iOS改变UINavigationBar上的BackBarButton图片

说是改变，其实是替换：

    UIButton *backButton = [UIButton buttonWithType:UIButtonTypeCustom];
    backButton.frame =  CGRectMake(0, 0, 60, 48);
    backButton.backgroundColor = [UIColor clearColor];
    [backButton setBackgroundImage:[UIImage imageNamed:@"bluebackbutton.png.png"] forState:UIControlStateNormal];
    [backButton addTarget:self action:@selector(PopViewcontroller) forControlEvents:UIControlEventTouchUpInside];
    UIBarButtonItem *btn=[[UIBarButtonItem alloc]initWithCustomView:backButton];
 
    [self.navigationItem setHidesBackButton:YES];
    self.navigationItem.leftBarButtonItem=btn;