【Android开发坑系列】之PopupWindow

1. PopupWindow在4.0之前的版本有个系统级别的BUG，必须借助一段自定义的fix代码来修复。其中mPopPm就是PopupWindow实例。
   java.lang.NullPointerException
   at android.widget.PopupWindow$1.onScrollChanged

   if (Build.VERSION.SDK_INT < Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
               try {
                   final Field fAnchor = PopupWindow.class.getDeclaredField("mAnchor");
                   fAnchor.setAccessible(true);
                   Field listener = PopupWindow.class.getDeclaredField("mOnScrollChangedListener");
                   listener.setAccessible(true);
                   final ViewTreeObserver.OnScrollChangedListener originalListener = (ViewTreeObserver.OnScrollChangedListener) listener
                           .get(mPopPm);
                   ViewTreeObserver.OnScrollChangedListener newListener =
                           new ViewTreeObserver.OnScrollChangedListener() {
                               public void onScrollChanged() {
                                   try {
                                       View mAnchor = (View) fAnchor.get(mPopPm);
                                       if (mAnchor == null) {
                                           return;
                                       } else {
                                           originalListener.onScrollChanged();
                                       }
                                   } catch (Exception e) {
                                       e.printStackTrace();
                                   }
                               }
                           };
                   listener.set(mPopPm, newListener);
               } catch (Exception e) {
                   e.printStackTrace();
               }

2. 在生成popupWindow的父Activity销毁之前，需要销毁popupWindow。否则会报内存泄露(leak)。也即在Activity的onDestroy()执行如下代码：

           if (mPopupWindow != null) {
               mPopupWindow.dismiss();
               mPopupWindow = null;
           }

3. 在<=2.3的系统上，慎用setFocusable(boolean)，一般设为mPopPm.setFocusable(false)