//利用GCD并行多個線程并且等待所有線程結束之後再執行其它任務
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{
// 并行執行的線程一
});
dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{
// 并行執行的線程二
});
dispatch_group_notify(group, dispatch_get_global_queue(0,0), ^{
// 彙總結果
});