Generic NSOperation subclass loses NSOperation functionality

Question

Today I've met one weird issue when I was trying to 'generalize' my 'CoreData importing operations'. It appeared that if I create a generic subclass of NSOperation the main() func won't be called.

Simple example:

class MyOperation<T: NSObject>: NSOperation {

    override func main() {
        println("My operation main was called")
    }
}

If you create an instance of this class and add it to the operationQueue you will see that it's main() isn't actually called.

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.

    self.operationQueue = NSOperationQueue()
    let operation = MyOperation<NSString>()
    self.operationQueue!.addOperation(operation)
}

Operation simply transits from ready to executing and to finished state without calling main().

If I remove generic annotation <T: NSObject> from MyOperation class it will work fine.

How is this possible? Am I missing something here?

Answer 1

The problem is caused by this simple rule:

Method in a generic class cannot be represented in Objective-C

As a result, when bridged to Objective-C, MyOperation looks like pure, with no methods are overridden, NSOperation subclass.

You can see this error by marking override func main() with @objc attribute.

@objc override func main() {  // < [!] Method in a generic class cannot be represented in Objective-C
    println("My operation main was called")
}