1.设置nav图片

//设置图片
[self.navigationController.navigationBar setBackgroundImage:[UIImage imageNamed:@"nav_bg"]forBarMetrics:UIBarMetricsDefault];
//去除导航栏下面的黑线
[self.navigationController.navigationBar setShadowImage:[UIImage new]];

2.设置透明导航栏
要遵循UINavigationControllerDelegate，UIGestureRecognizerDelegate两个代理。
设置代理

self.navigationController.delegate = self;
self.navigationController.interactivePopGestureRecognizer.delegate = self;
self.edgesForExtendedLayout = UIRectEdgeNone;

代理方法：

- (void)navigationController:(UINavigationController *)navigationController willShowViewController:(UIViewController *)viewController animated:(BOOL)animate{
    if([viewController isKindOfClass:[self class]]){
        [navigationController setNavigationBarHidden:YES animated:YES];
    }
    else{
        [navigationController setNavigationBarHidden:NO animated:YES];
    }
}

以上，就可以设置透明导航栏了。

3.在公共父类控制器中设置navtitle
首先在父类控制器的.h文件中声明类型为NSString的title

@property(nonatomic ,copy)NSString *navTitle;

然后是设置title代码在.m文件中

#pragma mark -- 设置title
- (void)setNavTitle:(NSString *)navTitle {
    _navTitle = navTitle;
    UIView *titleView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 200, 44)];
    UILabel *titleLabel = [UILabel new];
    titleLabel.frame = titleView.bounds;
    titleLabel.font = [UIFont systemFontOfSize:18.f];
    titleLabel.text = navTitle;
    titleLabel.textColor = RGB(51,51,51);
    titleLabel.textAlignment = NSTextAlignmentCenter;
    [titleView addSubview:titleLabel];
    self.navigationItem.titleView = titleView;
}