Since you're not using any parameters in your callback, you could just use a standard dispatch_block_t and since you just want to call back to it when your long process has completed, there's no need to keep track of it with a property. You could just do this:
- (void)doSomeLongThing:(dispatch_block_t)block
{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// Perform really long process in background queue here.
// ...
// Call your block back on the main queue now that the process
// has completed.
dispatch_async(dispatch_get_main_queue(), block);
});
}
Then you implement it just like you specified:
[downloader doSomeLongThing:^(void) {
// do something when it is finished
}];