@selector() in Swift?
nstimer, selector, swift
Solution
Swift itself doesn't use selectors — several design patterns that in Objective-C make use of selectors work differently in Swift. (For example, use optional chaining on protocol types or `is`/`as` tests instead of `respondsToSelector:`, and use closures wherever you can instead of `performSelector:` for better type/memory safety.)
But there are still a number of important ObjC-based APIs that use selectors, including timers and the target/action pattern. Swift provides the `Selector` type for working with these. (Swift automatically uses this in place of ObjC's `SEL` type.)
In Swift 2.2 (Xcode 7.3) and later (including Swift 3 / Xcode 8 and Swift 4 / Xcode 9):
You can construct a `Selector` from a Swift function type using the `#selector` expression.
let timer = Timer(timeInterval: 1, target: object,
selector: #selector(MyClass.test),
userInfo: nil, repeats: false)
button.addTarget(object, action: #selector(MyClass.buttonTapped),
for: .touchUpInside)
view.perform(#selector(UIView.insertSubview(_:aboveSubview:)),
with: button, with: otherButton)
The great thing about this approach? A function reference is checked by the Swift compiler, so you can use the `#selector` expression only with class/method pairs that actually exist and are eligible for use as selectors (see "Selector availability" below). You're also free to make your function reference only as specific as you need, as per the Swift 2.2+ rules for function-type naming.
(This is actually an improvement over ObjC's `@selector()` directive, because the compiler's `-Wundeclared-selector` check verifies only that the named selector exists. The Swift function reference you pass to `#selector` checks existence, membership in a class, and type signature.)
There are a couple of extra caveats for the function references you pass to the `#selector` expression:
- Multiple functions with the same base name can be differentiated by their parameter labels using the aforementioned syntax for function references (e.g. `insertSubview(_:at:)` vs `insertSubview(_:aboveSubview:)`). But if a function has no parameters, the only way to disambiguate it is to use an `as` cast with the function's type signature (e.g. `foo as () -> ()` vs `foo(_:)`).
- There's a special syntax for property getter/setter pairs in Swift 3.0+. For example, given a `var foo: Int`, you can use `#selector(getter: MyClass.foo)` or `#selector(setter: MyClass.foo)`.
General notes:
Cases where `#selector` doesn't work, and naming: Sometimes you don't have a function reference to make a selector with (for example, with methods dynamically registered in the ObjC runtime). In that case, you can construct a `Selector` from a string: e.g. `Selector("dynamicMethod:")` — though you lose the compiler's validity checking. When you do that, you need to follow ObjC naming rules, including colons (`:`) for each parameter.
Selector availability: The method referenced by the selector must be exposed to the ObjC runtime. In Swift 4, every method exposed to ObjC must have its declaration prefaced with the `@objc` attribute. (In previous versions you got that attribute for free in some cases, but now you have to explicitly declare it.)
Remember that `private` symbols aren't exposed to the runtime, too — your method needs to have at least `internal` visibility.
Key paths: These are related to but not quite the same as selectors. There's a special syntax for these in Swift 3, too: e.g. `chris.valueForKeyPath(#keyPath(Person.friends.firstName))`. See SE-0062 for details. And even more `KeyPath` stuff in Swift 4, so make sure you're using the right KeyPath-based API instead of selectors if appropriate.
You can read more about selectors under Interacting with Objective-C APIs in Using Swift with Cocoa and Objective-C.
Note: Before Swift 2.2, `Selector` conformed to `StringLiteralConvertible`, so you might find old code where bare strings are passed to APIs that take selectors. You'll want to run "Convert to Current Swift Syntax" in Xcode to get those using `#selector`.
Problem
I'm trying to create an `NSTimer` in `Swift` but I'm having some trouble. ``` NSTimer(timeInterval: 1, target: self, selector: test(), userInfo: nil, repeats: true) ``` `test()` is a function in the same class. I get an error in the editor: Could not find an overload for 'init' that accepts the supplied arguments When I change `selector: test()` to `selector: nil` the error disappears. I've tried: - `selector: test()` - `selector: test` - `selector: Selector(test())` But nothing works and I can't find a solution in the references.