Concurrency
Swift has built-in support for writing asynchronous and parallel code in a structured way.
Parallel code means multiple pieces of code run simultaneously.
- Concurrency will make the code harder to debug.
- Swift can help to catch problem at compile time.
Although it’s possible to write concurrent code without using Swift language support, that code tends to be hard to read.
listPhotos(inGallery: "Summer Vacation") { photoNames in
let sortedNames = photoNames.sorted()
let name = sortedNames[0]
downloadPhoto(named: name) { photo in
show(photo)
}
}Keywords:
asyncmake the function asynchronous.awaitmake function calling synchronous.
Defining and Calling Asynchronous Functions
The asynchronous functions or methods can run in another new ‘thread’, but we have no need to create a thread to do that, the Swift manage the asynchronous calling automatically.
func listPhotos(inGallery name: String) async -> [String] {
let result = // ... some asynchronous networking code ...
return result
}With the async keyword, the function will be executed in an asynchronous way when it’s been called (the async should be written before throws). The network request takes a relatively long time, so use the we async to let the rest of the app’s code keep running while this code waits for the picture to be ready.
Suspend Execution by await
Use await to make the caller being suspend. Waiting for the return of the function.That code also runs until the next suspension point, marked by await, or until it completes.
While this code’s execution is suspended, some other concurrent code in the same program runs. For example, maybe a long-running background task continues updating a list of new photo galleries.
let photoNames = await listPhotos(inGallery: "Summer Vacation")
let sortedNames = photoNames.sorted()
let name = sortedNames[0]
let photo = await downloadPhoto(named: name)
show(photo)The await make the code to be suspend that also called yielding the thread, so behind the scenes, the other code in the same thread will have opportunity to be executed.
Only certain place in the program can call asynchronous function or methods:
- Code in the body of an asynchronous function, method, or property.
- Code in the static
main()method of a structure, class, or enumeration that mark with@main. - Code in an unstructured child task.
A way to simulate the waiting for network request.
func listPhotos(inGallery name: String) async throws -> [String] {
let result = ["fine", "aaa"]
try await Task.sleep(nanoseconds: 5 * 10000) // Use the 'try await Task.sleep' to simulate
print("did")
return result
}
func downloadPhoto(names: [String]) async throws {
print("download")
try await Task.sleep(4 * 1_000_000)
}Asynchronous Sequences
Suspend every execution of the elements.
import Foundation
let handle = FileHandle.standardInput
for try await line in handle.bytes.lines {
print(line)
}Calling Asynchronous Functions in Parallel
In synchronous way, they are processed one by one.
let firstPhoto = await downloadPhoto(named: photoNames[0])
let secondPhoto = await downloadPhoto(named: photoNames[1])
let thirdPhoto = await downloadPhoto(named: photoNames[2])In asynchronous way, they are processed asynchronously, don’t wait for each mother.
async let firstPhoto = downloadPhoto(named: photoNames[0])
async let secondPhoto = downloadPhoto(named: photoNames[1])
async let thirdPhoto = downloadPhoto(named: photoNames[2])
// Async can't be used for the non-async properties.- Both
awaitandasync-letallow other code to run while they’re suspended. - The code can use both
awaitandasync-letin a same project.
Tasks and Task Groups
Structured concurrency
- Tasks are arranged in a hierarchy. Each task in a task group has the same parent task, and each task can have child tasks.
await withTaskGroup(of: Data.self) { taskGroup in
let photoNames = await listPhotos(inGallery: "Summer Vacation")
for name in photoNames {
taskGroup.addTask { await downloadPhoto(named: name) }
}
}Unstructured Concurrency
Two way to create unstructured task and get a task handle as the result.
- To create an unstructured task that runs on the current actor, call the
Task.init(priority: operation:)initializer. - To create an unstructured task that’s not part of the current actor, call the
Task.d etached(priority:operation:)class method.
let newPhoto = // ... some photo data ...
let handle = Task {
return await add(newPhoto, toGalleryNamed: "Spring Adventures")
}
let result = await handle.valueTask Cancellation
Task check can do some responds when the condition is satisfied. It depended on the work we do.
- Throwing error like
cancellationError - Returning
nilor an empty collection - Returning the partially completed work
Call Task.checkCancellation() , and it will throws CancellationError if the task has been cancelled. Call Task.isCancelled and handle the cancellation in the code.
Call Task.cancel() to propagate cancellation manually.
Actors
It’s similar to the class. It’s reference type. Allow only one task to access their mutable state at a time.
When a task(A) is interacting with an actor instance, the another task(B) can’t access this actor instance at the same time, and the task B will be suspend and waiting for access the property.
actor Home{
let label: String
var t: [Int]
private(set) var max: Int = 0
init(label: String, t: Int) {
self.label = label
self.t = [t]
if t > self.max {
self.max = t
}
}
}
extension Home{
func update(with temp: Int) {
t.append(temp)
if temp > max {
max = temp
}
}
}Use the actor
@main
struct DemoApp {
static func main() async {
var home = Home(label: "Ha", t: 10)
print(await home.max)
await home.update(with: 22)
print(await home.max)
}
}