---
title: "multicast(subject:)"
framework: combine
role: symbol
role_heading: Instance Method
path: "combine/publisher/multicast(subject:)"
---

# multicast(subject:)

Provides a subject to deliver elements to multiple subscribers.

## Declaration

```swift
func multicast<S>(subject: S) -> Publishers.Multicast<Self, S> where S : Subject, Self.Failure == S.Failure, Self.Output == S.Output
```

## Parameters

- `subject`: A subject to deliver elements to downstream subscribers.

## Discussion

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(_:), this method produces a publisher that shares the provided Subject among all the downstream subscribers. The following example uses a sequence publisher as a counter to publish three random numbers, generated by a map(_:) operator. It uses a multicast(subject:) operator with 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(subject: 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", 78)) // Stream 2 received: ("First", 78) // Stream 1 received: ("First", 78) // Random: receive value: (("Second", 98)) // Stream 2 received: ("Second", 98) // Stream 1 received: ("Second", 98) // Random: receive value: (("Third", 61)) // Stream 2 received: ("Third", 61) // Stream 1 received: ("Third", 61) 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

- [multicast(_:)](combine/publisher/multicast(_:).md)
- [share()](combine/publisher/share().md)