combineLatest(_:_:_:)

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

Declaration

func combineLatest<P, Q, R>(_ publisher1: P, _ publisher2: Q, _ publisher3: R) -> Publishers.CombineLatest4<Self, P, Q, R> where P : Publisher, Q : Publisher, R : Publisher, Self.Failure == P.Failure, P.Failure == Q.Failure, Q.Failure == R.Failure

Parameters

publisher1:
A second publisher to combine with the first publisher.
publisher2:
A third publisher to combine with the first publisher.
publisher3:
A fourth publisher to combine with the first publisher.

Return Value

A publisher that receives and combines elements from this publisher and three 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 the example below, combineLatest(_:_:_:) receives input from any of the publishers, 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>()
let pub4 = PassthroughSubject<Int, Never>()

cancellable = pub
    .combineLatest(pub2, pub3, pub4)
    .sink { print("Result: \($0).") }

pub.send(1)
pub.send(2)
pub2.send(2)
pub3.send(9)
pub4.send(1)

pub.send(3)
pub2.send(12)
pub.send(13)
pub3.send(19)
//
// Prints:
//  Result: (2, 2, 9, 1).
//  Result: (3, 2, 9, 1).
//  Result: (3, 12, 9, 1).
//  Result: (13, 12, 9, 1).
//  Result: (13, 12, 19, 1).

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