android fragment 使用popupwindow，弹出时在跳转前上一个Activity页面上，求解

Question

Context context = getContext();
View contentView = LayoutInflater.from(getActivity()).inflate(R.layout.exam_add1, null);
mPopWindow = new PopupWindow(contentView,
ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT, true);
View parent = getView();
mPopWindow.showAtLocation(getView(), Gravity.CENTER, 0, 0);
darkenBackground(0.2f);

页面逻辑是一个fragment Activity跳转到 下一个Fragment Activity,popupwindow是在当前fragment下操作显示,但显示在上一个Activity上，是什么原因，求教