combineLatest(_:)

Subscribes to an additional publisher and publishes a tuple upon receiving output from either publisher.

Declaration

func combineLatest<P>(_ other: P) -> Publishers.CombineLatest<Self, P> where P : Publisher, Self.Failure == P.Failure

Parameters

other:
Another publisher to combine with this one.

Return Value

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

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 pair 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.

In this example, PassthroughSubject pub1 and also pub2 emit values; as combineLatest(_:) receives input from either upstream publisher, it combines the latest value from each publisher into a tuple and publishes it.

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

cancellable = pub1
    .combineLatest(pub2)
    .sink { print("Result: \($0).") }

pub1.send(1)
pub1.send(2)
pub2.send(2)
pub1.send(3)
pub1.send(45)
pub2.send(22)

// Prints:
//    Result: (2, 2).    // pub1 latest = 2, pub2 latest = 2
//    Result: (3, 2).    // pub1 latest = 3, pub2 latest = 2
//    Result: (45, 2).   // pub1 latest = 45, pub2 latest = 2
//    Result: (45, 22).  // pub1 latest = 45, pub2 latest = 22

When all upstream publishers finish, this publisher finishes. If an upstream publisher never publishes a value, this publisher never finishes.