swift - 在 swift 单例中调用异步函数的正确方法

Question

我想为我的应用程序构建一个分析类，并且我正在使用单例。

如果我运行它，tagEvent 函数会立即运行，而不是首先运行 openSession()，因此 sessionId 返回 nil。我如何通过适当的初始化创建这样的类，并像单例实例一样在应用程序范围内使用它。

Analytics.swift

final class Analytics {
    public static let instance = Analytics()
    private var baseUrl = "http://localhost"
    public var deviceId: String?

    private init(){
       self.deviceId = SomeFunctionGetsDeviceID()
   }

   func openSession(){
       // make an API call to create a session and save sessionId to UserDefaults
       if let url = URL(string: self.baseUrl + "/session"){
           let params:[String:Any] = ["deviceId": "\(self.deviceId!)"]

           var request = URLRequest(url: url)
           request.setValue("application/json", forHTTPHeaderField:"Content-Type")
           request.httpMethod = "POST"
           request.httpBody = try! JSONSerialization.data(withJSONObject: params, options: [])

           AnalyticsSessionManager.sharedManager.request(request as URLRequestConvertible).validate().responseObject(completionHandler: { (response: DataResponse<SessionOpenResponse>) in
               if response.result.value != nil {
                   UserDefaults.standard.set(response.result.value?.sessionId, forKey: "sessionId")
               }
           })
       }
   }

   func closeSession(){
      // make an API call to close a session and delete sessionId from UserDefaults
     ...
   }

   func tagEvent(eventName: String, attributes: [String : String]? = nil) {
        if let url = URL(string: self.baseUrl + "/event"),
            let sessionId = UserDefaults.standard.string(forKey: "sessionId"){
            ...
            // make an API call to create an event with that sessionId
        }
    }
}

AppDelegate.swift

func application(_ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
    Analytics.instance.openSession()
    Analytics.instance.tagEvent(eventName: "App Launch", attributes: 
    ["userID":"1234"])
} 

Answer 1

我最好的猜测是 openSession 函数正在异步执行工作，并且 tagEvent 调用在异步代码完成之前进入。有几种解决方法:

1) 添加同步，这样 tagEvent 代码将等待 openSession 调用完成(如果正在进行)。如果不在进行中，或许应该自动调用openSession，等待完成，然后执行那个函数中的代码

2) 从 openSession 添加一个完成处理程序，在该外壳内您可以调用 tagEvent，例如:

func openSession(completionHandler: @escapaing (Bool) -> ()){
    // make an API call to create a session and save sessionId to UserDefaults
    ...
    UserDefaults.standard.set(someSessionID, forKey: "sessionId")

    // when done with async work
    completionHandler(true)
}

然后在您的应用委托(delegate)中:

Analytics.instance.openSession() { (success)
    Analytics.instance.tagEvent(eventName: "App Launch", attributes:["userID":"1234"])
}

3) * 这是我修复它的方式 * 我不会在类外调用 openSession。我会向 Analytics 类添加一个标志:

private var initialized = false

在openSession函数中，一切搞定后设置这个

initialized = true

在 tagEvent 函数中:

func tagEvent(eventName: String, attributes: [String : String]? = nil) {
    // Check for initialization
    if (!initialized) {
        openSession() { (success) in
            // perform your tagEvent code
        })
    } else {
        if let url = URL(string: self.baseUrl + "/event"),
            let sessionId = UserDefaults.standard.string(forKey: "sessionId"){
            ...
            // make an API call to create an event with that sessionId
        }
    }
}