如何将Firebase Remote Config闭包代码改写为async/await版本？

需求背景 #

需要将访问Firebase Remote Config的旧闭包代码改写为async/await版本，原代码如下：

final class RemoteConfigService {

    private let isDebugEnabled: Bool
    private let semaphore = DispatchSemaphore(value: 1)
    private let concurrentQueue = DispatchQueue.global(qos: .background)

    init(isDebugEnabled: Bool) {
        self.isDebugEnabled = isDebugEnabled
    }

    func fetch(
        defaultValuesPlistFile: PlistFile,
        expirationDuration: TimeInterval,
        onComplete: @escaping (RemoteConfig) -> Void,
        onError: @escaping (Error?) -> Void
    ) {
        concurrentQueue.async { [weak self] in
            guard let strongSelf = self else { return }
            
            strongSelf.semaphore.wait()

            let expirationDuration = !strongSelf.isDebugEnabled ? expirationDuration : 0
            let defaultValues = defaultValuesPlistFile.plist?.dictionary as? [String: NSObject]
            let remoteConfig = RemoteConfig.remoteConfig()
            remoteConfig.setDefaults(defaultValues)
            remoteConfig.fetch(withExpirationDuration: expirationDuration) { [remoteConfig] status, error in
                strongSelf.semaphore.signal()
                
                if status == .success || status == .throttled {
                    remoteConfig.activate { _, _ in
                        DispatchQueue.main.async {
                            onComplete(remoteConfig)
                        }
                    }
                } else {
                    DispatchQueue.main.async {
                        onError(error)
                    }
                }
            }
        }
    }
}

Xcode已为FirebaseRemoteConfig的闭包方法生成了async版本，但遇到三个核心问题：

• 原代码用信号量同步RemoteConfig访问，需在async/await版本保留该线程安全机制
• 原代码在后台队列执行配置获取，需在async代码中实现相同的后台执行逻辑
• 原代码通过DispatchQueue.main.async在主线程返回结果，新代码也要保证这一点

尝试的实现方案 #

初始async/await尝试 #

不确定是否保留了同步机制、是否在后台执行，且未处理主线程返回逻辑：

func fetch(defaultValuesPlistFile: PlistFile, expirationDuration: TimeInterval) async throws -> RemoteConfig {
        try await Task.detached(priority: .background) { [isDebugEnabled] in
            let expirationDuration = !isDebugEnabled ? expirationDuration : 0
            let defaultValues = defaultValuesPlistFile.plist?.dictionary as? [String: NSObject]
            let remoteConfig = RemoteConfig.remoteConfig()
            remoteConfig.setDefaults(defaultValues)
            
            let status = try await remoteConfig.fetch(withExpirationDuration: expirationDuration)
            
            guard status == .success || status == .throttled else {
                throw FeatureStatusError.unsupported
            }
            
            _ = try await remoteConfig.activate()
            
            return remoteConfig
        }.value
    }

思路更新：用Actor实现线程安全 #

利用Actor的串行执行特性替代信号量，天然保证RemoteConfig访问的线程安全：

actor RemoteConfigService {
    
    let remoteConfig = RemoteConfig.remoteConfig()
    
    private let isDebugEnabled: Bool
    
    init(isDebugEnabled: Bool) {
        self.isDebugEnabled = isDebugEnabled
    }
    
    func fetch(defaultValuesPlistFile: PlistFile, expirationDuration: TimeInterval) async throws -> RemoteConfig {
        
        let expirationDuration = !isDebugEnabled ? expirationDuration : 0
        let defaultValues = defaultValuesPlistFile.plist?.dictionary as? [String: NSObject]
        
        remoteConfig.setDefaults(defaultValues)
        
        let status = try await remoteConfig.fetch(withExpirationDuration: expirationDuration)
        
        guard status == .success || status == .throttled else {
            throw FeatureStatusError.unsupported
        }
        
        _ = try await remoteConfig.activate()
        
        return remoteConfig
    }
}

为兼容旧闭包代码，添加非隔离扩展处理主线程回调：

extension RemoteConfigService {
    nonisolated func fetch(
        defaultValuesPlistFile: PlistFile,
        expirationDuration: TimeInterval,
        onComplete: @escaping (RemoteConfig) -> Void,
        onError: @escaping (Error?) -> Void
    ) {
        Task {
            do {
                let remoteConfig = try await fetch(defaultValuesPlistFile: defaultValuesPlistFile, expirationDuration: expirationDuration)
                
                await MainActor.run {
                    onComplete(remoteConfig)
                }
            } catch {
                await MainActor.run {
                    onError(error)
                }
            }
        }
    }
}

思路更新2：优化Actor初始化逻辑 #

将默认配置初始化移至Actor构造方法，简化fetch方法逻辑：

final actor RemoteConfigService {
    
    let remoteConfig = RemoteConfig.remoteConfig()
    
    private let isDebugEnabled: Bool
    private let expirationDuration: TimeInterval
    
    init(isDebugEnabled: Bool, defaultValuesPlistFile: PlistFile, expirationDuration: TimeInterval) {
        self.isDebugEnabled = isDebugEnabled
        self.expirationDuration = expirationDuration
        
        let defaultValues = defaultValuesPlistFile.plist?.dictionary as? [String: NSObject]
        remoteConfig.setDefaults(defaultValues)
    }
    
    func fetch() async throws -> RemoteConfig {
        let expirationDuration = !isDebugEnabled ? expirationDuration : 0
        let status = try await remoteConfig.fetch(withExpirationDuration: expirationDuration)
        
        guard status == .success || status == .throttled else {
            throw FeatureStatusError.unsupported
        }
        
        _ = try await remoteConfig.activate()
        
        return remoteConfig
    }
}

最终方案解析 #

1. 线程安全保障：用Actor替代信号量，Actor天然保证内部方法串行执行，彻底避免多线程竞争，更符合Swift并发模型设计。
2. 后台执行逻辑：Firebase的async版本fetch和activate方法本身会在后台处理网络请求与配置激活，无需额外指定后台队列；若需明确优先级，可将调用包裹在Task(priority: .background)中。
3. 主线程返回结果：兼容旧闭包的扩展中，通过await MainActor.run确保回调在主线程执行；对于async/await版本的调用者，若需更新UI，可在调用后用await MainActor.run切换线程。

内容的提问来源于stack exchange，提问作者Michał Ziobro