How to invoke a class method using performSelector() on AnyClass in Swift?

Question

In ObjC you could simply invoke a class method using the class method from NSObject.

[Machine performSelector:@selector(calculate:) withObject:num];

But how do you do this in Swift 2.2?

@objc(Machine) // put it here, so you can simply copy/paste into Playground
class Machine: NSObject {
    static func calculate(param: NSNumber) -> String {
        if param.integerValue > 5 {
            return "42"
        }
        return "42" // there is only 1 answer to all the questions :D
    }
}

if let aClass = NSClassFromString("Machine") {
    let sel = #selector(Machine.calculate(_:))
    let num = NSNumber(integer: 1337)
    let answer = aClass.performSelector(sel, withObject: num) // compiler error
    // let answer = aClass.calculate(num)                     // <-- this works
    print(answer)
}

With this code I'm getting the following compiler error:

error: cannot invoke 'performSelector' with an argument list of type '(Selector, withObject: NSNumber)'

What am I missing here?

Riviera

It is definitely not pointless, what if you want to use the class from an external source? E.G. reading the class name from a string in a plist, are you going to write an if condition based on every possible object? – Thermolysis

Related: Access Private UIKit Function Without Using Bridging Header – Varela

Answer 1

AnyClass does not conform to NSObjectProtocol out of the box. I had to cast aClass as NSObjectProtocol to use performSelector (performSelector:withObject: is bridged to Swift as a method on NSObjectProtocol):

Swift 3:

if let aClass = NSClassFromString("Machine") {
    let sel = #selector(Machine.calculate(param:))
    let num = NSNumber(value: 1337)

    if let myClass = aClass as? NSObjectProtocol {
        if myClass.responds(to: sel) {
            let answer = myClass.perform(sel, with: num).takeRetainedValue() // this returns AnyObject, you may want to downcast to your desired type
            print(answer) // "42\n"
        }
    }
}

Swift 2.x:

(aClass as! NSObjectProtocol).performSelector(sel, withObject: num) // Unmanaged<AnyObject>(_value: 42) 

A little bit safer:

if let aClass = NSClassFromString("Machine") {
    let sel = #selector(Machine.calculate(_:))
    let num = NSNumber(integer: 1337)

    if let myClass = aClass as? NSObjectProtocol {
        if myClass.respondsToSelector(sel) {
            let answer = myClass.performSelector(sel, withObject: num).takeUnretainedValue()
            print(answer) // "42\n"
        }
    }
}

performSelector returns an Unmanaged object, that's why takeUnretainedValue() (or optionally takeRetainedValue() if you want to transfer memory ownership) are required.

Varela

That is funny, because casting it as? AnyObject worked for me – Thermolysis

@Thermolysis Casting to AnyObject will work because AnyClass is really a typealias for the metatype AnyObject.Type. You probably got a warning that the cast aways succeeds because they're the same type. Probably not a good idea to cast a class to an object though. – Varela

@Thermolysis You can only use performSelector on Objective-C types, not pure Swift types. Try it. That's why I check for NSObjectProtocol conformance. – Varela

I have no warning on my screen, and wouldn't NSClassFromString automatically fail for swift types? – Thermolysis

@Thermolysis Hmm, good point about NSClassFromString. But I still see a warning if I try to cast the class as AnyObject: if let myClass = aClass as? AnyObject { ... } I'm using Xcode 7.3. – Varela

yes, weird, use my code and you won't see is, maybe it is because it is AnyClass? ? – Thermolysis

ahh, I need to upgrade to 2.2 again, seems like my install went bad, I will get back to you on the warning – Thermolysis

thanks! I knew I was missing some conformance but didn't know which protocol and where to look it up ;) – Riviera

Finally got swift 2.2 working again, anyway, yes, still not warning, Also, since we know that NSClass is going to return a member of NSClass when valid, I do not think the second variable is needed, you can just check if aClass is a member of NSObjectProtocol, and see if it responds to the selector there – Thermolysis

@Thermolysis yes, this code can probably be optimized further like if let aClass = NSClassFromString("Machine") as? NSObjectProtocol { ... } but I left the answer expanded so users can see the flow of casting from AnyClass to NSObjectProtocol. I think it's easier to read like this and can always be optimized further as an exercise to the reader. – Varela

@Riviera Yes forgot that earlier, so I wanted to make my code a little bit safer in case another user finds it and decides to use it. – Varela

"You can only use performSelector on Objective-C types, not pure Swift types." But actually, at runtime, pure Swift types do support .respondsToSelector(), .performSelector(), etc. So casting to AnyObject (to overcome lack of knowledge of such support at compile-time) and using them does work, without needing for the type to conform to NSObjectProtocol. – Roast

@Roast Ah you're right! I actually just asked a question about this here. Even though pure Swift classes are represented as SwiftObject to the Objective-C runtime, you need to explicitly declare your object as AnyObject or NSObjectProtocol to fire Objective-C selectors on it. – Varela

Oddly, Swift 4 will emit a warning trying to cast AnyClass to NSObjectProtocol, which annoys me. At runtime it still works, but trying to keep clean project of warnings. Does anyone have an idea? – Sorb

@Sorb use if let myClass = aClass as AnyObject as? NSObjectProtocol { – Tore

Answer 2

If you want to perform calculate on machine, just do:

Machine.calculate(NSNumber(integer: 1337))

If you need to call perform selector, then do:

if let aClass = NSClassFromString("Machine") as? AnyObject
{
    let sel = #selector(Machine.calculate(_:))
    let num = NSNumber(integer: 1337)
    let answer = aClass.performSelector(sel, withObject: num)
    print(answer)
}

Thermolysis