【Android開發坑系列】之PopupWindow

1. PopupWindow在4.0以前的版本有個系統級別的BUG，必須藉助一段自定義的fix代碼來修復。其中mPopPm就是PopupWindow實例。
   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銷燬以前，須要銷燬popupWindow。不然會報內存泄露(leak)。也即在Activity的onDestroy()執行以下代碼：
   

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

3. 在<=2.3的系統上，慎用setFocusable(boolean)，通常設爲mPopPm.setFocusable(false)