dispatch_block_cancel

iOS8之后，提交到gcd队列中的dispatch block也可取消了，只需要简单的调用dispatch_block_cancel传入想要取消的block即可:

dispatch_queue_t queue = dispatch_queue_create("queue", DISPATCH_QUEUE_SERIAL);dispatch_block_t block1 = dispatch_block_create(0, ^{    NSLog(@"block1 begin");    [NSThread sleepForTimeInterval:1];    NSLog(@"block1 done");});dispatch_block_t block2 = dispatch_block_create(0, ^{    NSLog(@"block2 ");});dispatch_async(queue, block1);dispatch_async(queue, block2);dispatch_block_cancel(block2);