NSOperation mainQueue没有被执行

Question

class AsyncOperation : NSOperation {
    override func main () {
        print("In the user queue")
        NSOperationQueue.mainQueue().addOperationWithBlock({
            print("main block")
        })
    }
}
let queue = NSOperationQueue()
let operation = AsyncOperation();
queue.addOperation(operation)

它打印In the user queue但不打印main block。由于我要添加到主队列，我希望它能够被执行。我是否必须开始执行队列？