ListView适配器中的ViewHolder无法正常工作

Question

我一直在尝试使用ViewPager使ListView滚动更平滑，但是它一直崩溃。 我已经尝试了3种解决方案。 屏幕打开，显示加载指示器，但过一会儿（约3-5秒）它崩溃。 要从URL加载图像，我使用nostra13的Universal-Image-Loader lib 。 无需添加ViewHolder，一切运行良好。

我的LogCat和Adapter类中的getView + ViewHolder：

logcat的

    01-30 17:01:53.713: E/AndroidRuntime(12702): FATAL EXCEPTION: main
01-30 17:01:53.713: E/AndroidRuntime(12702): java.lang.NullPointerException
01-30 17:01:53.713: E/AndroidRuntime(12702):    at com.example.animalist.AnimalAdapter$1.onLoadingComplete(AnimalAdapter.java:123)
01-30 17:01:53.713: E/AndroidRuntime(12702):    at com.nostra13.universalimageloader.core.DisplayBitmapTask.run(DisplayBitmapTask.java:74)
01-30 17:01:53.713: E/AndroidRuntime(12702):    at android.os.Handler.handleCallback(Handler.java:587)
01-30 17:01:53.713: E/AndroidRuntime(12702):    at android.os.Handler.dispatchMessage(Handler.java:92)
01-30 17:01:53.713: E/AndroidRuntime(12702):    at android.os.Looper.loop(Looper.java:143)
01-30 17:01:53.713: E/AndroidRuntime(12702):    at android.app.ActivityThread.main(ActivityThread.java:4263)
01-30 17:01:53.713: E/AndroidRuntime(12702):    at java.lang.reflect.Method.invokeNative(Native Method)
01-30 17:01:53.713: E/AndroidRuntime(12702):    at java.lang.reflect.Method.invoke(Method.java:507)
01-30 17:01:53.713: E/AndroidRuntime(12702):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
01-30 17:01:53.713: E/AndroidRuntime(12702):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
01-30 17:01:53.713: E/AndroidRuntime(12702):    at dalvik.system.NativeStart.main(Native Method)
01-30 17:01:57.957: I/wpa_supplicant(15739): Reset vh_switch_counter due to receive LINKSPEED cmd

在AnimalAdapter.java中-getView + ViewHolder

 static class ViewHolder{
        TextView animalView;
        TextView areaView;
        ImageView notfound;
        ImageView animalPic;
        ProgressBar indicator;
    }


    public View getView(int position, View convertView, ViewGroup parent){

        ViewHolder holder = null;



        if(convertView == null){
              LayoutInflater mLayoutInflater = LayoutInflater.from(mContext);
              convertView = mLayoutInflater.inflate(R.layout.animal_row_item, null);
              holder = new ViewHolder();

             holder.animalView = (TextView) convertView.findViewById(R.id.animal_text);
              holder.areaView = (TextView) convertView.findViewById(R.id.area_text);
            holder.notfound = (ImageView) convertView.findViewById(R.id.notfoundimg);
              holder.animalPic = (ImageView)convertView.findViewById(R.id.animal_pic);
              holder.indicator = (ProgressBar)convertView.findViewById(R.id.progress);
               convertView.setTag(holder);
          }

        else{
            holder = (ViewHolder) convertView.getTag();

        }  

          final Animal animal = mAnimals.get(position);




          holder.notfound.setVisibility(View.INVISIBLE);
          holder.indicator.setVisibility(View.VISIBLE);
          holder.animalPic.setVisibility(View.INVISIBLE);

            //Setup a listener we can use to switch from the loading indicator to the Image once it's ready
            ImageLoadingListener listener = new ImageLoadingListener(){


                ViewHolder holder = null;
                @Override
                public void onLoadingStarted(String arg0, View arg1) {
                    // TODO Auto-generated method stub

                }

                @Override
                public void onLoadingCancelled(String arg0, View arg1) {
                    // TODO Auto-generated method stub

                }

                @Override
                public void onLoadingComplete(String arg0, View arg1, Bitmap arg2) {
                    holder.indicator.setVisibility(View.INVISIBLE);
                    holder.animalPic.setVisibility(View.VISIBLE);
                    holder.notfound.setVisibility(View.INVISIBLE);
                }

                @Override
                public void onLoadingFailed(String arg0, View view, FailReason arg2) {
                    holder.notfound.setVisibility(View.VISIBLE);
                    holder.indicator.setVisibility(View.INVISIBLE);
                    holder.animalPic.setVisibility(View.INVISIBLE);
                }

            };



          imageLoader.displayImage(animal.getImgUrl(), holder.animalPic,options, listener);
          holder.animalView.setText(animal.getAnimal());
          holder.areaView.setText(animal.getArea());





          convertView.setOnClickListener(new OnClickListener() {

                @Override
                public void onClick(View view) {

                    Intent intent = new Intent(getContext(), MoreActivity.class);

                    intent.putExtra("about", animal.getAbout());
                    intent.putExtra("animal", animal.getAnimal());
                    intent.putExtra("imgUrl", animal.getImgUrl());
                    getContext().startActivity(intent);
                }
          });

          return convertView;
      }

您有什么想法，如何改进？

提前致谢

Answer 1

你在堆栈跟踪中的事实

01-30 16:45:09.003: E/AndroidRuntime(12533):    at android.widget.ListView.measureHeightOfChildren(ListView.java:1264)
01-30 16:45:09.003: E/AndroidRuntime(12533):    at android.widget.ListView.onMeasure(ListView.java:1127)

建议您返回的convertView为null 。

关于第二个问题， ImageLoadingListener的ViewHolder实例始终为null 。 您应该注意相应地对其进行初始化

Answer 2

我认为您的逻辑不正确，请在您的getView()方法中进行测试：

if (vi == null){

但是您应该在此测试是否：

if (convertView == null) {

尝试更改它，应以这种方式工作。

----编辑----

初始化ViewHolder的方式对我来说似乎是可以的，然后通过findViewById()为每个实例分配一个View ，这是可以的-如果您收到设置可见性的NullPointerException ，这意味着由于某种原因findViewById()返回了空对象-即，它找不到名称为R.id.progress的ID（可能在您的布局中没有使用该名称的此类元素）。

----编辑----

这是适配器的getView()方法的示例：

public static class vhItem {
  TextView maintext;
  TextView topictext; 
  ImageView mainimage;
  TextView users; 
}

@Override
public View getView(final int position, View convertView, final ViewGroup parent) {
  vhItem viewHolder;

  if (convertView == null) {
    final LayoutInflater inflater = ((Activity) context).getLayoutInflater();
    convertView = inflater.inflate(resourceId, parent, false);

    viewHolder = new vhItem();
    viewHolder.maintext = (TextView) convertView.findViewById(R.id.chanlist_chan);
    viewHolder.topictext = (TextView) convertView.findViewById(R.id.chanlist_topic);
    viewHolder.mainimage = (ImageView) convertView.findViewById(R.id.chanlist_featured);
    viewHolder.users = (TextView) convertView.findViewById(R.id.chanlist_usercount);

    // I've set a contextual menu for each row this way, but it's not required
    ((Activity) context).registerForContextMenu(convertView);
  }
  else
    viewHolder = (vhItem) convertView.getTag();

  viewHolder.maintext.setText((String) getItem(position));
  viewHolder.mainimage.setTag(getItem(position));
  viewHolder.mainimage.setOnClickListener(new View.OnClickListener() {
    public void onClick(final View v) {
      AlertDialog.Builder builder = new AlertDialog.Builder(context);
      ...
      dialog.show();
    }
  });

  return convertView; 
}