【问题标题】：How to work with async functions swift? Completion handlers [duplicate]如何快速使用异步函数？完成处理程序[重复]
【发布时间】：2017-11-12 22:45:14
【问题描述】：

我试图等待函数处理以显示我的图像。我尝试了很多东西，但没有一个奏效。我知道这是一个异步函数，基本上我必须等待才能获得正确的值，但我不知道如何在这里修复这个函数。我希望你能帮助我。谢谢！

     func createListProductsGood(Finished() -> void) {

        refProducts.child("Products").queryOrderedByKey().observe(.childAdded, with: { snapshot in

            let prod = snapshot.value as! NSDictionary
            let active = snapshot.key
            let rejected = prod["NotInterested"] as! String
            let photoURL = prod["photoURL"] as! String
            var findit = false

           // print(rejected)

            if (rejected != self.userUID){
                //print(active)
                if rejected.contains(","){
                     var pointsArr = rejected.components(separatedBy: ",")
                        for x in pointsArr{
                                if x.trimmingCharacters(in: NSCharacterSet.whitespaces) == self.userUID {
                                   // print("dont show")
                                    findit = true
                                    return
                                }
                            }

                    if (findit == false){
                        if let url = NSURL(string: photoURL) {
                            if let data = NSData(contentsOf: url as URL) {
                                self.ProductId = active
                                self.productPhoto.image = UIImage(data: data as Data)
                            }}
                    }
                }else{
                    print(active)
                    if let url = NSURL(string: photoURL) {
                        if let data = NSData(contentsOf: url as URL) {
                            self.ProductId = active
                            self.productPhoto.image = UIImage(data: data as Data)

                        }}
                }


            }

               })

    finished()

}

已编辑：

这就是我的 viewDidLoad 的样子：

override func viewDidLoad() {
    super.viewDidLoad()
    setAcceptedOrRejected()
    createListProductsGood{_ in
    }


}



 func createListProductsGood(finished: @escaping (_ imageData: Data) -> Void) {

        refProducts.child("Products").queryOrderedByKey().observe(.childAdded, with: { snapshot in

            let prod = snapshot.value as! NSDictionary
            let active = snapshot.key
            let rejected = prod["NotInterested"] as! String
            let photoURL = prod["photoURL"] as! String
            var findit = false

           // print(rejected)

            if (rejected != self.userUID){
                //print(active)
                if rejected.contains(","){
                     var pointsArr = rejected.components(separatedBy: ",")
                        for x in pointsArr{
                                if x.trimmingCharacters(in: NSCharacterSet.whitespaces) == self.userUID {
                                   // print("dont show")
                                    findit = true
                                    return
                                }
                            }

                    if (findit == false){
                        if let url = NSURL(string: photoURL) {
                            if let data = NSData(contentsOf: url as URL) {
                                self.ProductId = active
                                DispatchQueue.main.async {
                                    self.productPhoto.image = UIImage(data: data as Data)
                                }
                            }}
                    }
                }else{
                    print(active)
                    if let url = NSURL(string: photoURL) {
                        if let data = NSData(contentsOf: url as URL) {
                            self.ProductId = active
                            DispatchQueue.main.async {
                             self.productPhoto.image = UIImage(data: data as Data)
                            }
                        }}
                }
        }
             })

}

这是我的第二种方法：

func setAcceptedOrRejected() {
    refProducts.child("Products").queryOrderedByKey().observe(.childAdded, with: { snapshot in

        let prod = snapshot.value as! NSDictionary

        if self.ProductId == snapshot.key{

            self.texto = prod["NotInterested"] as! String
            self.refProducts.child("Products").child(self.ProductId).updateChildValues(["NotInterested": self.texto + ", " + self.userUID])

        } })
}

【问题讨论】：

标签： swift firebase asynchronous completionhandler

【解决方案1】：

你应该改变：

func createListProductsGood(Finished() -> void) {

到：

func createListProductsGood(finished: @escaping (_ something: SomeType) -> Void) {

或者更具体一点：

func createListProductsGood(finished: @escaping (_ imageData: Data) -> Void) {

然后，无论您在函数中获得图像的任何位置，调用

finished(imageData)

这样您就可以通过闭包将 imageData 传递到需要的地方。

然后你像这样调用这个函数：

createListProductsGood{ imageData in 
    ...
   let image = UIImage(data: imageData)

   // update UI from main Thread:
   DispatchQueue.main.async {
   self.productPhoto.image = image
   }
}

还有：

不习惯使用Finished()，你应该使用finished()
使用void 是错误的。您必须使用Void 或()

如果您在使用闭包和完成处理程序时遇到问题，我建议您首先尝试使用简单的 UIAlertController。见here。尝试使用闭包创建一个动作，例如见here

编辑：

感谢 Leo 的 cmets：

func createListProductsGood(finished: @escaping(_ imageData: Data?, MyError?) -> Void) {

let value: Data?
let error = MyError.someError("The error message")

    refProducts.child("Products").queryOrderedByKey().observe(.childAdded, with: { snapshot in

        let prod = snapshot.value as! NSDictionary
        let active = snapshot.key
        let rejected = prod["NotInterested"] as! String
        let photoURL = prod["photoURL"] as! String
        var findit = false

        // print(rejected)

        if (rejected != self.userUID){
            //print(active)
            if rejected.contains(","){
                var pointsArr = rejected.components(separatedBy: ",")
                for x in pointsArr{
                    if x.trimmingCharacters(in: NSCharacterSet.whitespaces) == self.userUID {
                        // print("dont show")
                        findit = true
                        return
                    }
                }

                if (findit == false){
                    if let url = NSURL(string: photoURL) {
                        if let data = NSData(contentsOf: url as URL) {
                            self.ProductId = active // REMOVE
                            self.productPhoto.image = UIImage(data: data as Data) // REMOVE
                            finished(data, nil) //ADD
                        }else{
                            finished(nil,error) //ADD
                        }
                    }
                }
            }else{
                print(active)
                if let url = NSURL(string: photoURL) {
                    if let data = NSData(contentsOf: url as URL) {
                        self.ProductId = active // REMOVE
                        self.productPhoto.image = UIImage(data: data as Data) // REMOVE
                        finished(data,nil) //ADD
                    }else{
                        finished(nil,error) //ADD
                    }
                }
            }

        }
    })
}

然后你这样称呼它：

   createListProductsGood { imageData, error in guard let value = imageData, error == nil else { // present an alert and pass the error message return } 

   ...
   let image = UIImage(data: imageData)

   // update UI from main Thread:
   DispatchQueue.main.async { 
   self.ProductId = active

   self.productPhoto.image = image } }

基本上这种方式createListProductsGood 接受2 个闭包，一个用于如果图像存在，另一个用于如果返回错误。

【讨论】：