iOS向上滑动，导航栏颜色逐渐加深

淘宝，京东都有的功能。最开始导航栏是无色的，随着向上拖动，导航栏变为有色并且颜色逐渐加深。

//监听页面滑动-(void)scrollViewDidScroll:(UIScrollView *)scrollView{    if(scrollView.contentOffset.y<0) {        backView.hidden = YES;    }else{        backView.hidden = NO;        CGFloat alpha=scrollView.contentOffset.y/90.0f>1.0f?1:scrollView.contentOffset.y/90.0f;        backView.backgroundColor = [UIColor colorWithPatternImage:[self getImageWithAlpha:alpha]];    }    if (scrollView.contentOffset.y<230) {        _navScrollerView.alpha = 0;    }else{        _navScrollerView.alpha = 1;    }}#pragma handle image -mark-(UIImage *)getImageWithAlpha:(CGFloat)alpha{    UIColor *color=[UIColor colorWithRed:0.506f green:0.035f blue:0.576f alpha:alpha];    CGSize colorSize=CGSizeMake(1, 1);    UIGraphicsBeginImageContext(colorSize);    CGContextRef context = UIGraphicsGetCurrentContext();    CGContextSetFillColorWithColor(context, color.CGColor);    CGContextFillRect(context, CGRectMake(0, 0, 1, 1));    UIImage *img=UIGraphicsGetImageFromCurrentImageContext();    UIGraphicsEndImageContext();    return img;}