List all subclasses of one class

Question

Can I return a list of all subclasses of one class? For example:

class Mother {

}

class ChildFoo: Mother {

}

class ChildBar: Mother {

}

let motherSubclasses = ... // TODO
print(motherSubclasses) // should to return [ChildFoo.self, ChildBar.self]

Answer 1

An optimised version of Jean Le Moignan's code

    static func subclasses<T>(of theClass: T) -> [T] {
        var count: UInt32 = 0, result: [T] = []
        let allClasses = objc_copyClassList(&count)!
        let classPtr = address(of: theClass)

        for n in 0 ..< count {
            let someClass: AnyClass = allClasses[Int(n)]
            guard let someSuperClass = class_getSuperclass(someClass), address(of: someSuperClass) == classPtr else { continue }
            result.append(someClass as! T)
        }

        return result
    }

public func address(of object: Any?) -> UnsafeMutableRawPointer{
    return Unmanaged.passUnretained(object as AnyObject).toOpaque()
}

For every Type there is only one metaclass instance in runtime, so, pointers on them are unique. In some reason operator === is not allowed for AnyClass, but we can compare pointers directly

performance test:

        let start = CFAbsoluteTimeGetCurrent()
        let found = RuntimeUtils.subclasses(of:UIViewController.self)
        let diff = CFAbsoluteTimeGetCurrent() - start
        print("Took \(diff) seconds, \(found.count) found")

output:

String(describing: theClass): Took 1.0465459823608398 seconds, 174 found

address(of: theClass): Took 0.2642860412597656 seconds, 174 found