方式一

   /**
     * @param headLayout
     * 头部布局
     * @param imageview
     * 标题
     */
    Toolbar toolbar;
    View headView;

    public void setTitleAndHead(Toolbar toolbar, View headView) {
        this.toolbar = toolbar;
        this.headView = headView;
    }

    @Override
    protected void onScrollChanged(int l, int t, int oldl, int oldt) {
        if (headView != null && toolbar != null) {
            float headHeight = headView.getMeasuredHeight()
                    - toolbar.getMeasuredHeight();
            int alpha = (int) (((float) t / headHeight) * 255);
            if (alpha >= 255)
                alpha = 255;
            if (alpha <= 0)
                alpha = 0;
            toolbar.getBackground().mutate().setAlpha(alpha);
        }

        super.onScrollChanged(l, t, oldl, oldt);
    }

方式二

/**
     * 改变Toolbar的透明度
     */
    private void changeToolbarAlpha() {
        int scrollY = scrollView.getScrollY();
        //快速下拉会引起瞬间scrollY < 0
        if (scrollY < 0) {
            toolbar.getBackground().mutate().setAlpha(0);
            return;
        }
        //计算当前的透明度
        float radio = Math.min(1, scrollY / (ivImg.getHeight() - toolbar.getHeight() * 1f));
        //设置透明度
        toolbar.getBackground().mutate().setAlpha((int) (radio * 0xFF));
    }

方式三

@Override
 public void onScrollChange(NestedScrollView v, int scrollX, int scrollY, int oldScrollX, int oldScrollY) {
        int abs_y = Math.abs(scrollY);
         //如果在变化的范围内
         if ((scrollLength - abs_y) > 0) {
          // 算法：开始值 +（结束值 - 开始值）* 进度
         IntEvaluator evaluator = new IntEvaluator();

         // 总数-变动的值/总数 （ 变动的值范围：0~总数 ）
         // 结果比例：1 ~ 0
           float percent = (float) (scrollLength - abs_y) / scrollLength;

          //标题栏透明度
          int evaluate = evaluator.evaluate(percent, 255, 0);
           rv_bar.getBackground().mutate().setAlpha(evaluate);

         }else {
           rv_bar.getBackground().mutate().setAlpha(255);
          }