A cancellable completion for Swift.

Completion.swift

final class Completion<R> {
	
	private let closure: (R) -> Void
	private var cancelled = false

	/// `closure` is called upon completion, if not cancelled.
	init(closure: (R) -> Void) {
		self.closure = closure
	}
	
	/// `object` is held as a weak reference and passed to the closure upon completion, if not cancelled.
	init<O:AnyObject>(object: O, closure: (O, R) -> Void) {
		self.closure = { [weak object] r in
			guard let strongObject = object else {
				return
			}
			closure(strongObject, r)
		}
	}
	
	func cancel() {
		cancelled = true
	}
	
	func handler(result: R) {
		if cancelled {
			Logger.logVerbose("Completion handler called but doing nothing; cancelled.")
			return
		}
		closure(result)
	}
}

/* In use:

let completion: Completion<Result<String>>
completion = Completion(object: self) { me, result in  // self is weakly held
	me.receive(result)
}
webService.fetchStringWithCompletion(completion.handler)
completion.cancel() // block is never called
*/