点击显示popupWindow背景切换透明，popupWindow消失背景恢复
private void bgAlpha(float bgAlpha) {
WindowManager.LayoutParams lp = getWindow().getAttributes();
lp.alpha = bgAlpha;//0.0-1.0
getWindow().addFlags(WindowManager.LayoutParams.FLAG_DIM_BEHIND);
getWindow().setAttributes(lp);
点击显示popupWindow时调用
bgAlpha(0.5f);
popupWindow.setOnDismissListener(new PopupWindow.OnDismissListener() {
@Override
public void onDismiss() {
bgAlpha(1f);
}
});
