如何使用 withCheckedContinuation 使异步 function 可重入或防止覆盖延续？

Question

我有以下代码：

class Locator : NSObject, ObservableObject
{
    private let locationManager: CLLocationManager
    private var authorizationContinuation: CheckedContinuation<CLAuthorizationStatus, Never>?

    @Published var authorizationStatus: CLAuthorizationStatus
    @Published var location: CLLocation?
    @Published var error: Error?

    override init()
    {
        locationManager = CLLocationManager()
        authorizationStatus = locationManager.authorizationStatus

        super.init()

        locationManager.delegate = self
    }

    @MainActor func checkAuthorizationStatus() async -> CLAuthorizationStatus
    {
        let status = locationManager.authorizationStatus
        if status == .notDetermined
        {
            return await withCheckedContinuation
            { continuation in
                authorizationContinuation = continuation

                locationManager.requestWhenInUseAuthorization()
            }
        }
        else
        {
            authorizationStatus = status

            return status
        }
    }
}

extension Locator : CLLocationManagerDelegate
{
    func locationManagerDidChangeAuthorization(_ manager: CLLocationManager)
    {
        authorizationStatus = manager.authorizationStatus

        authorizationContinuation?.resume(returning: authorizationStatus)
    }

    func locationManager(_ manager: CLLocationManager, didFailWithError error: Error)
    {
        print(error)
        
        self.error = error
        location = nil
    }
}

function checkAuthorizationStatus()以authorizationContinuation的形式存储 state。

如果在authorizationContinuation?.resume(returning: authorizationStatus)之前第二次调用 checkAuthorizationStatus checkAuthorizationStatus() ) ，则此 state 将被覆盖，并且第一次异步调用将永远不会恢复。

是否有可能多次调用checkAuthorizationStatus()而这个 state 被覆盖了？ 如果是这样，是否可以预防，或者有什么办法可以解决？

Answer 1

在命令式代码库（例如 UIKit）中， AsyncStream是包装授权（和位置）事件的异步序列的自然模式。 也许像这个要点。

但是当处理遵循声明模式（即可观察对象）的代码库时，只需观察@Published值、 authorizationStatus就可以了。 无需等待checkAuthorizationStatus （关于此保存/覆盖延续的快刀斩乱麻）。

因此，如果尚未确定，请请求授权，但您不需要将其设为异步事件，因为您观察到的属性已经自动将授权状态更改传播到您的 UI。

而且，正如其他地方所讨论的那样，我们通常会将 class 与位置管理器放在主要角色上（因为除其他考虑外， 文档说CLLocationManager需要一个运行循环）：

@MainActor
class ViewModel: NSObject, ObservableObject {
    let locationManager = CLLocationManager()

    @Published var error: Error?
    @Published var location: CLLocation?
    @Published var authorizationStatus: CLAuthorizationStatus = .notDetermined

    override init() {
        super.init()

        locationManager.delegate = self
        authorizationStatus = locationManager.authorizationStatus
    }

    func requestWhenInUseAuthorization() {
        locationManager.requestWhenInUseAuthorization()
    }

    func startUpdatingLocation() {
        locationManager.startUpdatingLocation()
    }
}

extension ViewModel: CLLocationManagerDelegate {
    func locationManager(_ manager: CLLocationManager, didFailWithError error: Error) {
        self.error = error
    }

    func locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) {
        self.location = locations.last
        if error != nil { self.error = nil }
    }

    func locationManagerDidChangeAuthorization(_ manager: CLLocationManager) {
        self.authorizationStatus = manager.authorizationStatus

        if authorizationStatus == .notDetermined {
            requestWhenInUseAuthorization()
        }
    }
}