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我使用從ArrayAdapter擴展的自定義適配器實現了我的ListView。 我的問題有時是ListView緩慢加載。這意味着一個空白活動首先加載沒有ListView，然後ListView出來。在最壞的情況下，我被提示「強行關閉或等待」。我喜歡改善緩慢加載，因爲這對用戶來說很煩人。如何提高ListView的加載時間？
但有時，加載速度很快，幾乎立即。
但我想確保我的ListView設計是正確的，並且設計沒有任何緩慢加載的問題。所以這個討論對於那些面臨和我一樣的問題的其他人是有用的。
我的ListView的設計如下。
每個ListItem有三個組件，縮略圖，ID文本和箭頭圖像，如圖所示
。
在ListView的加載過程中，（1）所有ID文本被從數據庫中檢索並填充到ListArray List<String> listIDs
public class MyListFragment extends ListFragment implements OnItemClickListener {
dbHelper = new TrackerDBAdapter(getActivity());
dbHelpLatLong = new LatLogDBAdapter(getActivity());
dbHelpNotification = new NotificationDatabaseAdapter(getActivity());
dbHelper.open();
Cursor cursor = dbHelper.fetchAllTrackerInTheList();
listIDs = new ArrayList<String>();
activationStatus = new ArrayList<String>();
thisListFragmentContext = getActivity();
for (cursor.moveToFirst(); !cursor.isAfterLast(); cursor.moveToNext()) {
listIDs.add(cursor.getString(1));
}
dbHelper.close();
(2)Then my custom list adapter is called.
adapter = new customList_Adaptor(thisListFragmentContext,
R.layout.list_row, listIDs, this);
}
That is the loading process inside my `ListFragment`.
（3）下面的類是我的自定義ArrayAdapter和我實現加載縮略圖ImageView使用AsyncTask。我的查詢是
（i）我仍然從數據庫檢索ID文本，並加載箭頭圖像。我是否也應該將這些流程放入AsyncTask？ （ii）如果我需要它，我應該執行另一個AsyncTask或使用相同的AsyncTask用於縮略圖圖像加載？ （iii）其中，程序設計的哪個方面我仍然可以改進，對我的緩慢加載有疑慮？
public class customList_Adaptor extends ArrayAdapter<String>{
protected static final int CAMERA_REQUEST = 0;
private TrackerDBAdapter dbHelper;
private Context context;
private List<String> listIDs = new ArrayList<String>();
private List<String> activationState = new ArrayList<String>();
public MyListFragment mMyListFragment;
public customList_Adaptor(Context context, int textViewResourceId,
List<String> objects, List<String> activationStatus, MyListFragment mMyListFragment) {
super(context, textViewResourceId, objects);
this.setContext(context);
this.listIDs = objects;
this.activationState = activationStatus;
this.mMyListFragment= mMyListFragment;
dbHelper = new TrackerDBAdapter(context);
}
@Override
public int getCount() {
// TODO Auto-generated method stub
if(listIDs != null)
return listIDs.size();
else
return 0;
}
@Override
public String getItem(int arg0) {
// TODO Auto-generated method stub
if(listIDs != null)
return listIDs.get(arg0);
else
return null;
}
@Override
public long getItemId(int arg0) {
// TODO Auto-generated method stub
return arg0;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
View vi=convertView;
ViewHolder viewHolder=new ViewHolder();
LayoutInflater inflater = (LayoutInflater) this.getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
if(vi==null){
vi = inflater.inflate(R.layout.list_row, parent, false);
viewHolder.id=(TextView)vi.findViewById(R.id.title);
viewHolder.thumbnailImage=(ImageView)vi.findViewById(R.id.list_image);
viewHolder.activationStatus = (TextView)vi.findViewById(R.id.activated);
//lazy load image
BitmapWorkerTask task = new BitmapWorkerTask(viewHolder.thumbnailImage);
task.execute(position);
viewHolder.arrow=(ImageView)vi.findViewById(R.id.list_arrow);
vi.setTag(viewHolder);
}
else
viewHolder=(ViewHolder) vi.getTag();
viewHolder.thumbnailImage.setOnClickListener(new onMyClick(position));
// Setting all values in listview
viewHolder.id.setText(listIDs.get(position));
if(activationState.get(position).equals("Not activated yet")){
viewHolder.activationStatus.setText(activationState.get(position));
viewHolder.activationStatus.setTextColor(android.graphics.Color.GRAY);
}
else if(activationState.get(position).equals("Activated"))
viewHolder.activationStatus.setText("");
return vi;
}
public class onMyClick implements OnClickListener {
private final int pos;
public onMyClick(int pos) {
this.pos = pos;
}
@Override
public void onClick(View v) {
MyListFragment.clickedimageView = (ImageView) v.findViewById(R.id.list_image);
mMyListFragment.imagepos(pos);
}
}
public Context getContext() {
return context;
}
public void setContext(Context context) {
this.context = context;
}
//Lazy image update
class BitmapWorkerTask extends AsyncTask<Integer, Void, Bitmap> {
private final WeakReference<ImageView> imageViewReference;
private int data = 0;
public BitmapWorkerTask(ImageView imageView) {
// Use a WeakReference to ensure the ImageView can be garbage collected
imageViewReference = new WeakReference<ImageView>(imageView);
}
// Decode image in background.
@Override
protected Bitmap doInBackground(Integer... params) {
setData(params[0]);
Bitmap bitmap = null;
dbHelper.open();
Cursor mCursor = dbHelper.getImagebyIDnumber(getData());
byte[] img_bytes = mCursor.getBlob(13);
bitmap = BitmapFactory.decodeByteArray(img_bytes, 0, img_bytes.length);
dbHelper.close();
return bitmap;
}
// Once complete, see if ImageView is still around and set bitmap.
@Override
protected void onPostExecute(Bitmap bitmap) {
if (imageViewReference != null && bitmap != null) {
final ImageView imageView = imageViewReference.get();
if (imageView != null) {
imageView.setImageBitmap(bitmap);
}
}
}
public int getData() {
return data;
}
public void setData(int data) {
this.data = data;
}
}
}
public class ViewHolder {
TextView id;
TextView activationStatus;
ImageView thumbnailImage;
ImageView arrow;
}
我使用數據庫來保存ListItem的所有內容。因此從數據庫中檢索數據是加載時的響應速度慢。所以我嘗試使用AsyncTask來檢索ListItem內容。但問題是，如果在調用自定義適配器時後臺進程仍在運行，則不會顯示ListView。 – batuman