修改BTN的背景颜色的图片

- (void)viewDidLoad
{
    UIButton *backButton = [UIButton buttonWithType:UIButtonTypeCustom];
    backButton.frame =  CGRectMake(0, 0, 60, 24);
    backButton.backgroundColor = [UIColor clearColor];
    [backButton setBackgroundImage:[UIImage imageNamed:@"bar-button-back.png"] forState:UIControlStateNormal];
    [backButton addTarget:self action:@selector(backAction:) forControlEvents:UIControlEventTouchUpInside];
    
    UIBarButtonItem *backBarButton = [[UIBarButtonItem alloc] initWithCustomView:backButton];
    self.navigationItem.leftBarButtonItem = backBarButton;
}