改变导航栏下方默认横线的样式

用图片替换：

-(void)viewDidLoad{
[super viewDidLoad];
CALayer *border = [CALayer layer];
    border.borderColor = [UIColor colorWithPatternImage:[UIImage imageNamed:@"navigationBarUnderline"]].CGColor;
    border.borderWidth = 1;
    CALayer *layer = self.navigationController.navigationBar.layer;
    border.frame = CGRectMake(0, layer.bounds.size.height, layer.bounds.size.width, 2);
    [layer addSublayer:border];
｝