combineLatest(_:_:)

Subscribes to two additional publishers and publishes a tuple upon receiving output from any of the publishers.

Declaration

func combineLatest<P, Q>(_ publisher1: P, _ publisher2: Q) -> Publishers.CombineLatest3<Self, P, Q> where P : Publisher, Q : Publisher, Self.Failure == P.Failure, P.Failure == Q.Failure

Parameters

publisher1:
A second publisher to combine with the first publisher.
publisher2:
A third publisher to combine with the first publisher.

Return Value

A publisher that receives and combines elements from this publisher and two other publishers.

Discussion

Use combineLatest(_:_:) when you want the downstream subscriber to receive a tuple of the most-recent element from multiple publishers when any of them emit a value. To combine elements from multiple publishers, use zip(_:_:) instead. To receive just the most-recent element from multiple publishers rather than tuples, use merge(with:_:).

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.

All upstream publishers need to finish for this publisher to finish. If an upstream publisher never publishes a value, this publisher never finishes.

In this example, three instances of PassthroughSubject emit values; as combineLatest(_:_:) receives input from any of the upstream publishers, it combines the latest value from each publisher into a tuple and publishes it:

let pub = PassthroughSubject<Int, Never>()
let pub2 = PassthroughSubject<Int, Never>()
let pub3 = PassthroughSubject<Int, Never>()

cancellable = pub
    .combineLatest(pub2, pub3)
    .sink { print("Result: \($0).") }

pub.send(1)
pub.send(2)
pub2.send(2)
pub3.send(9)

pub.send(3)
pub2.send(12)
pub.send(13)
pub3.send(19)

// Prints:
//  Result: (2, 2, 9).
//  Result: (3, 2, 9).
//  Result: (3, 12, 9).
//  Result: (13, 12, 9).
//  Result: (13, 12, 19).

If any of the combined publishers terminates with a failure, this publisher also fails.