Android AsyncTaskLoader需要注意的问题

在使用AsyncTaskLoader的时候,遇到两个问题:
1.继承AsyncTaskLoader并实现了必要的方法后,发现loadInBackground()没有被执行
  在网上查找之后,得到如下解决方法:

继承AsyncTaskLoader后,需要重载以下方法供系统调用:

@Overrideprotected void onStartLoading() {// TODO Auto-generated method stubsuper.onStartLoading();forceLoad();}

2.使用问题1的解决方案后,Loader开始工作了,但是又遇到了新的问题,我在Activity里面写了一个ListView,ListView的数据通过AsyncTaskLoader
  来进行加载,当从当前Activity跳到另一个Activity并按back键返回时,此时没有问题,但是当点击ListView中的某一项时,报出以下的错误:
  E/AndroidRuntime(1141): java.lang.IllegalStateException: The content of the adapter has changed but ListView did not receive a notification. Make sure the content of your adapter is not modified from a background thread, but only from the UI thread. [in ListView(16908298, class android.widget.ListView) with Adapter(class com.study.ActivityAdapter)]
以下是我自定义AsyncTaskLoader的代码:

public class ActivityListLoader extends AsyncTaskLoader<Integer>{private Context context;private Bundle args;private String path;private List<Map<String, Object>> data;public ActivityListLoader(Context context,Bundle args) {super(context);this.context = context;this.args = args;path = args.getString("path");data = new ArrayList<Map<String,Object>>();}@Overridepublic Integer loadInBackground() {PackageManager pm = context.getPackageManager();Intent matchIntent = new Intent();matchIntent.setAction(Constants.ACTION_MAIN);matchIntent.addCategory(Constants.CATEGORY_STUDY);matchIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);List<ResolveInfo> list = pm.queryIntentActivities(matchIntent,0);String label = null;for(ResolveInfo info:list){label = info.loadLabel(pm).toString();if(label.startsWith(path)){String[] paths = path.split("/");String[] files = label.split("/");Map<String, Object> map = new HashMap<String, Object>();Intent intent = new Intent();intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);int pathlen = paths.length;if(path.equals("")){pathlen = 0;}if(files.length - pathlen == 1){map.put("isFile", true);intent.setComponent(new ComponentName(info.activityInfo.packageName, info.activityInfo.name));map.put("intent", intent);}else{map.put("isFile", false);intent.setClass(context, MainActivity.class);intent.putExtra("path", path+files[pathlen]+"/");map.put("intent", intent);}data.add(map);}}return 0;}@Overrideprotected void onStartLoading() {// TODO Auto-generated method stubsuper.onStartLoading();forceLoad();}public List<Map<String, Object>> getData(){return data;}}MainActivity的代码:public class MainActivity extends ListActivity implementsLoaderCallbacks<Integer> {private ListActivity context;private List<Map<String,Object>> datalist;enum Type {ShowProgress, DismissProgress,UpdateAdapter};private ProgressDialog progress;private Handler handler = new Handler() {@Overridepublic void handleMessage(Message msg) {Type type = (Type) msg.obj;switch (type) {case ShowProgress:progress.show();break;case DismissProgress:if(progress.isShowing())progress.dismiss();break;case UpdateAdapter:ActivityAdapter adapter = new ActivityAdapter(context, datalist);setListAdapter(adapter);adapter.notifyDataSetChanged();break;default:break;}}};@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);context = this;initProgressDialog();Intent intent = getIntent();String path = intent.getStringExtra("path");if (path == null)path = "";Bundle args = new Bundle();args.putString("path", path);getLoaderManager().initLoader(0, args, this);Message msg = Message.obtain(handler, 0, Type.ShowProgress);msg.sendToTarget();setListAdapter(new ActivityAdapter(this, new ArrayList<Map<String,Object>>()));getListView().setOnItemClickListener(new OnItemClickListener() {@Overridepublic void onItemClick(AdapterView<?> parent, View view,int position, long id) {@SuppressWarnings("unchecked")Map<String, Object> map = (Map<String, Object>) getListAdapter().getItem(position);Intent intent = (Intent)map.get("intent");startActivity(intent);}});}@Overridepublic boolean onCreateOptionsMenu(Menu menu) {// Inflate the menu; this adds items to the action bar if it is present.getMenuInflater().inflate(R.menu.main, menu);return true;}private void initProgressDialog(){progress = new ProgressDialog(this);progress.setMessage("加载所有的Activity信息");progress.setCanceledOnTouchOutside(false);}@Overridepublic Loader<Integer> onCreateLoader(int id, Bundle args) {return new ActivityListLoader(context, args);}@Overridepublic void onLoadFinished(Loader<Integer> loader, Integer data) {ActivityListLoader aloader = (ActivityListLoader) loader;datalist = aloader.getData();LogUtils.e("debug");Message msg = Message.obtain(handler, 0, Type.DismissProgress);msg.sendToTarget();Message msgUpdate = Message.obtain(handler, 0, Type.UpdateAdapter);msgUpdate.sendToTarget();}@Overridepublic void onLoaderReset(Loader<Integer> loader) {LogUtils.e("debug");Message msgUpdate = Message.obtain(handler, 0, Type.UpdateAdapter);msgUpdate.sendToTarget();}}

更新ListView数据的操作都是发生在UI线程,所以应该是系统在我们不知道的地方更新了数据而没有通知ListView.
查看Activity.java的源码可以看到:

protected void onStart() {if (DEBUG_LIFECYCLE) Slog.v(TAG, "onStart " + this);mCalled = true;if (!mLoadersStarted) {mLoadersStarted = true;if (mLoaderManager != null) {mLoaderManager.doStart(); //系统会执行Loader的onStartLoading函数} else if (!mCheckedForLoaderManager) {mLoaderManager = getLoaderManager(null, mLoadersStarted, false);}mCheckedForLoaderManager = true;}getApplication().dispatchActivityStarted(this);}

Activity恢复的时候,会重新调用onStart函数,从而最终调用到我们Loader里的onStartLoading函数,
而我们的onStartLoading函数是这样:

@Overrideprotected void onStartLoading() {// TODO Auto-generated method stubsuper.onStartLoading();forceLoad();}

也就是说,不管如何,都会在后台将数据重新加载一遍,因为是系统调用的,所以没有通知ListView,造成数据的不同步.
到这里,解决方案就呼之欲出了:
改造如下:

@Overrideprotected void onStartLoading() {// TODO Auto-generated method stubsuper.onStartLoading();if(data.size() == 0)forceLoad();}