Anthony Dito Anthony Dito - 6 months ago 31
Swift Question

Understanding Swift 2.2 Selector Syntax - #selector()

I am switching over the syntax of my project toward Swift 2.2 (which xCode helps me do automatically); however, I do not understand the new

#selector()
syntax.

As an example:

timer = NSTimer.scheduledTimerWithTimeInterval(1.0, target: self,
selector: #selector(MyVC.timerCalled(_:)), //new selector syntax!
userInfo: nil, repeats: true)


This has the selector
#selector(MyVC.timerCalled(_:))


What does the
_:
signify? Can you add other variables into this selector? Say,
#MyVC.timerCalled(_:whateverVar)
.
General info on what is different in this syntax as opposed to the string based implementation from earlier versions of Swift are greatly appreciated.

Thanks in advance

Answer

The bit in parenthesis is a mechanism for identifying the argument list for the selector that you want.

I recommend you look at the Generalized Naming proposal from Swift Evolution. It covers cases where you have a number of functions that differ only by their parameter labels and need to refer to them. The example from that document is:

extension UIView {
  func insertSubview(view: UIView, at index: Int)
  func insertSubview(view: UIView, aboveSubview siblingSubview: UIView)
  func insertSubview(view: UIView, belowSubview siblingSubview: UIView)
}

If you wanted to get a function value for one of those the result is ambiguous:

let fn = someView.insertSubview // ambiguous: could be any of the three methods

The solution implemented is to add the argument labels, without any type information to the code that generates the function value to disambiguate which you want:

let fn = someView.insertSubview(_:at:)
let fn1 = someView.insertSubview(_:aboveSubview:)

See how the labels are added in the parens?

This proposal played a role in the one that most directly applies to your question:

Referencing the Objective-C selector of a method

In this particular case the selector you want to refer to is timerCalled: which is a function of one parameter that has no label. Hence (_:). The underscore means the label is not specified and the colon.