ScrollView里面基于某个View弹出PopupWindow，PopupWindow不会跟着View滚动？

遇到这样一个需求：根布局为ScrollView，内部有一个TextView，叫它A，需要在A正上方弹出一个PopupWindow。
初看其实很简单，根据A的位置和大小，算出PopupWindow的位置，调用showAtLocation方法即可实现在A正上方弹出。
代码如下：

showPopAlongView(textView);

private void showPopAlongView(View v) {
        View view = View.inflate(this, R.layout.pop_layout, null);
        view.measure(View.MeasureSpec.UNSPECIFIED, View.MeasureSpec.UNSPECIFIED);
        int measureHeight = view.getMeasuredHeight();
        int measureWidth = view.getMeasuredWidth();
        PopupWindow popupWindow = new PopupWindow(view, ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);
        popupWindow.setFocusable(false);
        popupWindow.setBackgroundDrawable(new ColorDrawable(0x00000000));
        popupWindow.setOutsideTouchable(false);
        int[] location = new int[2];
        v.getLocationOnScreen(location);
        popupWindow.showAtLocation(v, Gravity.NO_GRAVITY, location[0] + v.getMeasuredWidth() / 2 - measureWidth / 2, location[1] - measureHeight);
    }

如上，就可以在textView正上方显示出一个PopupWindow。看起来一切都好，但是当ScrollView开始上下滚动的时候，就有问题了：

可以发现，我们的PopupWindow并没有跟随一起滚动，这是因为在调用showAtLocation的时候，PopupWindow的位置已经确定了，所以并不会跟着A的滚动而滚动。
这样的效果产品经理肯定不会同意的，所以就需要我们来动态更新PopupWindow的位置了。
来看PopupWindow的一个方法：

    /** * Updates the position and the dimension of the popup window. * <p> * Width and height can be set to -1 to update location only. Calling this * function also updates the window with the current popup state as * described for {@link #update()}. * * @param x the new x location * @param y the new y location * @param width the new width in pixels, must be >= 0 or -1 to ignore * @param height the new height in pixels, must be >= 0 or -1 to ignore */
    public void update(int x, int y, int width, int height) {
        update(x, y, width, height, false);
    }

用这个方法可以更新PopupWindow的尺寸和位置。我们这里只需要让PopupWindow始终在A的正上方，所以重点就是确定随着ScrollView的滚动PopupWindow的位置。

首先定义变量来保存PopupWindow的初始位置和大小

    private int orginalX, originalY;

    private int popWidth, popHeight;

ScrollView设置滚动监听，计算出PopupWindow新的y坐标，调用update方法更新。

    scrollView.setOnScrollChangeListener(this);

    /** * Called when the scroll position of a view changes. * * @param v The view whose scroll position has changed. * @param scrollX Current horizontal scroll origin. * @param scrollY Current vertical scroll origin. * @param oldScrollX Previous horizontal scroll origin. * @param oldScrollY Previous vertical scroll origin. */
    @Override
    public void onScrollChange(View v, int scrollX, int scrollY, int oldScrollX, int oldScrollY) {
        int currentY = originalY - scrollY;
        popupWindow.update(orginalX, currentY, popWidth, popHeight);
    }