此方法能够自己定义返回button，且不影响返回手势。

新方法：

 self.navigationItem.leftBarButtonItem = [[UIBarButtonItem alloc]
                                         initWithImage:img
                                         style:UIBarButtonItemStylePlain
                                         target:self
                                         action:@selector(onBack:)];
self.navigationController.interactivePopGestureRecognizer.delegate = (id<UIGestureRecognizerDelegate>)self;

老方法：

    UIBarButtonItem *backButton = [[UIBarButtonItem alloc] initWithTitle:@"" style:UIBarButtonItemStylePlain target:self action:nil];
    self.navigationController.navigationBar.topItem.backBarButtonItem = backButton;