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Swift
Swift 3.0
Selector Syntax
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Selector syntax for swift 3.0

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Introduction

Swift 3.0 replaced string-based selectors with the #selector expression, giving compile-time checking for method references used in target-action patterns and notification observers. Before Swift 3, selectors were raw strings like "buttonTapped:" that would crash at runtime if misspelled. The #selector syntax catches these errors at compile time and auto-completes in Xcode. Any method referenced by #selector must be exposed to Objective-C with @objc.

Basic #selector Syntax

swift
1import UIKit
2
3class ViewController: UIViewController {
4
5    override func viewDidLoad() {
6        super.viewDidLoad()
7
8        let button = UIButton(type: .system)
9        button.setTitle("Tap Me", for: .normal)
10
11        // Swift 3.0+ selector syntax
12        button.addTarget(self, action: #selector(buttonTapped), for: .touchUpInside)
13
14        view.addSubview(button)
15    }
16
17    // Must be @objc to be used with #selector
18    @objc func buttonTapped() {
19        print("Button was tapped")
20    }
21}

The #selector(buttonTapped) expression resolves to a Selector value at compile time. If buttonTapped does not exist or is not marked @objc, the compiler emits an error.

Selectors with Parameters

swift
1class ViewController: UIViewController {
2
3    override func viewDidLoad() {
4        super.viewDidLoad()
5
6        // Method with one parameter
7        let tap = UITapGestureRecognizer(target: self, action: #selector(handleTap(_:)))
8        view.addGestureRecognizer(tap)
9
10        // Timer with selector
11        Timer.scheduledTimer(timeInterval: 1.0,
12                             target: self,
13                             selector: #selector(timerFired(_:)),
14                             userInfo: nil,
15                             repeats: true)
16    }
17
18    @objc func handleTap(_ sender: UITapGestureRecognizer) {
19        print("Tapped at \(sender.location(in: view))")
20    }
21
22    @objc func timerFired(_ timer: Timer) {
23        print("Timer fired")
24    }
25}

When the method takes a parameter, include the argument label in the #selector expression. #selector(handleTap(_:)) matches func handleTap(_ sender: UITapGestureRecognizer).

Disambiguating Overloaded Methods

swift
1class MyClass: NSObject {
2
3    @objc func process() {
4        print("No arguments")
5    }
6
7    @objc func process(data: String) {
8        print("String: \(data)")
9    }
10
11    @objc func process(data: Int) {
12        print("Int: \(data)")
13    }
14
15    func setupSelectors() {
16        // Disambiguate by specifying the full signature
17        let sel1 = #selector(process as () -> Void)
18        let sel2 = #selector(process(data:) as (String) -> Void)
19        let sel3 = #selector(process(data:) as (Int) -> Void)
20
21        print(sel1, sel2, sel3)
22    }
23}

When multiple methods share the same base name, cast to the specific function signature to tell the compiler which overload you mean.

Selectors for Properties (Getter/Setter)

swift
1class Person: NSObject {
2    @objc var name: String = ""
3
4    func demo() {
5        // Getter selector
6        let getter = #selector(getter: Person.name)
7
8        // Setter selector
9        let setter = #selector(setter: Person.name)
10
11        print(getter)  // "name"
12        print(setter)  // "setName:"
13    }
14}

Use #selector(getter:) and #selector(setter:) to reference property accessors, commonly used with KVO and key-value observing APIs.

Notification Observers

swift
1class ViewController: UIViewController {
2
3    override func viewDidLoad() {
4        super.viewDidLoad()
5
6        NotificationCenter.default.addObserver(
7            self,
8            selector: #selector(keyboardWillShow(_:)),
9            name: UIResponder.keyboardWillShowNotification,
10            object: nil
11        )
12    }
13
14    @objc func keyboardWillShow(_ notification: Notification) {
15        guard let frame = notification.userInfo?[UIResponder.keyboardFrameEndUserInfoKey] as? CGRect else {
16            return
17        }
18        print("Keyboard height: \(frame.height)")
19    }
20
21    deinit {
22        NotificationCenter.default.removeObserver(self)
23    }
24}

Migration from Swift 2 to Swift 3

swift
1// Swift 2 (deprecated)
2button.addTarget(self, action: Selector("buttonTapped"), for: .touchUpInside)
3// or
4button.addTarget(self, action: "buttonTapped", for: .touchUpInside)
5
6// Swift 3+ (correct)
7button.addTarget(self, action: #selector(buttonTapped), for: .touchUpInside)

String-based selectors compile but produce a deprecation warning in Swift 3 and are removed in later versions. Always use #selector for compile-time safety.

@objc Requirement

swift
1class ViewController: UIViewController {
2
3    // This works — exposed to Objective-C
4    @objc func validSelector() { }
5
6    // This does NOT work with #selector
7    func swiftOnlyMethod() { }
8
9    // Private @objc methods work fine
10    @objc private func privateAction() { }
11
12    func setup() {
13        // Compiles
14        let sel1 = #selector(validSelector)
15
16        // Compiler error: argument of #selector refers to instance method
17        // that is not exposed to Objective-C
18        // let sel2 = #selector(swiftOnlyMethod)
19
20        // Compiles — @objc private is valid
21        let sel3 = #selector(privateAction)
22    }
23}

Every method referenced by #selector must be @objc. In Swift 4+, methods are not implicitly @objc even if the class inherits from NSObject, so the annotation is always required.

Common Pitfalls

  • Forgetting @objc: #selector only works with methods exposed to Objective-C. Omitting @objc produces a compile error. In Swift 4+, inheriting from NSObject alone is not enough.
  • Wrong argument labels: #selector(handleTap) fails if the method is func handleTap(_ sender: UITapGestureRecognizer). You need #selector(handleTap(_:)) with the argument label.
  • Using string selectors: Selector("methodName") bypasses compile-time checking and crashes at runtime if the method does not exist. Always use #selector.
  • Ambiguous overloads without cast: When multiple methods share a name, #selector(process) is ambiguous. Disambiguate with #selector(process as () -> Void).
  • Structs and enums: #selector requires Objective-C compatibility, which means the containing type must be a class (inheriting from NSObject). Structs and enums cannot use #selector.

Summary

  • Use #selector(methodName) for compile-time checked selectors in Swift 3+
  • Include argument labels for methods with parameters: #selector(handleTap(_:))
  • Disambiguate overloads with type casts: #selector(method as (String) -> Void)
  • Use #selector(getter:) and #selector(setter:) for property accessors
  • All selector targets must be marked @objc — Swift 4+ does not infer this
  • Never use string-based Selector("...") — it bypasses compile-time safety
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