iOS 返回事件 《方便记忆》

    if (self.navigationController) {
        if (self.navigationController.viewControllers.count > 1) {
            [self.navigationController popViewControllerAnimated:YES];
        }else{
            [self.navigationController dismissViewControllerAnimated:YES completion:nil];
        }
    }else{
        [self dismissViewControllerAnimated:YES completion:nil];
    }