combineLatest(_:_:)

Subscribes to an additional publisher and invokes a closure upon receiving output from either publisher.

Declaration

func combineLatest<P, T>(_ other: P, _ transform: @escaping (Self.Output, P.Output) -> T) -> Publishers.Map<Publishers.CombineLatest<Self, P>, T> where P : Publisher, Self.Failure == P.Failure

Parameters

other:
Another publisher to combine with this one.
transform:
A closure that receives the most-recent value from each publisher and returns a new value to publish.

Return Value

A publisher that receives and combines elements from this and another publisher.

Discussion

Use combineLatest<P,T>(_:) to combine the current and one additional publisher and transform them using a closure you specify to publish a new value to the downstream.

The combined publisher passes through any requests to all upstream publishers. However, it still obeys the demand-fulfilling rule of only sending the request amount downstream. If the demand isn’t .unlimited, it drops values from upstream publishers. It implements this by using a buffer size of 1 for each upstream, and holds the most-recent value in each buffer.

In the example below, combineLatest() receives the most-recent values published by the two publishers, it multiplies them together, and republishes the result:

let pub1 = PassthroughSubject<Int, Never>()
let pub2 = PassthroughSubject<Int, Never>()

cancellable = pub1
    .combineLatest(pub2) { (first, second) in
        return first * second
    }
    .sink { print("Result: \($0).") }

pub1.send(1)
pub1.send(2)
pub2.send(2)
pub1.send(9)
pub1.send(3)
pub2.send(12)
pub1.send(13)
//
// Prints:
//Result: 4.    (pub1 latest = 2, pub2 latest = 2)
//Result: 18.   (pub1 latest = 9, pub2 latest = 2)
//Result: 6.    (pub1 latest = 3, pub2 latest = 2)
//Result: 36.   (pub1 latest = 3, pub2 latest = 12)
//Result: 156.  (pub1 latest = 13, pub2 latest = 12)

All upstream publishers need to finish for this publisher to finish. If an upstream publisher never publishes a value, this publisher never finishes. If any of the combined publishers terminates with a failure, this publisher also fails.