ios - Firebase 观察方法不会返回并继续

Question

我正在为登录页面编写一些代码，我们在其中获取用户名并找到关联的密码。暂时我已经说过“如果用户名下存在电子邮件，则完成 segue”。但是，当我调用检查电子邮件的方法 getEmail 时，它似乎永远不会以完整的电子邮件地址正确退出。 print(email) 返回正确的电子邮件地址，所以我知道我已经检索到它并且它是正确的。不过，我似乎从来没有摆脱过这种方法。真的卡在这里了!这是我的代码:

    func getEmail(name: String) -> String{
    var email = ""
    ref = Database.database().reference()
    self.ref.child("Users").child(name).observeSingleEvent(of: .value, with: { (snapshot) in
        if let user = snapshot.value as? [String:Any] {
            print("email retrieved");
            email = user["email"] as! String;
            print(email)
            return;
        }
        else{
            print("email could not be retrieved from the user.");

        }
    }){ (error) in
        print("Could not retrieve object from database because: ");
        print((Any).self);
    }
    return email;
}

func validate(){
    if(Username.text == ""){
        EmptyStringAlert();
    }

    let email = getEmail(name: Username.text!);
    print(email)
    if(email == ""){
        return;
    }
    performSegue(withIdentifier: "LoginSuccess", sender: nil)
}

Answer 1

对 Firebase 的调用是异步的，因此您必须在函数中使用完成来获取数据。尝试这样的事情:

func getEmail(name: String, completion: @escaping (Bool, Any?, Error?) -> Void) {
    var email = ""
    ref = Database.database().reference()
    self.ref.child("Users").child(name).observeSingleEvent(of: .value, with: { (snapshot) in
            if let user = snapshot.value as? [String:Any] {
                email = user["email"] as! String
                completion(true, email, nil)
            }
            else {
                completion(false, nil, nil)
            }
    }){ (error) in
        completion(false, nil, error)
    }
}

func validate(){
    if(Username.text == ""){
        EmptyStringAlert();
    }

    getEmail(name: Username.text!) { (success, response, error) in
        guard success, let email = response as? String else {
            print(error ?? "Failed getEmail..")
            return
        }
        if(email == "") {
            return
        }
        performSegue(withIdentifier: "LoginSuccess", sender: nil)
    }
}