UINavigationBar 背景透明与渐变

1、UINavigationBar 背景透明（按键还看得到）

    [self.navigationController.navigationBar setBackgroundImage:[UIImage new] forBarMetrics:UIBarMetricsDefault];

    [self.navigationController.navigationBar setShadowImage:[UIImage new]];

2、列表上拉，UINavigation背景颜色逐渐变深

思路：上拉的时候不断改变背景颜色的alpha值

-(void)scrollViewDidScroll:(UIScrollView *)scrollView{

    CGFloat yoffset = scrollView.contentOffset.y;

    if (yoffset >0) {

        CGFloat alpha = MIN(1, yoffset/150); //150：上拉150距离背景图片alpha=1

        UIImage *image = [self createImageWithColor:[UIColor colorWithRed:73/255.0f green:174/255.0f blue:175/255.0f alpha:alpha]];

        [self.navigationController.navigationBar setBackgroundImage:image forBarMetrics:UIBarMetricsDefault];

    }

}

- (UIImage *) createImageWithColor: (UIColor *) color

{

    CGRect rect = CGRectMake(0.0f,0.0f,1.0f,1.0f);

    UIGraphicsBeginImageContext(rect.size);

    CGContextRef context =UIGraphicsGetCurrentContext();

    CGContextSetFillColorWithColor(context, [color CGColor]);

    CGContextFillRect(context, rect);

    UIImage *myImage =UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    return myImage;

}