Wait until an object is available

Question

I believe my use case is fairly common, but I couldn't find any documentation that would make me 100% sure about what I am doing. Any pointer is appreciated.

At some point in my app, I begin to download an object. Afterwards, the user can click a button. If the object is done downloading, I want to execute some code. Otherwise I want to wait until the object is downloaded and execute the same piece of code. If the user does not click the button, I don't want to do anything. The downloaded object is lost.

My basic idea was to do something like this:

NSObject *myObj = nil;

- (void)download {
  [self downloadObj:^(NSObject *obj){
    myObj = obj;
  }];
}

- (void)buttonClicked {
  waitOrExecuteDirectly:^{
    // Some code with myObj
  }
}

Of course, the first problem is "how do I wait?"

So I tried with

- (void)buttonClicked {
  if(myObj) {
    // Some code
  } else {
    // Wait then do the exact same code
  }
}

But I think the trickier problem is "what happens if the objects finishes downloading right after the "if" is calculated and before the "else" block is entered?".

I tried to encapsulate the download within an NSOperation and use the completionBlock property. But if the operation has already finished when I set the callback, the completionBlock is never called. I do not want to set the callback in the "download" method because the user might not click on the button.

Is there a built-in mechanism that allows me to give a completion callback to a task that will wait or execute directly depending on the task status? If not, what would be the best practice to do it by myself? Use a NSLock when setting and reading myObj?

Answer 1

Here is the code example:

    - (void)download {
        dispatch_semaphore_t sema = dispatch_semaphore_create(0);
        [self downloadObj:^(NSObject *obj){
            myObj = obj;
            dispatch_semaphore_signal(sema);
        }];
        dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
        dispatch_release(sema);
    }