Swift 闭包不设置变量

Question

我试图在闭包内部设置闭包外部的变量，但它最终并没有被设置。但是，我将变量设置为 的值 正在打印到控制台。此外，在设置返回变量并自行打印之后，正确的值将打印到控制台。当我返回变量时出现问题；它的值与初始化时的值保持不变。这是一些伪代码:

let str: String = {
    var ret: String = "default value"

    functionWithClosure(with: (some_name) in {
        if let name = some_name {
            ret = name
            print(name) // prints successfully
            print(ret_name) // also prints successfully
        }
    })

    return ret // returns "default value"
}()

这是不起作用的实际代码:

let name: String = {
    var ret_name = "default value"

    if let uid = FIRAuth.auth()?.currentUser?.uid {
        FIRDatabase.database().reference().child("users").child(uid).observeSingleEvent(of: .value, with: { (snapshot) in
            if let dictionary = snapshot.value as? [String: AnyObject] {
                if let name = dictionary["name"] as? String {
                    ret_name = name
                    print(ret_name)
                }
            }
        })
    }

    return ret_name
}()

Answer 1

.observeSingleEvent 正在异步工作。

你可以这样做:

func getRetname(completion: @escaping(_ retName: String) -> Void) {
    if let uid = FIRAuth.auth()?.currentUser?.uid {
        FIRDatabase.database().reference().child("users").child(uid).observeSingleEvent(of: .value, with: { (snapshot) in
        if let dictionary = snapshot.value as? [String: AnyObject] {
            if let name = dictionary["name"] as? String {
                ret_name = name
                print(ret_name)
                completion(ret_name)
            }
        }
    })
}

然后，您可以在任何地方使用它:

getRetname(completion: { ret_name in 
    // ret_name - your data
})

希望对你有帮助