如何快速使用异步函数？完成处理程序[重复]
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【中文标题】如何快速使用异步函数？完成处理程序[重复]【英文标题】：How to work with async functions swift? Completion handlers [duplicate] 【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】：
【参考方案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】：
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