Structured Concurrency
Structured concurrency means related pieces of work are grouped together neatly, so they all finish (or all get cancelled) as a tidy unit, instead of scattering off on their own.
Concurrent Work with async let
async let starts an asynchronous operation immediately and lets it run concurrently with the rest of the function, until its value is awaited.
Example: Concurrent Work with async let
func fetchFirstName() async -> String {
return "Ada"
}
func fetchLastName() async -> String {
return "Lovelace"
}
async let first = fetchFirstName()
async let last = fetchLastName()
let fullName = await "\(first) \(last)"
print(fullName)Login to try C/C++/Java/PHP code in the editor
Grouping Tasks with withTaskGroup
withTaskGroup lets you add a dynamic number of child tasks and collect their results together as a structured, awaitable unit.
Example: Grouping Tasks with withTaskGroup
func square(_ n: Int) async -> Int {
return n * n
}
let total = await withTaskGroup(of: Int.self) { group in
for number in 1...4 {
group.addTask {
await square(number)
}
}
var sum = 0
for await result in group {
sum += result
}
return sum
}
print("Sum of squares: \(total)")Login to try C/C++/Java/PHP code in the editor
Common Mistakes
- Launching several independent
Task { }blocks for related work instead of grouping them withasync letor a task group, making cancellation and error propagation harder to manage. - Forgetting
async letbindings run concurrently as soon as declared, but each one must still beawaited to retrieve its result. - Using a task group but forgetting to
awaitall its child tasks before the group'swith...TaskGroupblock ends.
Chapter Summary
async letstarts multiple asynchronous operations concurrently within one function scope.- Each
async letbinding must be awaited to retrieve its value. - Task groups (
withTaskGroup) manage a dynamic number of concurrent child tasks together. - Structured concurrency ties a task's lifetime to the scope that created it, making cancellation and error handling predictable.
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