Android之Loader理解
在看Android的文档时，看到了这么一个东西: Loader
究竟是什么东西呢?
Introduced in Android 3.0, loaders make it easy to asynchronously load data in an activity or fragment. Loaders have these characteristics:
1、They are available to every Activity and Fragment. //支持Activity和Fragment
2、They provide asynchronous loading of data. //异步下载
3、They monitor the source of their data and deliver new results when the content changes. //当数据源改变时能及时通知客户端
4、They automatically reconnect to the last loader's cursor when being recreated after a configuration change. Thus, they don't need to re-query their data. //发生configuration change时自动重连接
看来这东西蛮强大的，开始我的探索之路吧.
先简单看一下它的用法先:
/** * Demonstration of the use of a CursorLoader to load and display contacts * data in a fragment. */public class LoaderCursor extends Activity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); FragmentManager fm = getFragmentManager(); // Create the list fragment and add it as our sole content. if (fm.findFragmentById(android.R.id.content) == null) { CursorLoaderListFragment list = new CursorLoaderListFragment(); fm.beginTransaction().add(android.R.id.content, list).commit(); } } public static class CursorLoaderListFragment extends ListFragment implements LoaderManager.LoaderCallbacks<Cursor> { // This is the Adapter being used to display the list's data. SimpleCursorAdapter mAdapter; // If non-null, this is the current filter the user has provided. String mCurFilter; @Override public void onActivityCreated(Bundle savedInstanceState) { 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(0, null, this); } @Override public void onListItemClick(ListView l, View v, int position, long id) { // Insert desired behavior here. Log.i("FragmentComplexList", "Item clicked: " + id); } // These are the Contacts rows that we will retrieve. static final String[] CONTACTS_SUMMARY_PROJECTION = new String[] { Contacts._ID, Contacts.DISPLAY_NAME, Contacts.CONTACT_STATUS, Contacts.CONTACT_PRESENCE, Contacts.PHOTO_ID, Contacts.LOOKUP_KEY, }; public Loader<Cursor> onCreateLoader(int id, Bundle args) { // This is called when a new Loader needs to be created. This // sample only has one Loader, so we don't care about the ID. // First, pick the base URI to use depending on whether we are // currently filtering. Uri baseUri; if (mCurFilter != null) { baseUri = Uri.withAppendedPath(Contacts.CONTENT_FILTER_URI, Uri.encode(mCurFilter)); } else { baseUri = Contacts.CONTENT_URI; } // Now create and return a CursorLoader that will take care of // creating a Cursor for the data being displayed. String select = "((" + Contacts.DISPLAY_NAME + " NOTNULL) AND (" + Contacts.HAS_PHONE_NUMBER + "=1) AND (" + Contacts.DISPLAY_NAME + " != '' ))"; return new CursorLoader(getActivity(), baseUri, CONTACTS_SUMMARY_PROJECTION, select, null, Contacts.DISPLAY_NAME + " COLLATE LOCALIZED ASC"); } public void onLoadFinished(Loader<Cursor> loader, Cursor data) { // Swap the new cursor in. (The framework will take care of closing the // old cursor once we return.) mAdapter.swapCursor(data); // The list should now be shown. if (isResumed()) { setListShown(true); } else { setListShownNoAnimation(true); } } public void onLoaderReset(Loader<Cursor> loader) { // This is called when the last Cursor provided to onLoadFinished() // above is about to be closed. We need to make sure we are no // longer using it. mAdapter.swapCursor(null); } }}
public <D> Loader<D> initLoader(int id, Bundle args, LoaderManager.LoaderCallbacks<D> callback) { if (mCreatingLoader) { throw new IllegalStateException("Called while creating a loader"); } LoaderInfo info = mLoaders.get(id); if (info == null) { // Loader doesn't already exist; create. info = createAndInstallLoader(id, args, (LoaderManager.LoaderCallbacks<Object>)callback); if (DEBUG) Log.v(TAG, " Created new loader " + info); } else { if (DEBUG) Log.v(TAG, " Re-using existing loader " + info); info.mCallbacks = (LoaderManager.LoaderCallbacks<Object>)callback; } if (info.mHaveData && mStarted) { // If the loader has already generated its data, report it now. info.callOnLoadFinished(info.mLoader, info.mData); } return (Loader<D>)info.mLoader;}private LoaderInfo createAndInstallLoader(int id, Bundle args, LoaderManager.LoaderCallbacks<Object> callback) { try { mCreatingLoader = true; LoaderInfo info = createLoader(id, args, callback); installLoader(info); return info; } finally { mCreatingLoader = false; } }private LoaderInfo createLoader(int id, Bundle args, LoaderManager.LoaderCallbacks<Object> callback) { LoaderInfo info = new LoaderInfo(id, args, (LoaderManager.LoaderCallbacks<Object>)callback); Loader<Object> loader = callback.onCreateLoader(id, args); info.mLoader = (Loader<Object>)loader; return info; } void installLoader(LoaderInfo info) { mLoaders.put(info.mId, info); if (mStarted) { // The activity will start all existing loaders in it's onStart(), // so only start them here if we're past that point of the activitiy's // life cycle info.start(); } }void start() { if (mLoader != null) { if (!mListenerRegistered) { mLoader.registerListener(mId, this); mListenerRegistered = true; } mLoader.startLoading(); } }
Loader：public final void startLoading() { mStarted = true; mReset = false; mAbandoned = false; onStartLoading(); }CursorLoader:protected void onStartLoading() { if (mCursor != null) { deliverResult(mCursor); } if (takeContentChanged() || mCursor == null) { forceLoad(); } }AsynTaskLoader:protected void onForceLoad() { super.onForceLoad(); cancelLoad(); mTask = new LoadTask(); if (DEBUG) Slog.v(TAG, "Preparing load: mTask=" + mTask); executePendingTask(); }AsyncTaskLoader:final class LoadTask extends AsyncTask<Void, Void, D> implements Runnable { private final CountDownLatch mDone = new CountDownLatch(1); // Set to true to indicate that the task has been posted to a handler for // execution at a later time. Used to throttle updates. boolean waiting; /* Runs on a worker thread */ @Override protected D doInBackground(Void... params) { if (DEBUG) Slog.v(TAG, this + " >>> doInBackground"); try { D data = AsyncTaskLoader.this.onLoadInBackground(); return data; } catch (OperationCanceledException ex) { } } /* Runs on the UI thread */ @Override protected void onPostExecute(D data) { if (DEBUG) Slog.v(TAG, this + " onPostExecute"); try { AsyncTaskLoader.this.dispatchOnLoadComplete(this, data); } finally { mDone.countDown(); } } }AsyncTaskLoader：protected D onLoadInBackground() { return loadInBackground();}CursorLoader:public Cursor loadInBackground() { try { Cursor cursor = getContext().getContentResolver().query(mUri, mProjection, mSelection, mSelectionArgs, mSortOrder, mCancellationSignal); if (cursor != null) { // Ensure the cursor window is filled cursor.getCount(); registerContentObserver(cursor, mObserver); } return cursor; } finally { synchronized (this) { mCancellationSignal = null; } }
public void onChange(boolean selfChange) { onContentChanged();}public void onContentChanged() { if (mStarted) { forceLoad(); //这里重新发送请求. } else { // This loader has been stopped, so we don't want to load // new data right now... but keep track of it changing to // refresh later if we start again. mContentChanged = true; }}
Activity: protected void onStart() { if (DEBUG_LIFECYCLE) Slog.v(TAG, "onStart " + this); mCalled = true; if (!mLoadersStarted) { mLoadersStarted = true; if (mLoaderManager != null) { mLoaderManager.doStart(); } else if (!mCheckedForLoaderManager) { mLoaderManager = getLoaderManager(null, mLoadersStarted, false); } mCheckedForLoaderManager = true; } getApplication().dispatchActivityStarted(this); }LoaderManagerImp： void doStart() { if (DEBUG) Log.v(TAG, "Starting in " + this); if (mStarted) { RuntimeException e = new RuntimeException("here"); e.fillInStackTrace(); Log.w(TAG, "Called doStart when already started: " + this, e); return; } mStarted = true; // Call out to sub classes so they can start their loaders // Let the existing loaders know that we want to be notified when a load is complete for (int i = mLoaders.size()-1; i >= 0; i--) { mLoaders.valueAt(i).start(); } }