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ViewsHow to constrain concurrency (like `maxConcurrentOperationCount`) with Swift Concurrency?
I am trying to perform a series of network requests and would like to limit the number of concurrent tasks in the new Swift Concurrency system. With operation queues we would use maxConcurrentOperationCount. In Combine, flatMap(maxPublishers:_:). What is the equivalent in the new Swift Concurrency system?
E.g., it is not terribly relevant, but consider:
func download() async throws {
try await withThrowingTaskGroup(of: Void.self) { group in
for i in 0..<20 {
let source = sourceUrl(for: i)
let destination = destinationUrl(for: i)
group.addTask {
let (url, _) = try await self.session.download(from: source)
try? FileManager.default.removeItem(at: destination)
try FileManager.default.moveItem(at: url, to: destination)
}
}
try await group.waitForAll()
}
}
That results in all the requests running concurrently:
The fact that URLSession is not honoring httpMaximumConnectionsPerHost is interesting, but not the salient issue here. I am looking for, more generally, how to constrain the degree of concurrency in a series of asynchronous tasks running in parallel.
1 answers
Answer question0
One can insert a group.next() call inside the loop after reaching a certain count, e.g.:
func download() async throws {
try await withThrowingTaskGroup(of: Void.self) { group in
for i in 0..<20 {
let source = sourceUrl(for: i)
let destination = destinationUrl(for: i)
if i >= 6 { // max of six at a time
try await group.next()
}
group.addTask {
let (url, _) = try await self.session.download(from: source)
try? FileManager.default.removeItem(at: destination)
try FileManager.default.moveItem(at: url, to: destination)
}
}
try await group.waitForAll()
}
}
That results in no more than six at a time:
