函数返回值中的F＃类型错误

Question

我正在尝试在F＃中实现以下代码段：

    // Method style
void Callback (NSNotification notification)
{
    Console.WriteLine ("Received a notification UIKeyboard", notification);
}

void Setup ()
{
    NSNotificationCenter.DefaultCenter.AddObserver (UIKeyboard.WillShowNotification, Callback);
}

我做了以下事情：

let toggleKeyboard(notification : NSNotification) = 
    Console.WriteLine ("Received a notification UIKeyboard", notification)

NSNotificationCenter.DefaultCenter.AddObserver(UIKeyboard.WillShowNotification, toggleKeyboard) |> ignore

这似乎是一个简单的实现，但我得到一个类型错误：

This expression was expected to have type 'Action<NSNotification>' but here has type ''a -> unit' (FS0001) (Dissertation)

我不确定如何让我的方法返回一个Action<NSNotification>类型。

Answer 1

有时F＃会自动将函数Func<T>为Action<T>或Func<T>类型，但您也可以将函数显式地包装在Action<T>如下所示：

let foo x = printfn "%s" x
let action = System.Action<string>(foo)
action.Invoke("hey") // prints hey

或者在你的情况下：

...AddObserver(UIKeyboard.WillShowNotification, System.Action<NSNotification>(toggleKeyboard)) |> ignore