快速返回完成块中的变量

Question

func observeActiveTrackingState() -> String{

    let pathToObserveState = databaseRef.child("locationState").child("\(userID!)")

    pathToObserveState.observeSingleEvent(of: .value, with: { (snapshot) in

        let snapshotValue = snapshot.value as? NSDictionary

        print("Snapshot Returned (ObserveActiveTrackingState) = \(snapshotValue?["state"] as! String)")

        let currentState = snapshotValue?["state"] as! String

        return currentState //*****GIVES ME AN ERROR*****

        })
    //IF I PUT THE RETURN HERE HOW DO I CALL THE CURRENTSTATE VARIABLE
}

我有一个名为“observeActiveTrackingState”的函数，它应该返回一个字符串，通常在添加返回时它会返回我想要的字符串变量，但是因为我需要将返回语句放在下面

pathToObserveState.observeSingleEvent(of: .value, with: { (snapshot) in


    }) 

它返回一个错误并说它需要在 firebase 的完成块之外发生。如何修复此代码，以便在调用函数时获得所需的变量

Answer 1

你应该使用一个闭包来接受你想要返回的类型的参数：

func observeActiveTrackingState(_ completion: (String) -> ()) {

    let pathToObserveState = databaseRef.child("locationState").child("\(userID!)")

    pathToObserveState.observeSingleEvent(of: .value, with: { (snapshot) in

        let snapshotValue = snapshot.value as? NSDictionary

        print("Snapshot Returned (ObserveActiveTrackingState) = \(snapshotValue?["state"] as! String)")

        let currentState = snapshotValue?["state"] as! String

        completion(currentState)

    })
}

//

你可以创建一个全局闭包变量并调用它：

    let state: (String) -> ()

    func observeActiveTrackingState() {

        let pathToObserveState = databaseRef.child("locationState").child("\(userID!)")

        pathToObserveState.observeSingleEvent(of: .value, with: { (snapshot) in

            let snapshotValue = snapshot.value as? NSDictionary

            print("Snapshot Returned (ObserveActiveTrackingState) = \(snapshotValue?["state"] as! String)")

            let currentState = snapshotValue?["state"] as! String

            self.state(currentState)

        })
    }