ios实现导航栏渐变

//状态条高度
#define TG_TANGO_STATUSBAR_HEIGHT [[UIApplication sharedApplication] statusBarFrame].size.height
//导航条高度
#define TG_TANGO_NAVBAR_HEIGHT 44.0
//导航条+状态条高度
#define TG_TANGO_SYSTEMTOP_HEIGHT (TG_TANGO_STATUSBAR_HEIGHT + TG_TANGO_NAVBAR_HEIGHT)

self.navigationController.navigationBar.barTintColor = [UIColor orangeColor];

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

    CGFloat offsetY = scrollView.contentOffset.y;

    if (offsetY <= 0) {
        CGFloat alpha = -offsetY / SYSTEMTOP_HEIGHT;
        [self setNavAlpha:alpha];
    } else {
        [self setNavAlpha:0.0];
    }
}

- (void)setNavAlpha:(CGFloat)alpha {
    //修改barTintColor的alpha
    self.navigationController.navigationBar.alpha = alpha;
    //修改navigationBar的第一层subview的alpha也可以
    //self.navigationController.navigationBar.subviews.firstObject.alpha = alpha;
    //不起作用
    //self.navigationController.navigationBar.barTintColor = [UIColor colorWithRed:1.0 green:0.5 blue:0.0 alpha:alpha];
}