GCD-group、notify、wait

1 dispatch_group_notify，dispatch_group_async执行完后再执行notify；

2 dispatch_group_wait后代码等group执行完，再执行；

例1 dispatch_group_notify

- (void)groupAsyncSummit_notify{

    dispatch_group_t group = dispatch_group_create();

    dispatch_group_async(group, _mainQueue, ^{

        [self doSometing:@"1"];

        });

    dispatch_group_async(group, _mainQueue, ^{

        [self doSometing:@"2"];

    });

    dispatch_group_notify(group, _globalQueue, ^{

        [self doSometing:@"notify"];

    });

    dispatch_group_async(group, _globalQueue, ^{

        [self doSometing:@"3"];

        [self doSometing:@"3-1"];

    });

    [self doSometing:@"4"];

}

输出：43并发，再3-1、1并发，再执行2，最后执行notify；

说明：

dispatch_group_async搭_globalQueue，异步并发执行；

dispatch_group_async搭 _mainQueue，异步串行，加在_mainQueue队列的最后；

例2 dispatch_group_wait，

- (void)groupAsyncSummit_wait{

    dispatch_group_t group = dispatch_group_create();

    dispatch_group_async(group, _globalQueue, ^{

        [self doSometing:@"1"];

    });

    dispatch_group_async(group, _globalQueue, ^{

        [self doSometing:@"2"];

    });

    dispatch_group_async(group, _globalQueue, ^{

        [self doSometing:@"3"];

    });

    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

    [self doSometing:@"4"];

}

输出：1 2 3 并发执行 ， 4最后执行

说明：dispatch_group_wait后代码等group执行完，再执行；