---
title: "init(_:bufferingPolicy:_:)"
framework: swift
role: symbol
role_heading: Initializer
path: "swift/asyncstream/init(_:bufferingpolicy:_:)"
---

# init(_:bufferingPolicy:_:)

Constructs an asynchronous stream for an element type, using the specified buffering policy and element-producing closure.

## Declaration

```swift
init(_ elementType: Element.Type = Element.self, bufferingPolicy limit: AsyncStream<Element>.Continuation.BufferingPolicy = .unbounded, _ build: (AsyncStream<Element>.Continuation) -> Void)
```

## Parameters

- `elementType`: The type of element the AsyncStream produces.
- `limit`: A Continuation.BufferingPolicy value to set the stream’s buffering behavior. By default, the stream buffers an unlimited number of elements. You can also set the policy to buffer a specified number of oldest or newest elements.
- `build`: A custom closure that yields values to the AsyncStream. This closure receives an AsyncStream.Continuation instance that it uses to provide elements to the stream and terminate the stream when finished.

## Discussion

Discussion The AsyncStream.Continuation received by the build closure is appropriate for use in concurrent contexts. It is thread safe to send and finish; all calls to the continuation are serialized. However, calling this from multiple concurrent contexts could result in out-of-order delivery. The following example shows an AsyncStream created with this initializer that produces 100 random numbers on a one-second interval, calling yield(_:) to deliver each element to the awaiting call point. When the for loop exits, the stream finishes by calling the continuation’s finish() method. let stream = AsyncStream<Int>(Int.self,                               bufferingPolicy: .bufferingNewest(5)) { continuation in     Task.detached {         for _ in 0..<100 {             await Task.sleep(1 * 1_000_000_000)             continuation.yield(Int.random(in: 1...10))         }         continuation.finish()     } }

// Call point: for await random in stream {     print(random) }

## See Also

### Creating a Continuation-Based Stream

- [AsyncStream.Continuation.BufferingPolicy](swift/asyncstream/continuation/bufferingpolicy.md)
- [AsyncStream.Continuation](swift/asyncstream/continuation.md)