init(_:bufferingPolicy:_:)
Constructs an asynchronous stream for an element type, using the specified buffering policy and element-producing closure.
Declaration
init(_ elementType: Element.Type = Element.self, bufferingPolicy limit: AsyncThrowingStream<Element, Failure>.Continuation.BufferingPolicy = .unbounded, _ build: (AsyncThrowingStream<Element, Failure>.Continuation) -> Void) where Failure == any ErrorParameters
- elementType:
The type of element the
AsyncThrowingStreamproduces. - limit:
The maximum number of elements to hold in the buffer. By default, this value is unlimited. Use a
Continuation.BufferingPolicyto buffer a specified number of oldest or newest elements. - build:
A custom closure that yields values to the
AsyncThrowingStream. This closure receives anAsyncThrowingStream.Continuationinstance that it uses to provide elements to the stream and terminate the stream when finished.
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 are 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)
}