**通过自定义NavigationController 重写push方法实现对back按钮的全局修改，当需要在局部控制器设置back按钮样式时直接可以覆盖 **
首先创建自定义UICustomNavigationController 继承系统的UINavigationController
//重写UICustomNavigationController的:
-(void)pushViewController:(UIViewController*)viewController animated:(BOOL)animated 方法：
@implementation HNNavigationController
-(void)pushViewController:(UIViewController*)viewController animated:(BOOL)animated {
[super pushViewController:viewController animated:animated];
//NavigationController中包含的第一个UIViewController也是push方法进来的 但是一般这个controller不需要添加back button
if(self.viewControllers.count > 1) {
UIButton*backButton=[UIButton buttonWithType:UIButtonTypeCustom];
[backButton setBackgroundImage:[UIImage imageNamed:@"navigationbar_back"]forState:UIControlStateNormal];
[backButton setBackgroundImage:[UIImage imageNamed:@"navigationbar_back_highlighted"]forState:UIControlStateHighlighted];
backButton.frame = CGRectMake(0,0,30,30);
[backButton addTarget:self action:@selector(back) forControlEvents:UIControlEventTouchUpInside];
viewController.navigationItem.leftBarButtonItem=[[UIBarButtonItem alloc]initWithCustomView:backButton];
}
}
//back方法
-(void)back {
[self popViewControllerAnimated:YES];
}
