This NSOperationQueue category method lets you use a "cancelable" block with NSOperation. By passing the containing operation to the block, the block can return early or respond in other ways if the operation's state changes. The vanilla -addOperationWithBlock: has no reference to the operation, so it's impossible to know, let alone leverage, it…

gistfile1.txt

// Passes the new operation to block so it can check for cancellation and exit early
- (NSOperation *)bhs_addOperationWithCancelableBlock:(void (^)(NSOperation *))block {
    NSBlockOperation *op = [[NSBlockOperation alloc] init];
    __weak NSBlockOperation *weakOp = op;
    [op addExecutionBlock:^{
        block(weakOp);
    }];
    [self addOperation:op];
    return op;
}

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[myQueue bhs_addOperationWithCancelableBlock:^(NSOperation *op) {
    if ([op isCancelled]) return;
    
    // Do some work

    if ([op isCancelled]) return;

    // Rinse, repeat as needed
}];