[swift-evolution] Type-safe selectors

[Michel Fortin]

On Fri, Dec 4, 2015, at 06:46 PM, Michel Fortin wrote:

>> Implicit conversions cannot happen safely in the direction SubType to BaseType for
>> the arguments, including the target object. That makes Joe Groff's
>> approach the only type-safe solution: make an extension of the base
>> object and generate a method that does what you want. Which means that
>> instead of this:
>> 
>> 	view.action = MyObject.doSomething
>> 
>> you could write this:
>> 
>> 	view.action = { (target: AnyObject) in {
>> 		(target as! MyObject).doSomething()
>> 	}
>> 
>> ...which is safe. Maybe the compiler should just auto-generate that
>> boilerplate for you.
> 
> How can you write that? A @convention(selector) can't actually contain
> any executable code [...]

Like this:

	view.action = "_doSomething_UniqueSelector1234_currentModuleName_blahblah"

	extension NSObject {
		func _doSomething_UniqueSelector1234_currentModuleName_blahblah() {
			let target = self
			(target as! MyObject).doSomething()
		}
	}

(Joe Groff suggested it first.) This simply assumes the receiver will derive from NSObject. You also need to set a non-nil target. And note that the closure is context-free, meaning you can't capture variables with it.

I wonder how many methods like this you can add to NSObject before it starts to impact objc_msgSend dispatch performance...


-- 
Michel Fortin
michel.fortin at michelf.ca
https://michelf.ca