这个方法用在返回按钮是自定义的时候
UIView *emptyView = [[UIView alloc] init];;
UIBarButtonItem *emptyButton = [[UIBarButtonItem alloc] initWithCustomView:emptyView];
[self.navigationItem setLeftBarButtonItem:emptyButton animated:YES];
```
## 这个方法只有在返回按钮是系统的时候才有效果
[self.navigationItem setHidesBackButton:YES animated:YES];
