android - 在 GridView 中加载异步图像 (Android)

Question

我有一个带有图片/文本的 GridView，从不同的服务器加载(我只知道 URL)。我试图根据 the tutorial of ryac 修改我的代码.

在我的 Activity 文件中，我将 GAdapter 设置为我的 GridView，如下所示:

GridView mGridMain = (GridView)findViewById(R.id.gvMain);
mGridMain.setAdapter(new GAdapter(this, listAppInfo));

我已经修改了自己的适配器并尝试适配它:

public class GAdapter extends BaseAdapter {

 private Context mContext;
 private List<SiteStaff> mListAppInfo;
 private HashMap<Integer, ImageView> views;

 /**
  * @param context
  * @param list
  */
 public GAdapter(Context context, List<SiteStaff> list) {
  mContext = context;
  mListAppInfo = list;  
 }

 @Override
 public int getCount() {
  return mListAppInfo.size();
 }

 @Override
 public Object getItem(int position) {
  return mListAppInfo.get(position);
 }

 @Override
 public long getItemId(int position) {
  return position;
 }

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

  SiteStaff entry = mListAppInfo.get(position);

  if(convertView == null) {
   LayoutInflater inflater = LayoutInflater.from(mContext);
   convertView = inflater.inflate(R.layout.list_g, null);
  }

  ImageView v;

  // get the ImageView for this position in the GridView..
  v = (ImageView)convertView.findViewById(R.id.ivIcon);

  //set default image
  v.setImageResource(android.R.drawable.ic_menu_gallery);

  TextView tvName = (TextView)convertView.findViewById(R.id.tvName);
  tvName.setText(entry.getName());


  Bundle b = new Bundle ();
  b.putString("file", "http://www.test.com/mypict.jpg");
  b.putInt("pos", position);

  // this executes a new thread, passing along the file
  // to load and the position via the Bundle..
  new LoadImage().execute(b);

  // puts this new ImageView and position in the HashMap.
  views.put(position, v);


  // return the view to the GridView..
  // at this point, the ImageView is only displaying the
  // default icon..
  return v;

 }




 // this is the class that handles the loading of images from the cloud
 // inside another thread, separate from the main UI thread..
 private class LoadImage extends AsyncTask<Bundle, Void, Bundle> {

  @Override
  protected Bundle doInBackground(Bundle... b) {

   // get the file that was passed from the bundle..
   String file = b[0].getString("file");

   URL UrlImage;
   Bitmap bm=null;
   try {
    UrlImage = new URL (file);
    HttpURLConnection connection;
    connection = (HttpURLConnection) UrlImage.openConnection(); 
    bm= BitmapFactory.decodeStream(connection.getInputStream());

   } catch (MalformedURLException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
   } catch (IOException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
   }   


   // now that we have the bitmap (bm), we'll
   // create another bundle to pass to 'onPostExecute'.
   // this is the method that is called at the end of 
   // our task. like a callback function..
   // this time, we're not passing the filename to this
   // method, but the actual bitmap, not forgetting to
   // pass the same position along..

   Bundle bundle = new Bundle();
   bundle.putParcelable("bm", bm);
   bundle.putInt("pos", b[0].getInt("pos"));
   bundle.putString("file", file); // this is only used for testing..

   return bundle;
  }

  @Override
  protected void onPostExecute(Bundle result) {
   super.onPostExecute(result);

   // just a test to make sure that the position and
   // file name are matching before and after the
   // image has loaded..
   Log.d("test", "*after: " + result.getInt("pos") + " | " + result.getString("file"));

   // here's where the photo gets put into the
   // appropriate ImageView. we're retrieving the
   // ImageView from the HashMap according to
   // the position..
   ImageView view = views.get(result.getInt("pos"));

   // then we set the bitmap into that view. and that's it.
   view.setImageBitmap((Bitmap) result.getParcelable("bm"));
  }

 }

}

但是显示如下错误:

11-16 06:16:08.285: E/AndroidRuntime(517): FATAL EXCEPTION: main
11-16 06:16:08.285: E/AndroidRuntime(517): java.lang.NullPointerException
11-16 06:16:08.285: E/AndroidRuntime(517):  at common.adapter.GAdapter.getView(GAdapter.java:139)
11-16 06:16:08.285: E/AndroidRuntime(517):  at android.widget.AbsListView.obtainView(AbsListView.java:1315)
11-16 06:16:08.285: E/AndroidRuntime(517):  at android.widget.GridView.onMeasure(GridView.java:932)
11-16 06:16:08.285: E/AndroidRuntime(517):  at android.view.View.measure(View.java:8171)
11-16 06:16:08.285: E/AndroidRuntime(517):  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3132)
11-16 06:16:08.285: E/AndroidRuntime(517):  at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1012)
11-16 06:16:08.285: E/AndroidRuntime(517):  at android.widget.LinearLayout.measureVertical(LinearLayout.java:381)
11-16 06:16:08.285: E/AndroidRuntime(517):  at android.widget.LinearLayout.onMeasure(LinearLayout.java:304)
11-16 06:16:08.285: E/AndroidRuntime(517):  at android.view.View.measure(View.java:8171)
11-16 06:16:08.285: E/AndroidRuntime(517):  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3132)
11-16 06:16:08.285: E/AndroidRuntime(517):  at android.widget.FrameLayout.onMeasure(FrameLayout.java:245)
11-16 06:16:08.285: E/AndroidRuntime(517):  at android.view.View.measure(View.java:8171)

到目前为止，我从未在 Android 中使用过线程。如果有人能帮助我，我将不胜感激。

Answer 1

我在你做的最后输入了 getView()

views.put(position, v);

但它views 从未初始化。这就是您得到 java.lang.NullPointerException

的原因

为避免这种情况，请在您的 GAdapter 构造函数中初始化 HashMap 对象

public GAdapter(Context context, List<SiteStaff> list) {
  mContext = context;
  mListAppInfo = list;  
  views = new HashMap<Integer, ImageView>();

 }

如果您检查您在问题中引用的示例，您会发现

public ImageAdapter (Context c, String f, ArrayList<HashMap<String, Object>> p) {
        ...
        views = new HashMap<Integer, ImageView>();
    }