如何取消GCD任务

第一种：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）

代码示例

- (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：

2017-07-08 13:59:39.935 beck.wang[2796:284866] block2 <NSThread: 0x6180000758c0>{number = 3, name = (null)}
2017-07-08 13:59:44.940 beck.wang[2796:284889] block1 <NSThread: 0x618000074f80>{number = 4, name = (null)}

同样的，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。

　　第二种：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（如直接返回return）。

- (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＝YES
        sleep(3);
        if(isCancel){
            NSLog(@"任务004已被取消 %@",[NSThread currentThread]);
        }else{
            NSLog(@"任务004 %@",[NSThread currentThread]);
        }
    });
}

打印结果：

2017-07-08 15:33:54.017 beck.wang[3022:333990] 任务002 <NSThread: 0x60800007f740>{number = 4, name = (null)}
2017-07-08 15:33:54.017 beck.wang[3022:333989] 任务001 <NSThread: 0x600000261d80>{number = 3, name = (null)}
2017-07-08 15:33:54.017 beck.wang[3022:333992] 任务003 <NSThread: 0x618000261800>{number = 5, name = (null)}
2017-07-08 15:34:02.266 beck.wang[3022:334006] 任务004已被取消 <NSThread: 0x608000267100>{number = 6, name = (null)}

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