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GCD使用 dispatch_group_notify、dispatch_group_enter、dispatch_group_leave处理多线程同步操作

一、简介

dispatch_group_enter：通知group，下面的任务马上要放到group中执行。

dispatch_group_leave：通知group，任务完成了，该任务要从group中移除。

通常两者穿插于任务中;

二、测试

- (void)syncActionTest{    dispatch_group_t group = dispatch_group_create();    dispatch_group_enter(group);    //模拟多线程耗时操作    dispatch_group_async(group, dispatch_get_global_queue(0, 0), ^{        sleep(3);        NSLog(@"%@----Block1结束",[NSThread currentThread]);        dispatch_group_leave(group);    });    NSLog(@"%@----1结束",[NSThread currentThread]);    dispatch_group_async(group, dispatch_get_global_queue(0, 0), ^{        sleep(3);        NSLog(@"%@----Block2结束",[NSThread currentThread]);        dispatch_group_leave(group);    });    NSLog(@"%@----2结束",[NSThread currentThread]);    dispatch_group_notify(group, dispatch_get_global_queue(0, 0), ^{        dispatch_async(dispatch_get_main_queue(), ^{            NSLog(@"%@---全部结束",[NSThread currentThread]);        });    });}

三、应用

常见如 : 多图片上传, 在上传完所有图片后将imgUrl数组上传到服务器

dispatch_group_t group = dispatch_group_create();    __weak DespritionViewController *weakSelf = self;    for (UIImage *image in self.arrDataSources) {        dispatch_group_enter(group);        [ZDUserHttpRequest uploadImage:image andType:@"1" completion:^(NSString *picUrl) {            dispatch_group_leave(group);            if (picUrl) {                [weakSelf.picUrlArr addObject:picUrl];            } else {                [SVProgressHUD showErrorWithStatus:@"上传失败,请重试"];            }        } failed:^(NSURLSessionDataTask *task, NSError *error) {            dispatch_group_leave(group);        }];    }    dispatch_group_notify(group, dispatch_get_global_queue(0, 0), ^{        dispatch_async(dispatch_get_main_queue(), ^{            do something...        });    });

结论:

在开启了多线程执行任务时，若使用了dispatch_group_notify、dispatch_group_enter、dispatch_group_leave，也能有效的保证了等所有的子线程任务处理完后，有一个处理最后结果的地方。