F#:添加和删除事件处理程序 [英] F#: Add and remove event handlers

问题描述

我正在尝试添加&从Silverlight FrameworkElement中删除事件处理程序，但我只是语法不正确.

I'm trying to add & remove an event handler from a Silverlight FrameworkElement, but I just can't get the syntax correct.

我想做这样的事情:

let clickedEventHandler = fun args -> printfn "Click!"
fe.MouseLeftButtonDown.AddHandler( clickedEventHandler)
...
fe.MouseLeftButtonDown.RemoveHandler(clickedEventHandler)

正确的方法是什么?

推荐答案

如上所述，您需要创建一个委托实例:

As you already mentioned, you need to create an instance of delegate:

let ch = new MouseButtonEventHandler(fun obj args ->
  printfn "Click!") 

然后，您可以将ch用作AddHandler或RemoveHanlder的参数.原因是，F#函数值实际上未表示为任何委托类型.它具有自己的类型，不是委托类型(另请参阅另一篇SO讨论).如果查看F#交互式(或VS IntelliSense)中的类型，则可以看到区别:

Then you can use ch as an argument to AddHandler or RemoveHanlder. The reason is, that F# function value isn't actually represented as any delegate type. It has it's own type, which isn't a delegate (see also another SO discussion). You can see the difference if you look at the types in F# interactive (or VS IntelliSense):

> (fun (o:obj) (a:MouseButtonEventArgs) -> printfn "Click!") 
val it : obj -> MouseButtonEventArgs -> unit = (...)

> new MouseButtonEventHandler(fun o a -> printfn "Click!") 
val it : MouseButtonEventHandler = (...)

可能令人困惑的是，F#还允许在调用方法时从lambda函数值隐式转换为兼容的委托类型，但是仅当您直接将lambda函数作为参数(我认为)时，此方法才有效:

What may be confusing is that F# also allows implicit conversion from lambda function value to a compatible delegate type when calling a method, however this works only when you give a lambda function directly as the argument (I think):

fe.MouseLeftButtonDown.AddHandler(fun obj args -> 
    printfn "Click!")        

在F#中仍然需要委托的另一个原因是，委托具有定义明确的比较语义(将它们与任何其他引用类型进行比较).对于函数，F#规范没有说明它们何时相等.

Another reason why you still need delegates in F# is that delegates have a well-defined comparison semantics (they are compared as any other reference types). For functions, the F# specification doesn't say when they are equal.

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