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)}
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