Ios 在 Swift 中 @MainActor 似乎被 withCheckedContinuation 而不是其他异步调用继承是巧合吗

在指定了@MainActor的Swift方法中，该方法调用withCheckedContinuation并进行其他异步调用，似乎在@MainActor线程上调用了withCheckedContinuation，但其他异步调用并非如此.

这是巧合吗？

可以将以下代码放入XCode为您创建的默认iOS App的ContentView.swft中:

import SwiftUI

func printCurrentThread(_ label: String) {
    print("\(label) called on \(Thread.current)")
}

class SomeNonThreadSafeLibrary {
    func getFromServer(msg: String, completionHandler: @escaping (String) -> Void) {
        DispatchQueue.main.asyncAfter(deadline: .now() + 2.0) {
            completionHandler(msg)
        }
    }
}

class AppController: ObservableObject {
    @Published var something1: String = "not set"
    @Published var something2: String = "not set"
    var someNonThreadSafeLibrary: SomeNonThreadSafeLibrary? = SomeNonThreadSafeLibrary()

    func someOtherAsyncFunction(calledFrom: String) async {
        printCurrentThread("\(calledFrom) -> someOtherAsyncFunction")
    }

    func getSomethingAsynchronously() async -> String {
        printCurrentThread("getSomethingAsynchronously")
        await someOtherAsyncFunction(calledFrom: "getSomethingAsynchronously")
        return await withCheckedContinuation { continuation in
            printCurrentThread("getSomethingAsynchronously -> withCheckedContinuation")
            do {
                try Task.checkCancellation()
                self.someNonThreadSafeLibrary?.getFromServer(msg: "getSomethingAsynchronously") { result in
                    printCurrentThread("getSomethingAsynchronously -> getFromServer closure")
                    continuation.resume(returning: result)
                }
            }
            catch {}
        }
    }

    @MainActor
    func getSomethingAsynchronouslyWithMainActor() async -> String {
        printCurrentThread("getSomethingAsynchronouslyWithMainActor")
        await someOtherAsyncFunction(calledFrom: "getSomethingAsynchronouslyWithMainActor")
        return await withCheckedContinuation { continuation in
            printCurrentThread("getSomethingAsynchronouslyWithMainActor -> withCheckedContinuation")
            do {
                try Task.checkCancellation()
                self.someNonThreadSafeLibrary?.getFromServer(msg: "getSomethingAsynchronouslyWithMainActorWithMainActor") { result in
                    printCurrentThread("getSomethingAsynchronouslyWithMainActor -> getFromServer closure")
                    continuation.resume(returning: result)
                }
            }
            catch {}
        }
    }

    @MainActor
    func getValueForSomething() async {
        something1 = await getSomethingAsynchronously()
    }

    @MainActor
    func getValueForSomethingWithMainActor() async {
        something2 = await getSomethingAsynchronouslyWithMainActor()
    }

}

struct ContentView: View {
    @StateObject private var appController = AppController()
    var body: some View {
        VStack {
            Text("something1 is \(appController.something1)")
            Text("something2 is \(appController.something2)")
        }
        .task {
            await appController.getValueForSomething()
        }
        .task {
            await appController.getValueForSomethingWithMainActor()
        }
        .padding()
    }
}

代码的输出如下所示...

在线程1上调用带@MainActor的函数，在任意线程上调用不带@MainActor的函数，正如预期的那样:

getSomethingAsynchronouslyWithMainActor called on <_NSMainThread: 0x600001910400>{number = 1, name = main}
getSomethingAsynchronously called on <NSThread: 0x60000195c300>{number = 7, name = (null)}

然后，从这些方法中的每一个调用不做任何事情的函数someOtherAsyncFunction，并且在两种情况下(即，在调用函数上具有或不具有@MainActor)，它在任意线程上被调用.这表明函数上的@MainActor不会被其内部调用的函数继承:

getSomethingAsynchronously -> someOtherAsyncFunction called on <NSThread: 0x60000195c300>{number = 7, name = (null)}
getSomethingAsynchronouslyWithMainActor -> someOtherAsyncFunction called on <NSThread: 0x6000019582c0>{number = 6, name = (null)}

然而，在闭包为withCheckedContinuation的特定情况下，当@MainActor在调用函数上时，它似乎确实在线程1上运行:

 getSomethingAsynchronously -> withCheckedContinuation called on <NSThread: 0x60000195c300>{number = 7, name = (null)}
 getSomethingAsynchronouslyWithMainActor -> withCheckedContinuation called on <_NSMainThread: 0x600001910400>{number = 1, name = main}

所以...withCheckedContinuation似乎通过它的调用函数在@MainActor上运行，这只是一个巧合，还是故意的？为什么只有withCheckedContinuation，而不是someOtherAsyncFunction？

如果它是设计出来的，我从哪里可以获得关于它在哪里指定的更多信息？