利用OnPreDrawListener监听绘制前的时机，在这个点控件已经完全ready（完成可measure, layout），此时可以获取到动画所必须的参数，也保证动画的流畅。

ShowButton.java

btnMenu.setVisibility(View.VISIBLE);
    btnClose.setVisibility(View.VISIBLE);
    btnRefresh.setVisibility(View.VISIBLE);
    btnMenu.getViewTreeObserver().addOnPreDrawListener(new ViewTreeObserver.OnPreDrawListener() {
      @Override
      public boolean onPreDraw() {
        btnMenu.getViewTreeObserver().removeOnPreDrawListener(this);
        ViewGroup.MarginLayoutParams params =
            (ViewGroup.MarginLayoutParams) btnClose.getLayoutParams();
        int marginBottom = params.bottomMargin;
        int translationY = btnClose.getHeight() + marginBottom;
        btnClose.setTranslationY(translationY);
        btnRefresh.setTranslationY(translationY);
        btnMenu.setTranslationY(translationY);

        Interpolator interpolator = new OvershootInterpolator();
        ObjectAnimator btnCloseAnimator =
            ObjectAnimator.ofFloat(btnClose, "translationY", 0);
        btnCloseAnimator.setDuration(ANIMATION_DURATION);
        btnCloseAnimator.setInterpolator(interpolator);

        ObjectAnimator btnRefreshAnimator =
            ObjectAnimator.ofFloat(btnRefresh, "translationY", 0);
        btnRefreshAnimator.setInterpolator(interpolator);
        btnRefreshAnimator.setDuration(ANIMATION_DURATION);

        ObjectAnimator btnMenuAnimator =
            ObjectAnimator.ofFloat(btnMenu, "translationY", 0);
        btnMenuAnimator.setInterpolator(interpolator);
        btnMenuAnimator.setDuration(ANIMATION_DURATION);

        AnimatorSet animatorSet = new AnimatorSet();
        animatorSet.play(btnCloseAnimator);
        animatorSet.play(btnRefreshAnimator).after(50);
        animatorSet.play(btnMenuAnimator).after(100);
        animatorSet.start();
        return true;
      }
    });