pragma mark -- 修改导航栏渐变色 colorWithAlphaComponent防止污染子控件透明度

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

   CGFloat alpha = scrollView.contentOffset.y/100;
   if (alpha > 1) {
       alpha = 1;
   }else if (alpha<0){
       UIColor *color = [UIColor colorWithHexString:@"#ff8833"];
       self.navigationBar.backgroundColor = [color colorWithAlphaComponent:-alpha];
   }else{
       UIColor *color = [UIColor colorWithHexString:@"#ff8833"];
       self.navigationBar.backgroundColor = [color colorWithAlphaComponent:alpha];
   }
}