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

Answer 1

Workaround: You can create NSOperation subclass (no generic), override main and call you own 'execute' func, which can be overriden by generic subclasses. Example:

class SwiftOperation : NSOperation {

    final override func main() {
        execute()
    }

    func execute() {
    }

}

class MyOperation<T> : SwiftOperation {

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

}

Answer 2

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")
}

Answer 3

In Xcode 7 generic NSOperation has been fixed: if I run this code in a playground it works:

protocol SomeProtocol {

    // markup protocol
}

class GenericOperation<SomeTypeImplementingProtocol: SomeProtocol>: NSOperation {

    let referenceToSomeTypeImplementingProtocol: SomeTypeImplementingProtocol

    init(referenceToSomeTypeImplementingProtocol: SomeTypeImplementingProtocol) {

        self.referenceToSomeTypeImplementingProtocol = referenceToSomeTypeImplementingProtocol
    }

    override func main() {

        debugPrint("The GenericOperation main() method was called.")

    }
}

class TypeImplementingSomeProtocol: SomeProtocol {


    init() {

    }
}


let operationQueue = NSOperationQueue()

let typeImplementingSomeProtocolInstance = TypeImplementingSomeProtocol()


let operation = GenericOperation<TypeImplementingSomeProtocol>(referenceToSomeTypeImplementingProtocol: typeImplementingSomeProtocolInstance)


operationQueue.addOperation(operation)