Android 3.0中提供了一个新概念Loaders，这两天看了之前Contacts的源码，其中自己写了通话记录部分，发现源码里面有很多LoaderManager的使用，现在做一个总结，记录一下LoaderManager的基本用法：
这个其实是分为两部分：LoaderManager和自定义Loader，比如像加载联系人，短信这些系统提供ContentProvider的数据时，其实就是返回一个标准cursor,那我们就没必要自定义loader 直接用loaderManager来管理这个CursorLoader,这是一种简单普遍的情况。还有一种就是大家要加载非cursor类型的数据时，就必须自定义loader了。
1. LoaderManager和CursorLoader
举例用一个listFragment加载联系人，那么让它实现LoaderManager.LoaderCallbacks<Cursor> 接口，必须重写下面3个方法：
onCreateLoader： 这个是创建一个CursorLoader并返回，我们在里面new一个CursorLoader并返回就OK了
onLoadFinished： 这个是加载完成后更新UI，在这里就是setAdapter了 而这个加载过程其实就是在CursorLoader里面完成的，只不过系统帮我们完成了，而假如自定义loader的话就要自己完成，这就是区别！
onLoaderReset： loader的重置，在这里一般让UI不显示数据就行
就这么3个方法就可以完成数据加载了，核心就是把CursorLoader构造出来，至于加载过程，数据监听更新这些就有系统去完成吧！
public class ContactsFragment extends ListFragment implements LoaderManager.LoaderCallbacks
<Cursor>{
private SimpleCursorAdapter mAdapter;
private final String[] CONTACTS_PROJECTION = new String[]{Contacts._ID,
Co<span style="font-size:14px;">ntacts.DISPL</span>AY_NAME,
Contacts.CONTACT_STATUS,
Contacts.CONTACT_PRESENCE,
Contacts.PHOTO_ID,
Contacts.LOOKUP_KEY,
};
@Override
public void onActivityCreated(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onActivityCreated(savedInstanceState);
setEmptyText("当前没有联系人");
mAdapter = new SimpleCursorAdapter(getActivity(),
android.R.layout.simple_list_item_2, null,
new String[] { Contacts.DISPLAY_NAME, Contacts.CONTACT_STATUS },
new int[] { android.R.id.text1, android.R.id.text2 }, 0);
setListAdapter(mAdapter);
getLoaderManager().initLoader(1, null, this);
}
@Override
public Loader<Cursor> onCreateLoader(int arg0, Bundle arg1) {
// TODO Auto-generated method stub
String select = "((" + Contacts.DISPLAY_NAME + " NOTNULL) AND ("
+ Contacts.HAS_PHONE_NUMBER + "=1) AND ("
+ Contacts.DISPLAY_NAME + " != '' ))";
return new CursorLoader(getActivity(), Contacts.CONTENT_URI,
<span style="font-size:14px;"> </span>CONTACTS_PROJECTION, select, null, Contacts.DISPLAY_NAME + " COLLATE LOCALIZED ASC");
}
@Override
public void onLoadFinished(Loader<Cursor> arg0, Cursor arg1) {
// TODO Auto-generated method stub
mAdapter.swapCursor(arg1);
}
@Override
public void onLoaderReset(Loader<Cursor> arg0) {
// TODO Auto-generated method stub
mAdapter.swapCursor(null);
}
}
2. 自定义Loader
但有些拿不到cursor这样的数据，我们就需要自定义Loader，其实在Loader里面只需要重写两个方法：一个onStartLoading，还有一个loadInBackground，但为了代码更加完善一些，再重写另外几个方法： onStopLoading onReset onCanceled，可以类比activity的生命周期，然后参考google文档进行重写，先贴出代码：
public class MySmsLoader extends AsyncTaskLoader<List<SmsEntry>> {
private PackageManager mPm;
private InstalledAppsObserver mAppsObserver;
public MySmsLoader(Context context) {
super(context);
// TODO Auto-generated constructor stub
mPm = getContext().getPackageManager();
Log.d("dml", "new一个MySmsLoader");
}
@Override
public List<SmsEntry> loadInBackground() {
Log.d("dml", "loadInBackground--------" );
// TODO Auto-generated method stub
// Retrieve all installed applications.
List<ApplicationInfo> apps = mPm.getInstalledApplications(0);
Log.d("dml", "apps >>>>>>>>>>" + apps);
if (apps == null) {
Log.d("dml", "apps null");
apps = new ArrayList<ApplicationInfo>();
}
// Create corresponding array of entries and load their labels.
List<SmsEntry> entries = new ArrayList<SmsEntry>(apps.size());
for (int i = 0; i < apps.size(); i++) {
SmsEntry entry = new SmsEntry();
entry.setFrom_name(i+"::::::::");
entry.setSend_time(apps.get(i).packageName);
entries.add(entry);
}
Log.d("dml", "entries---->>>>>>>>>----" + entries);
return entries;
}
@Override
protected void onStartLoading() {
// TODO Auto-generated method stub
super.onStartLoading();
if (mAppsObserver == null) {
mAppsObserver = new InstalledAppsObserver(this);
}
forceLoad();
}
@Override
protected void onStopLoading() {
// The Loader has been put in a stopped state, so we should attempt to
// cancel the current load (if there is one).
cancelLoad();
// Note that we leave the observer as is; Loaders in a stopped state
// should still monitor the data source for changes so that the Loader
// will know to force a new load if it is ever started again.
}
@Override
protected void onReset() {
// Ensure the loader is stopped.
onStopLoading();
// The Loader is being reset, so we should stop monitoring for changes.
if (mAppsObserver != null) {
getContext().unregisterReceiver(mAppsObserver);
mAppsObserver = null;
}
}
@Override
public void onCanceled(List<SmsEntry> apps) {
// Attempt to cancel the current asynchronous load.
super.onCanceled(apps);
// The load has been canceled, so we should release the resources
// associated with 'mApps'.
releaseResources(apps);
}
private void releaseResources(List<SmsEntry> apps) {
// For a simple List, there is nothing to do. For something like a Cursor,
// we would close it in this method. All resources associated with the
// Loader should be released here.
}
}
onStartLoading：注册一些监听器到loader上，并且执行一次forceLoad(); 否则loader不会开始工作
loadInBackground：不用说，在这里就是加载数据并且返回，其实这个数据就返回到了LoaderManager的
onLoadFinished方法第二个参数
onStopLoading：停止加载数据，但不要停止监听也不要释放数据，就可以随时重启loader
onReset：先确保已经停止加载数据了，然后释放掉监听器并设为null
onCanceled： 在这里可以释放资源，假如是list就不需要做什么了，但是象cursor或者打开了什么文件就应该关闭一下
这几个生命周期方法还不是非常清楚，但就象activity的生命周期一样，可以里面的细节不是很了解 但我们还是可以很熟练的使用activity，除了这几个还有一个deliverResult()，确实没弄懂，留到以后在体会吧！
下面就是监听器的代码，注意是在构造方法中绑定到loader：
public class InstalledAppsObserver extends BroadcastReceiver {
private static final String TAG = "ADP_InstalledAppsObserver";
private static final boolean DEBUG = true;
private MySmsLoader mLoader;
public InstalledAppsObserver(MySmsLoader loader) {
mLoader = loader;
// Register for events related to application installs/removals/updates.
IntentFilter filter = new IntentFilter(Intent.ACTION_PACKAGE_ADDED);
filter.addAction(Intent.ACTION_PACKAGE_REMOVED);
filter.addAction(Intent.ACTION_PACKAGE_CHANGED);
filter.addDataScheme("package");
mLoader.getContext().registerReceiver(this, filter);
// Register for events related to sdcard installation.
IntentFilter sdFilter = new IntentFilter();
sdFilter.addAction(Intent.ACTION_EXTERNAL_APPLICATIONS_AVAILABLE);
sdFilter.addAction(Intent.ACTION_EXTERNAL_APPLICATIONS_UNAVAILABLE);
mLoader.getContext().registerReceiver(this, sdFilter);
}
@Override
public void onReceive(Context context, Intent intent) {
if (DEBUG) Log.i(TAG, "+++ The observer has detected an application change!" +
" Notifying Loader... +++");
// Tell the loader about the change.
mLoader.onContentChanged();
}
}
剩下的工作就和之前一样了，在LoaderManager里面拿到loader返回的数据，更新UI：
public class SmsFragment extends ListFragment implements LoaderCallbacks
<List<SmsEntry>>{
private SmsAdapter adapter;
@Override
public void onActivityCreated(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onActivityCreated(savedInstanceState);
Log.d("dml", "进入SmsFragment ");
adapter = new SmsAdapter(getActivity());
setEmptyText("No applications");
setListAdapter(adapter);
setListShown(false);
getLoaderManager().initLoader(2, null, this);
}
@Override
public Loader<List<SmsEntry>> onCreateLoader(int arg0, Bundle arg1) {
// TODO Auto-generated method stub
Log.d("dml", " onCreateLoader");
return new MySmsLoader(getActivity());
}
@Override
public void onLoadFinished(Loader<List<SmsEntry>> arg0,
List<SmsEntry> arg1) {
// TODO Auto-generated method stub
Log.d("dml", " onLoadFinished>>>>>>>arg1: " + arg1);
adapter.setData(arg1);
if (isResumed()) {
setListShown(true);
} else {
setListShownNoAnimation(true);
}
}
@Override
public void onLoaderReset(Loader<List<SmsEntry>> arg0) {
// TODO Auto-generated method stub
Log.d("dml", " onLoaderReset");
adapter.setData(null);
}
class SmsAdapter extends ArrayAdapter<SmsEntry>{
private LayoutInflater inflater;
public SmsAdapter(Context context) {
super(context,android.R.layout.simple_list_item_2);
// TODO Auto-generated constructor stub
Log.d("dml", " new SmsAdapter--------");
inflater = (LayoutInflater) context.getSystemService
(context.LAYOUT_INFLATER_SERVICE);
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
// TODO Auto-generated method stub
Log.d("dml", "getView-------- ");
View view = convertView;
ViewHolder holder;
if(view==null){
view = inflater.inflate(R.layout.item, parent,false);
holder = new ViewHolder();
holder.name = (TextView) view.findViewById(R.id.name);
holder.date = (TextView)view.findViewById(R.id.date);
view.setTag(holder);
}
else{
holder = (ViewHolder)view.getTag();
}
SmsEntry item = getItem(position);
Log.d("dml", "item-------- " + item);
holder.name.setText(item.from_name);
holder.date.setText(item.send_time);
return view;
}
private class ViewHolder{
TextView name;
TextView date;
}
public void setData(List<SmsEntry> data){
clear();
if(data!=null){
Log.d("dml", "data-------- " + data);
for(int i=0;i<data.size();i++){
add(data.get(i));
}
}
}
}
}
可以看出这里的数据结构由原来的cursor变成了自定义的SmsEntry,另外在这里要学会自定义ArrayAdapter的用法，显示数据