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?
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.