自定义NavgationBar按钮

iOS 上UINavigationController视图压栈形式，可以在当前视图无限制push许多视图，然而一些会觉得自带的push按钮不够美观，而且当上的上一个页面title很长的时候，那个返回按钮就很长,怎样定义一个自己返回按钮呢？像第三幅图片一样。。。

######需要push的视图里面定义一个事件

- (IBAction)pushVC:(id)sender {
NavViewController *navVC = (NavViewController *)[self.storyboard     instantiateViewControllerWithIdentifier:@"NavViewController"];
navVC.title = NSLocalizedString(@"设置", nil);
[self.navigationController pushViewController:navVC animated:YES];
} 

######替换返回按钮，定义自定义按钮

- (void)viewDidLoad
{
[super viewDidLoad];

UIButton *backBtn = [UIButton buttonWithType:UIButtonTypeCustom];
backBtn.frame = CGRectMake(0, 0, 44, 44);

[backBtn setImage:[UIImage imageNamed:@"back.png"] forState:UIControlStateNormal];
[backBtn addTarget:self action:@selector(doBack:) forControlEvents:UIControlEventTouchUpInside];

UIBarButtonItem *backItem = [[UIBarButtonItem alloc] initWithCustomView:backBtn];
self.navigationItem.leftBarButtonItem = backItem;
 
// Do any additional setup after loading the view.
}

-(void)doBack:(id)sender
{
[self.navigationController popViewControllerAnimated:YES];
}