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Doug
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Installed for Copilot with `gh skill install` and manually created a symlink for Claude to access all Copilot skills.
102 lines
3.0 KiB
Markdown
102 lines
3.0 KiB
Markdown
# Structured concurrency
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## `async let` vs task groups
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Use `async let` when you have a fixed number of independent operations that return different types, e.g. fetching the news, the weather, and an app update at the same time. Use task groups when you have a dynamic number of operations of the same type, e.g. downloading all images in an array of URLs.
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## Task groups over loops
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It’s generally a bad idea to use unstructured tasks in a loop; prefer task groups.
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```swift
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// WRONG: No cancellation propagation, no way to await all results, leaked tasks on failure.
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for url in urls {
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Task { try await fetch(url) }
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}
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// RIGHT: Structured, cancellable, collects results.
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let results = try await withThrowingTaskGroup { group in
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for url in urls {
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group.addTask { try await fetch(url) }
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}
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var collected = [Data]()
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for try await result in group {
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collected.append(result)
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}
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return collected
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}
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```
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## `withDiscardingTaskGroup` (Swift 5.9+)
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When child tasks don't return meaningful results (fire-and-forget), use `withDiscardingTaskGroup` instead of `withTaskGroup`. It avoids accumulating unused results in memory.
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```swift
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// Preferred for side-effect-only child tasks
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await withDiscardingTaskGroup { group in
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for connection in connections {
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group.addTask { await connection.sendHeartbeat() }
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}
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}
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```
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## Limiting concurrency
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Task groups launch all child tasks eagerly, which may be undesirable. Consider limiting concurrency manually when it is appropriate:
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```swift
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try await withThrowingTaskGroup { group in
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let maxConcurrent = 4
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var iterator = urls.makeIterator()
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// Start initial batch
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for _ in 0..<maxConcurrent {
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guard let url = iterator.next() else { break }
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group.addTask { try await fetch(url) }
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}
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// As each finishes, start the next
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for try await result in group {
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process(result)
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if let url = iterator.next() {
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group.addTask { try await fetch(url) }
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}
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}
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}
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```
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## Error handling with partial results
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When one child task throws, the group cancels all remaining children. If you need partial results, catch errors inside each child task:
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```swift
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await withTaskGroup(of: (URL, Result<Data, Error>).self) { group in
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for url in urls {
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group.addTask {
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do {
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return (url, .success(try await fetch(url)))
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} catch {
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return (url, .failure(error))
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}
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}
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}
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for await (url, result) in group {
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switch result {
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case .success(let data): handle(data)
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case .failure(let error): log(error, for: url)
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}
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}
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}
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```
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## Inferring the type of task groups
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Swift is usually able to infer the type of task groups, but not always. Simple types like `String`, `URL`, `Data`, etc, usually work fine, but the example above uses `withTaskGroup(of: (URL, Result<Data, Error>).self)` and that is an example of the specific type being required – Swift would not be able to infer that.
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