NSOperation and NSOperationQueue callback

Question

I have a class. Inside this class I pass an NSOperation into the NSOperationQueue that lives in my global variables.

Now my NSOperation is finished. I just want to know that it is finished in my class and to have the operation pass data to that class. How is this typically done?

Answer 1

I use the delegate pattern - that was the approach recommended to me by the guys at an Apple Developer Conference.

The scaffolding:

1. Set up a protocol MyOperationDelegate with a setResult:(MyResultObject *) result method.
2. Have whoever needs the result implement that protocol.
3. Add @property id<MyOperationDelegate> delegate; to the NSOperation subclass you've created.

The work:

1. When you create your operation, but before you queue it, tell it who should receive the result. Often, this is the object that creates the operation: [myOperation setDelegate: self];
2. At the end of your operation's main function, call [delegate setResult: myResultObject]; to pass the result on.

Answer 2

Another alternative... if your need to do some work when the operation is complete, you could wrap that work up in a block and use a dependancy. This is very simple, especially with NSBlockOperation.

NSOperationQueue* myQueue = [[NSOperationQueue alloc] init];

NSBlockOperation* myOp = [NSBlockOperation blockOperationWithBlock:^{
    // Do work
}];

NSBlockOperation* myOp2 = [NSBlockOperation blockOperationWithBlock:^{
    // Do work
}];

// My Op2 will only start when myOp is complete
[myOp2 addDependency:myOp];

[myQueue addOperation:myOp];
[myQueue addOperation:myOp2];

Also you can use setCompletionBlock

[myOp setCompletionBlock:^{
    NSLog(@"MyOp completed");
}];