Swift无法将类型的值转换为预期的参数类型

Question

override func viewDidLoad() {
    super.viewDidLoad()
    let ref = FIRDatabase.database().reference()
    ref.observe(.value, with: {
    snapshot in
        self.label1.text = (snapshot.value as AnyObject)["label"] as! String
    }, withCancel: {
    error in
    print(error.description)
    })
    // Do any additional setup after loading the view, typically from a nib.
}

行}, withCancel: {显示编译器错误：

  

无法转换类型＆＃39;（_） - ＆gt;的值（）＆＃39;预期的参数类型   ＆＃39;（（错误） - ＆gt; Void）＆＃39;

Answer 1

试试这个

override func viewDidLoad() {
    super.viewDidLoad()
    let ref = FIRDatabase.database().reference()
    ref.observe(.value, with: {
    snapshot in
        self.label1.text = (snapshot.value as AnyObject)["label"] as! String
    }, withCancel: {
    (error:Error) -> Void in
    print(error.description)
    })
    // Do any additional setup after loading the view, typically from a nib.
}