How do I resolve “ambiguous use of” compile error with Swift #selector syntax?

[NOTE This answer was originally formulated under Swift 2.2. It has been revised for Swift 4, involving two important language changes: the first method parameter external is no longer automatically suppressed, and a selector must be explicitly exposed to Objective-C.]

You can work around this problem by casting your function reference to the correct method signature:

let selector = #selector(test as () -> Void)

(However, in my opinion, you should not have to do this. I regard this situation as a bug, revealing that Swift’s syntax for referring to functions is inadequate. I filed a bug report, but to no avail.)

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Just to summarize the new #selector syntax:

The purpose of this syntax is to prevent the all-too-common runtime crashes (typically “unrecognized selector”) that can arise when supplying a selector as a literal string. #selector() takes a function reference, and the compiler will check that the function really exists and will resolve the reference to an Objective-C selector for you. Thus, you can’t readily make any mistake.

(EDIT: Okay, yes you can. You can be a complete lunkhead and set the target to an instance that doesn’t implement the action message specified by the #selector. The compiler won’t stop you and you’ll crash just like in the good old days. Sigh…)

A function reference can appear in any of three forms:

• The bare name of the function. This is sufficient if the function is unambiguous. Thus, for example:

  @objc func test(_ sender:AnyObject?) {}
  func makeSelector() {
      let selector = #selector(test)
  }
  

  There is only one test method, so this #selector refers to it even though it takes a parameter and the #selector doesn’t mention the parameter. The resolved Objective-C selector, behind the scenes, will still correctly be "test:" (with the colon, indicating a parameter).

• The name of the function along with the rest of its signature. For example:

  func test() {}
  func test(_ sender:AnyObject?) {}
  func makeSelector() {
      let selector = #selector(test(_:))
  }
  

  We have two test methods, so we need to differentiate; the notation test(_:) resolves to the second one, the one with a parameter.

• The name of the function with or without the rest of its signature, plus a cast to show the types of the parameters. Thus:

  @objc func test(_ integer:Int) {}
  @nonobjc func test(_ string:String) {}
  func makeSelector() {
      let selector1 = #selector(test as (Int) -> Void)
      // or:
      let selector2 = #selector(test(_:) as (Int) -> Void)
  }
  

  Here, we have overloaded test(_:). The overloading cannot be exposed to Objective-C, because Objective-C doesn’t permit overloading, so only one of them is exposed, and we can form a selector only for the one that is exposed, because selectors are an Objective-C feature. But we must still disambiguate as far as Swift is concerned, and the cast does that.

  (It is this linguistic feature that is used — misused, in my opinion — as the basis of the answer above.)

Also, you might have to help Swift resolve the function reference by telling it what class the function is in:

• If the class is the same as this one, or up the superclass chain from this one, no further resolution is usually needed (as shown in the examples above); optionally, you can say self, with dot-notation (e.g. #selector(self.test), and in some situations you might have to do so.

• Otherwise, you use either a reference to an instance for which the method is implemented, with dot-notation, as in this real-life example (self.mp is an MPMusicPlayerController):

  let pause = UIBarButtonItem(barButtonSystemItem: .pause, 
      target: self.mp, action: #selector(self.mp.pause))
  

  …or you can use the name of the class, with dot-notation:

  class ClassA : NSObject {
      @objc func test() {}
  }
  class ClassB {
      func makeSelector() {
          let selector = #selector(ClassA.test)
      }
  }
  

  (This seems a curious notation, because it looks like you’re saying test is a class method rather than an instance method, but it will be correctly resolved to a selector nonetheless, which is all that matters.)