Loaders

Loaders，获取数据的东西。

总体流程是：通过getLoaderManager().initLoader(0,null,this)获得Loader，如果没有，那么就会调用接口函数获取，注意：有ID，所以要在获取的接口函数里进行相应的选择。

会自动更新数据；

一个主要依赖Activity或者Fragment存在的。

通过getLoaderManager().initLoader(0,null,this)方法初始化LOADER。

要继承LoaderManager.LoaderCallbacks，包括：

LoaderManager.LoaderCallbacks includes these methods:

onCreateLoader() — Instantiate and return a new Loader for the given ID.
onLoadFinished() — Called when a previously created loader has finished its load.
onLoaderReset() — Called when a previously created loader is being reset, thus making its data unavailable.
These methods are described in more detail in the following sections.

　　第一个用于创建LOADER，一般来说，只有init的时候ID对应的LOADER不在的时候才会运行该方法。

　　第二个一般用来获取数据后的填充适配器。

　　第三个用于重置。

　　Loader可以自定义，一般使用继承AsyncTaskLoader的类。

/*** A custom Loader that loads all of the installed applications.*/
public static class AppListLoader extends AsyncTaskLoader<List<AppEntry>> {final InterestingConfigChanges mLastConfig = new InterestingConfigChanges();final PackageManager mPm;List<AppEntry> mApps;PackageIntentReceiver mPackageObserver;public AppListLoader(Context context) {super(context);// Retrieve the package manager for later use; note we don't// use 'context' directly but instead the save global application// context returned by getContext().mPm = getContext().getPackageManager();}/*** This is where the bulk of our work is done.  This function is* called in a background thread and should generate a new set of* data to be published by the loader.*/@Override public List<AppEntry> loadInBackground() {// Retrieve all known applications.List<ApplicationInfo> apps = mPm.getInstalledApplications(PackageManager.GET_UNINSTALLED_PACKAGES |PackageManager.GET_DISABLED_COMPONENTS);if (apps == null) {apps = new ArrayList<ApplicationInfo>();}final Context context = getContext();// Create corresponding array of entries and load their labels.List<AppEntry> entries = new ArrayList<AppEntry>(apps.size());for (int i=0; i<apps.size(); i++) {AppEntry entry = new AppEntry(this, apps.get(i));entry.loadLabel(context);entries.add(entry);}// Sort the list.
        Collections.sort(entries, ALPHA_COMPARATOR);// Done!return entries;}/*** Called when there is new data to deliver to the client.  The* super class will take care of delivering it; the implementation* here just adds a little more logic.*/@Override public void deliverResult(List<AppEntry> apps) {if (isReset()) {// An async query came in while the loader is stopped.  We// don't need the result.if (apps != null) {onReleaseResources(apps);}}List<AppEntry> oldApps = mApps;mApps = apps;if (isStarted()) {// If the Loader is currently started, we can immediately// deliver its results.super.deliverResult(apps);}// At this point we can release the resources associated with// 'oldApps' if needed; now that the new result is delivered we// know that it is no longer in use.if (oldApps != null) {onReleaseResources(oldApps);}}/*** Handles a request to start the Loader.*/@Override protected void onStartLoading() {if (mApps != null) {// If we currently have a result available, deliver it// immediately.
            deliverResult(mApps);}// Start watching for changes in the app data.if (mPackageObserver == null) {mPackageObserver = new PackageIntentReceiver(this);}// Has something interesting in the configuration changed since we// last built the app list?boolean configChange = mLastConfig.applyNewConfig(getContext().getResources());if (takeContentChanged() || mApps == null || configChange) {// If the data has changed since the last time it was loaded// or is not currently available, start a load.
            forceLoad();}}/*** Handles a request to stop the Loader.*/@Override protected void onStopLoading() {// Attempt to cancel the current load task if possible.
        cancelLoad();}/*** Handles a request to cancel a load.*/@Override public void onCanceled(List<AppEntry> apps) {super.onCanceled(apps);// At this point we can release the resources associated with 'apps'// if needed.
        onReleaseResources(apps);}/*** Handles a request to completely reset the Loader.*/@Override protected void onReset() {super.onReset();// Ensure the loader is stopped
        onStopLoading();// At this point we can release the resources associated with 'apps'// if needed.if (mApps != null) {onReleaseResources(mApps);mApps = null;}// Stop monitoring for changes.if (mPackageObserver != null) {getContext().unregisterReceiver(mPackageObserver);mPackageObserver = null;}}/*** Helper function to take care of releasing resources associated* with an actively loaded data set.*/protected void onReleaseResources(List<AppEntry> apps) {// For a simple List<> there is nothing to do.  For something// like a Cursor, we would close it here.
    }
}