Generic Instance Method

multicast(_:)

Applies a closure to create a subject that delivers elements to subscribers.

Declaration

func multicast<S>(_ createSubject: @escaping () -> S) -> Publishers.Multicast<Self, S> where S : Subject, Self.Failure == S.Failure, Self.Output == S.Output

Parameters

createSubject

A closure to create a new Subject each time a subscriber attaches to the multicast publisher.

Discussion

Use a multicast publisher when you have multiple downstream subscribers, but you want upstream publishers to only process one receive(_:) call per event. This is useful when upstream publishers are doing expensive work you don’t want to duplicate, like performing network requests.

In contrast with multicast(subject:), this method produces a publisher that creates a separate Subject for each subscriber.

The following example uses a sequence publisher as a counter to publish three random numbers, generated by a map(_:) operator. It uses a multicast(_:) operator whose closure creates a PassthroughSubject to share the same random number to each of two subscribers. Because the multicast publisher is a ConnectablePublisher, publishing only begins after a call to connect().

let pub = ["First", "Second", "Third"].publisher
    .map( { return ($0, Int.random(in: 0...100)) } )
    .print("Random")
    .multicast { PassthroughSubject<(String, Int), Never>() }

cancellable1 = pub
   .sink { print ("Stream 1 received: \($0)")}
cancellable2 = pub
   .sink { print ("Stream 2 received: \($0)")}
pub.connect()

// Prints:
// Random: receive value: (("First", 9))
// Stream 2 received: ("First", 9)
// Stream 1 received: ("First", 9)
// Random: receive value: (("Second", 46))
// Stream 2 received: ("Second", 46)
// Stream 1 received: ("Second", 46)
// Random: receive value: (("Third", 26))
// Stream 2 received: ("Third", 26)
// Stream 1 received: ("Third", 26)

In this example, the output shows that the print(_:to:) operator receives each random value only one time, and then sends the value to both subscribers.

See Also

Working with Multiple Subscribers

func multicast<S>(subject: S) -> Publishers.Multicast<Self, S>

Provides a subject to deliver elements to multiple subscribers.

func share() -> Publishers.Share<Self>

Shares the output of an upstream publisher with multiple subscribers.