1.dispatch_block_cancel

iOS8之后可以调用dispatch_block_cancel来取消（需要注意必须用dispatch_block_create创建dispatch_block_t，dispatch_block_cancel也只能取消尚未执行的任务，对正在执行的任务不起作用。）

- (void)gcdBlockCancel{
    dispatch_queue_t queue = dispatch_queue_create("com.gcdtest.www", DISPATCH_QUEUE_CONCURRENT);
    dispatch_block_t block1 = dispatch_block_create(0, ^{
        sleep(5);
        NSLog(@"block1 %@",[NSThread currentThread]);
    });
    dispatch_block_t block2 = dispatch_block_create(0, ^{
        NSLog(@"block2 %@",[NSThread currentThread]);
    });
    dispatch_block_t block3 = dispatch_block_create(0, ^{
        NSLog(@"block3 %@",[NSThread currentThread]);
    });
    dispatch_async(queue, block1);
    dispatch_async(queue, block2);
    dispatch_block_cancel(block3);
}

打印结果：

2018-03-0210:10:30.925com.hua[2796:284866] block2 {number =3, name = (null)}
2018-03-0210:10:30.930com.hua[2796:284889] block1 {number =4, name = (null)}

2.定义外部变量，用于标记block是否需要取消

该方法是模拟NSOperation，在执行block前先检查isCancelled = YES ？在block中及时的检测标记变量，当发现需要取消时，终止后续操作。

- (void)gcdCancel{
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0);
    __block BOOL isCancel = NO;
    dispatch_async(queue, ^{
        NSLog(@"任务001 %@",[NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"任务002 %@",[NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"任务003 %@",[NSThread currentThread]);
        isCancel = YES;
    });
    dispatch_async(queue, ^{
        // 模拟：线程等待3秒，确保任务003完成 isCancel＝YESsleep(3);
        if(isCancel){
            NSLog(@"任务004已被取消 %@",[NSThread currentThread]);
        }else{
            NSLog(@"任务004 %@",[NSThread currentThread]);
        }
    });
}

打印结果：

2018-03-0210:15:20.825com.hua[3022:333990] 任务002 {number =4, name = (null)}
2018-03-0210:10:20.826com.hua[3022:333989] 任务001 {number =3, name = (null)}
2018-03-0210:10:20.827com.hua[3022:333992] 任务003 {number =5, name = (null)}
2018-03-0210:10:20.828com.hua[3022:334006] 任务004已被取消 {number =6, name = (null)}