---
title: "init(_:bufferingPolicy:_:)"
framework: swift
role: symbol
role_heading: Initializer
path: "swift/asyncthrowingstream/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: AsyncThrowingStream<Element, Failure>.Continuation.BufferingPolicy = .unbounded, _ build: (AsyncThrowingStream<Element, Failure>.Continuation) -> Void) where Failure == any Error
```

## Parameters

- `elementType`: The type of element the AsyncThrowingStream produces.
- `limit`: The maximum number of elements to hold in the buffer. By default, this value is unlimited. Use a Continuation.BufferingPolicy to buffer a specified number of oldest or newest elements.
- `build`: A custom closure that yields values to the AsyncThrowingStream. This closure receives an AsyncThrowingStream.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. If the random number is divisible by 5 with no remainder, the stream throws a MyRandomNumberError. let stream = AsyncThrowingStream<Int, Error>(Int.self,                                              bufferingPolicy: .bufferingNewest(5)) { continuation in     Task.detached {         for _ in 0..<100 {             await Task.sleep(1 * 1_000_000_000)             let random = Int.random(in: 1...10)             if random % 5 == 0 {                 continuation.finish(throwing: MyRandomNumberError())                 return             } else {                 continuation.yield(random)             }         }         continuation.finish()     } }

// Call point: do {     for try await random in stream {         print(random)     } } catch {     print(error) }

## See Also

### Creating a Continuation-Based Stream

- [AsyncThrowingStream.Continuation.BufferingPolicy](swift/asyncthrowingstream/continuation/bufferingpolicy.md)
- [AsyncThrowingStream.Continuation](swift/asyncthrowingstream/continuation.md)