GCD-group、notify、wait

1 dispatch_group_notify，dispatch_group_async執行完後再執行notify；

2 dispatch_group_wait後代碼等group執行完，再執行；

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"];

}

輸出：43并發，再3-1、1并發，再執行2，最後執行notify；

說明：

dispatch_group_async搭_globalQueue，異步并發執行；

dispatch_group_async搭 _mainQueue，異步串行，加在_mainQueue隊列的最後；

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"];

}

輸出：1 2 3 并發執行 ， 4最後執行

說明：dispatch_group_wait後代碼等group執行完，再執行；