Swift await works by capturing the context and suspending the execution until the called async method returns. Another thing that works similarly by capturing the surrounding context is an escaping closure. So async-await calls can be imagined as equivalent to escaping closure. Whenever you see a method with async mentally replace that method with a escaping completion handler.
func run() async
func run(_ completion: @escaping () -> Void)
Examples
A simple scenario with equivalent code
class Foo {
func before() {}
func after() {}
func doStuff() async {}
func doStuff(_ completion: @escaping () -> Void) {}
}
extension Foo {
func run() async {
before()
await doStuff()
after()
}
}
extension Foo {
func run(_ completion: @escaping () -> Void) {
before()
doStuff {
self.after()
completion()
}
}
}
And then it only makes sense how the return values work
func doStuff() async -> Int
let ret = await doStuff()
print("\(ret)")
func doStuff(_ completion: @escaping (Int) -> Void)
doStuff { ret in
print("\(ret)")
}
Or the error propagation
func div(_ a: Int, _ b: Int) async throws -> Double {
if b == 0 {
throw FooError.divideByZero
}
return Double(a) / Double(b)
}
do {
let ret = try await div(1, 0)
print("\(ret)")
} catch {
print("\(error)")
}
func div(_ a: Int, _ b: Int, _ completion: @escaping (Double) -> Void) throws {
if b == 0 {
throw FooError.divideByZero
}
completion(Double(a) / Double(b))
}
do {
try div(1, 0) { ret in
print("\(ret)")
}
} catch {
print("\(error)")
}
Converting async to sync
Notice how the async call naturally propagates to the callee. So a call to doStuff() async makes run() async as well. This also makes sense for completion handers in most cases. And if we wish to not propagate the asynchronous behavior, or in other words, we wish to convert an async method into a sync method, we need to use a Task which takes in a completion handler and is equivalent to wrapping within DispatchQueue.async { ... }
func run() {
before()
Task {
await doStuff()
after()
}
}
func run() {
before()
DispatchQueue.global().async {
self.doStuff {
self.after()
}
}
}
Chaining calls
If we do wish to call async methods one after the other, the mental model remains the same
func doStuff() async {}
func doMoreStuff() async {}
before()
await doStuff()
await doMoreStuff()
after()
func doStuff(_ completion: @escaping () -> Void) {}
func doMoreStuff(_ completion: @escaping () -> Void) {}
before()
doStuff {
self.doMoreStuff {
self.after()
}
}
But if the tasks need to be run in parallel the async-await provides a construct with async let
func run() async {
before()
async let task1: Void = doStuff()
async let task2: Void = doMoreStuff()
_ = await [task1, task2]
after()
}
This is then equivalent to using DispatchGroup
func run(_ completion: @escaping () -> Void) {
before()
let group = DispatchGroup()
group.enter()
doStuff { group.leave() }
group.enter()
doMoreStuff { group.leave() }
group.notify(queue: DispatchQueue.global()) {
self.after()
completion()
}
}
Conclusion
Having a better mental model for async-await helps with appreciating what sort of pitfalls it saves us from, and also how to migrate from completion handlers to async await.
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