### 知乎导航栏效果

转自 http://www.jianshu.com/p/ae5e72dbb8dd

    UILabel *navTitleLabel = [BMFriendControlsTool friendTitleLabel];
    self.navTitleLabel = navTitleLabel;
    [self.navigationController.navigationBar addSubview:navTitleLabel];
    navTitleLabel.backgroundColor = [UIColor clearColor];
    navTitleLabel.frame = CGRectMake((SCREEN_WIDTH-200)*0.5, 44, 200, 44);
    self.navigationController.navigationBar.layer.masksToBounds = YES;

- (void)scrollViewDidScroll:(UIScrollView *)scrollView{
    CGFloat offsetY = scrollView.contentOffset.y;
    
    CGFloat delta = offsetY - self.lastOffsetY;
    
    NSLog(@"offsetY:%lf,,,delta:%lf",offsetY,delta);
    NSLog(@"self.nav.frame:%@",NSStringFromCGRect(self.navTitleLabel.frame));
    
    if (delta >= 30 && delta <= 70) {
        self.navTitleLabel.y = 70-delta;
        self.navTitleLabel.alpha = 1.f;
    }else if (delta >= 70) {
        self.navTitleLabel.y = 0;
    }else if (delta >= 0) {
        CGFloat alpha = delta-40 / (30);
        alpha = MAX(alpha, 0.f);
        alpha = MIN(alpha, 1.f);
        self.navTitleLabel.alpha = alpha;
    }
}