iOS在异步任务完成后做事情

dispatch_group_t group = dispatch_group_create();

dispatch_group_enter(group);

开启异步任务

      successBlock{

          dispatch_group_leave(group);

}

failedBlock{

           dispatch_group_leave(group);

}



dispatch_group_notify(group, dispatch_get_main_queue(), ^{

异步任务完成后 需要做的事情....

});