ios Swift 中的回调函数语法
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Callback function syntax in Swift
提问by Floyd Resler
I am attempting pass a function to another function and then have the passed function executed passing to it a variable.
我正在尝试将一个函数传递给另一个函数，然后执行传递给它的函数，并将一个变量传递给它。
Here is my code:
这是我的代码：
func showStandardPrompt(prompt:String,view: UIViewController,numberInput: Bool, callback: (()->(String))?) {
let alert = UIAlertController(title: "Input Data", message: prompt, preferredStyle: .Alert)
alert.addTextFieldWithConfigurationHandler { (textField) in
if numberInput {
textField.keyboardType = .NumberPad
}
}
let OKAction = UIAlertAction(title: "OK", style: .Default) { (action) in
let field = alert.textFields![0] as UITextField
callback?(field.text!)
}
alert.addAction(OKAction)
let CancelAction = UIAlertAction(title: "Cancel", style: .Default, handler: nil)
alert.addAction(CancelAction)
view.presentViewController(alert,animated: true, completion: nil)
}
The error I get is in
我得到的错误是
callback?(field.text!)
The error is "Cannot convert value type of 'String' to expected argument type '()'. I know it's a matter of syntax - just don't know what it should be.
错误是“无法将 'String' 的值类型转换为预期的参数类型 '()'。我知道这是语法问题 - 只是不知道它应该是什么。
回答by Dan Beaulieu
Rob's answer is correct, though I'd like to share an example of a simple working callback / completion handler, you can download an example project below and experiment with the getBoolValue's input.
Rob 的回答是正确的，虽然我想分享一个简单的工作回调/完成处理程序的示例，但您可以下载下面的示例项目并尝试使用getBoolValue的输入。
Swift 2:
斯威夫特 2：
func getBoolValue(number : Int, completion: (result: Bool)->()) {
if number > 5 {
completion(result: true)
} else {
completion(result: false)
}
}
getBoolValue(8) { (result) -> () in
// do stuff with the result
print(result)
}
Important to understand:
重要的是要理解：
(String)->() // takes a String returns void
()->(String) // takes void returns a String
回答by Anand Verma
try below code updated for Swift 3
尝试下面为 Swift 3 更新的代码
func getBoolValue(number : Int, completion: (Bool)->()) {
if number > 5 {
completion(true)
} else {
completion(false)
}
}
getBoolValue(number : 8, completion:{ result in
print(result)
})
回答by álvaro Agüero
I use this way :
我用这种方式：
ViewController1
视图控制器1
destination.callback = { (id) -> Void in
print("callback")
print(id)
}
ViewController2
视图控制器2
var callback: ((_ id: Int) -> Void)?
callback?(example_id)
回答by rob mayoff
You've declared callbackto take no arguments, and then you're trying to pass it an argument.
你已经声明callback不接受任何参数，然后你试图向它传递一个参数。
You specified type (()->(String))?, which is an optional function that takes no arguments and returns a String.
您指定了 type (()->(String))?，它是一个可选函数，不接受任何参数并返回一个String.
Perhaps you mean to specify ((String)->())?, which is an optional function that takes a Stringand returns nothing.
也许您的意思是指定((String)->())?，这是一个可选函数，它接受 aString并且不返回任何内容。
回答by stonesam92
In the parameters for showStandardPrompt, you declare callbackto have the type ()->(String), which is a function which takes no parameters and returns a String.
在 for 的参数中showStandardPrompt，您声明callback了 type ()->(String)，它是一个不带参数并返回 a 的函数String。
You then call it with field.text!as a parameter, which obviously conflicts with the previously given type.
然后将其field.text!作为参数调用，这显然与之前给定的类型冲突。
You need to correct the type given to callbackto take a Stringargument and return nothing:
您需要更正给定的类型callback以获取String参数并且不返回任何内容：
(String)->()
