如何在swift中为函数类型定义一个可选变量，它可以是nil？

Question

I would like to have a variable to hold a function, but which is initialized to nil. I'm trying to make it an optional, but I get an error.

var OnClick: (UIButton!) -> ()? = nil

I get this error

Could not find an overload for '__conversion' that accepts the supplied arguments

If I remove the ?, I also get the same error.

Answer 1

你只需将它包装在括号中：

var OnClick: ((UIButton!) -> ())? = nil

Answer 2

Adding to Cezary's correct answer here. If someone need in detail explanation. Here you go.

Explaination

var OnClick: (UIButton!) -> ()?

defines a variable named OnClick which stores a function type. The function accepts a Implicitly Unwrapped Optional and return optional Void . Note that optional is not the function itself but return type of the function. So you can't assign nil.

var OnClick: ((UIButton!) -> ())?

defines a variable named OnClick which stores a optional function . The function is such that accepts Implicitly Unwrapped Optional and return Void .