首次发射后如何终止 Swift Combine 响应？答案

【问题标题】：How to terminate Swift Combine response after first emit?首次发射后如何终止 Swift Combine 响应？
【发布时间】：2020-06-10 15:22:29
【问题描述】：

我正在向发布者请求单个值，并希望在收到响应后终止。下面我只是在第一次之后释放取消令牌，有没有更好的方法来做到这一点？

extension MyInteractor {
    private static var locationPermissionToken: Cancellable?

    func requestLocationPermission(completion: @escaping (Result<Void, LocationError>) -> Void) {
        Self.locationPermissionToken = locationProxy.authorizationPublisher
            .sink { status in
                Self.locationPermissionToken = nil
                status ? completion(.success(())) : completion(.failure(.deniedLocationServices))
            }

        locationProxy.requestAuthorization()
    }
}

【问题讨论】：

您的发布商是自定义发布商吗？是这样的话，我建议以一种在发出单个值后立即发送完成的方式来实现它。如果它不是自定义发布者，因此您无法修改其实现，那么通过取消 Cancellable 引用手动取消订阅是您最好的选择。
这是一个自定义实现，我在另一个问题中有部分源代码你可以在这里看到：stackoverflow.com/questions/62307501/…

标签： swift reactive-programming combine

【解决方案1】：

考虑到您有一个自定义 Publisher，我建议修改它的实现，以便它在发出值后立即发送完成。

由于您使用的是PassthroughSubject 而不是符合Publisher 的自定义类型，因此您可以创建一个自定义方法来发送值和完成。然后你需要调用这个方法，而不是从你的类型内部调用authorizationSubject.send。

private func emitAndComplete(authorizationStatus: Bool) {
    Self.authorizationSubject.send(authorizationStatus)
    Self.authorizationSubject.send(completion: .finished)
}

完整修改代码：

@available(OSX 10.15, iOS 13, tvOS 13, watchOS 6, *)
public class LocationProxy: NSObject {
    private lazy var manager = CLLocationManager()

    private static let authorizationSubject = PassthroughSubject<Bool, Never>()
    public private(set) lazy var authorizationPublisher: AnyPublisher<Bool, Never> = Self.authorizationSubject.eraseToAnyPublisher()

    var isAuthorized: Bool { CLLocationManager.isAuthorized }

    func requestAuthorization(for type: LocationAPI.AuthorizationType = .whenInUse) {
        // Handle authorized and exit
        guard !isAuthorized(for: type) else {
            emitAndComplete(authorizationStatus: true)
            return
        }

        // Request appropiate authorization before exit
        defer {
            #if os(macOS)
            if #available(OSX 10.15, *) {
                manager.requestAlwaysAuthorization()
            }
            #elseif os(tvOS)
            manager.requestWhenInUseAuthorization()
            #else
            switch type {
            case .whenInUse:
                manager.requestWhenInUseAuthorization()
            case .always:
                manager.requestAlwaysAuthorization()
            }
            #endif
        }

        // Handle mismatched allowed and exit
        guard !isAuthorized else {
            // Process callback in case authorization dialog not launched by OS
            // since user will be notified first time only and ignored subsequently
            emitAndComplete(authorizationStatus: false)
            return
        }

        // Handle denied and exit
        guard CLLocationManager.authorizationStatus() == .notDetermined else {
            emitAndComplete(authorizationStatus: false)
            return
        }
    }

    public func locationManager(_ manager: CLLocationManager, didChangeAuthorization status: CLAuthorizationStatus) {
        guard status != .notDetermined else { return }
        emitAndComplete(authorizationStatus: isAuthorized)
    }

    private func emitAndComplete(authorizationStatus: Bool) {
        Self.authorizationSubject.send(authorizationStatus)
        Self.authorizationSubject.send(completion: .finished)
    }
}

【讨论】：