NSOperation cancel pattern and retain cycle avoidance

gistfile1.m

/*
 Source: Building Concurrent User Interfaces on iOS
 WWDC2012 Session 211 by Andy Matuschak
*/

NSOperationQueue *queue = [[NSOperation alloc] init];
NSBlockOperation *op = [[NSBlockOperation alloc] init];
__weak NSBlockOperation *weakOp = op;
[op addExecutionBlock:^{
    for (int i = 0; i < 10000; i++) {
        if ([weakOp isCancelled]) break;
        processData(data[i]);
    }
}];
[queue addOperation:op];