删除navigationBar下边的黑线

if ([self.navigationController.navigationBar respondsToSelector:@selector(setBackgroundImage:forBarMetrics:)]) {
    NSArray *arr=self.navigationController.navigationBar.subviews;
    for (UIView *view in arr) {
        if ([view isKindOfClass:[UIImageView class]]) {
            NSArray *subArray=view.subviews;
            for (UIView *imgView in subArray) {
                if ([imgView isKindOfClass:[UIImageView class]]) {
                    imgView.hidden=YES;
                }
            }
        }
    }
}