receives the oldest value from each publisher, uses the Int from numbersPub and publishes a string that repeats the String from lettersPub that many times.
let numbersPub = PassthroughSubject<Int, Never>()
let lettersPub = PassthroughSubject<String, Never>()
cancellable = numbersPub
.zip(lettersPub) { anInt, aLetter in
String(repeating: aLetter, count: anInt)
}
.sink { print("\($0)") }
numbersPub.send(1) // numbersPub: 1 lettersPub: zip output: <none>
numbersPub.send(2) // numbersPub: 1,2 lettersPub: zip output: <none>
numbersPub.send(3) // numbersPub: 1,2,3 lettersPub: zip output: <none>
lettersPub.send("A") // numbersPub: 1,2,3 lettersPub: "A" zip output: "A"
lettersPub.send("B") // numbersPub: 2,3 lettersPub: "B" zip output: "BB"
// Prints:
// A
// BBIf either upstream publisher finishes successfully or fails with an error, the zipped publisher does the same.