Android Loader(一) 概述
Android Loader(三) 结合CursorLoader分析Loader相关源码
CursorLoader是AsyncTaskLoader的子类,它可以异步查询ContentProvider中得数据,在ContentProvider中数据变化时,自动重新查询。
通过CursorLoader加载ContentProvider中数据有以下2步:
1. 实现LoaderManager.LoaderCallbacks<Cursor>接口。
2. 重写onCreateLoader,onLoadFinished,onLoaderReset方法。
下面用一个API DEMO例子说明:
以下代码实现一个模糊查询手机中联系人,并显示在listview上的功能,由于联系人的数据系统已经通过Contacts的ContentProvider提供,访问联系人数据直接调用系统URI即可。
public class LoaderCursor extends Activity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); FragmentManager fm = getFragmentManager(); 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 OnQueryTextListener, LoaderManager.LoaderCallbacks<Cursor> { SimpleCursorAdapter mAdapter; // If non-null, this is the current filter the user has provided. String mCurFilter; @Override public void onActivityCreated(Bundle savedInstanceState) { super.onActivityCreated(savedInstanceState); // Give some text to display if there is no data. In a real // application this would come from a resource. setEmptyText("No phone numbers"); // We have a menu item to show in action bar. setHasOptionsMenu(true); // Create an empty adapter we will use to display the loaded data. 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); // Start out with a progress indicator. setListShown(false); // Prepare the loader. Either re-connect with an existing one, // or start a new one. getLoaderManager().initLoader(0, null, this); } @Override public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) { // Place an action bar item for searching. MenuItem item = menu.add("Search"); item.setIcon(android.R.drawable.ic_menu_search); item.setShowAsAction(MenuItem.SHOW_AS_ACTION_IF_ROOM | MenuItem.SHOW_AS_ACTION_COLLAPSE_ACTION_VIEW); SearchView sv = new SearchView(getActivity()); sv.setOnQueryTextListener(this); item.setActionView(sv); } public boolean onQueryTextChange(String newText) { // Called when the action bar search text has changed. Update // the search filter, and restart the loader to do a new query // with this filter. String newFilter = !TextUtils.isEmpty(newText) ? newText : null; // Don't do anything if the filter hasn't actually changed. // Prevents restarting the loader when restoring state. if (mCurFilter == null && newFilter == null) { return true; } if (mCurFilter != null && mCurFilter.equals(newFilter)) { return true; } mCurFilter = newFilter; getLoaderManager().restartLoader(0, null, this); return true; } @Override public boolean onQueryTextSubmit(String query) { // Don't care about this. return true; } @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); } } }在onActivityCreated方法中调用getLoaderManager().initLoader(0, null, this);初始化Loader,在 Fragment中实现了LoaderManager.LoaderCallbacks<Cursor>接口,
onCreateLoader以ContentProvider的URI为参数,创建并返回一个CursorLoader。 在查询完毕后,会调用 onLoadFinished回调,所以在调用SimpleCursorAdapter的swapCursor(data),将查询后的数据显示在ListView上。最后,在Loader的数据不再使用时,onLoaderReset被调用, mAdapter.swapCursor(null); 保证旧的Cursor数据不再被使用。在 onQueryTextChange方法中,调用 getLoaderManager().restartLoader(0, null, this); 会使Loader重新创建, onCreateLoader-》onLoadFinished的回调方法会被重新执行。
下面从ContentProvider实现开始,实现一个CursorLoader的完整例子:
下面的程序实现的功能:从记录城市的表CITY表中查询出城市名称,显示在ListView上。
工程目录:
先定义表结构:
CityTable.java:
public class CityTable { public static final String TABLE_NAME = "city"; public static final String _ID = "_id"; public static final String NAME = "name"; public static final Uri CONTENT_URI = Uri.parse("content://" + CityProvider.AUTHORITY + "/" + TABLE_NAME); public static final Uri CONTENT_BASE_URI = Uri.parse("content://" + CityProvider.AUTHORITY + "/" + TABLE_NAME + "/"); }
DataBaseHelper.java:
public class DataBaseHelper extends SQLiteOpenHelper { private static final String DATABASE_NAME = "city.db"; private static final int DATABASE_VERSION = 1; public DataBaseHelper(Context context, int version) { super(context, DATABASE_NAME, null, DATABASE_VERSION); } @Override public void onCreate(SQLiteDatabase db) { db.execSQL("CREATE TABLE " + CityTable.TABLE_NAME + " (" + CityTable._ID + " INTEGER PRIMARY KEY," + CityTable.NAME + " TEXT" + ");"); } @Override public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { db.execSQL("DROP TABLE IF EXISTS notes"); onCreate(db); } }再实现提供CityTale数据的ContentProvider:
CityProvider.java:
public class CityProvider extends ContentProvider { public static final String AUTHORITY = "com.example.cityloader.provider"; private static final int MAIN = 1; private static final int MAIN_ID = 2; private DataBaseHelper mDataBaseHelper; private final UriMatcher mUriMatcher; public static final String CONTENT_TYPE = "vnd.android.cursor.dir/vnd.com.example.cityloader.citytable"; public static final String CONTENT_ITEM_TYPE = "vnd.android.cursor.item/vnd.com.example.cityloader.citytable"; public CityProvider() { mUriMatcher = new UriMatcher(UriMatcher.NO_MATCH); mUriMatcher.addURI(AUTHORITY, CityTable.TABLE_NAME, MAIN); mUriMatcher.addURI(AUTHORITY, CityTable.TABLE_NAME + "/#", MAIN_ID); } @Override public boolean onCreate() { mDataBaseHelper = new DataBaseHelper(getContext()); return true; } @Override public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) { switch (mUriMatcher.match(uri)) { case MAIN: break; case MAIN_ID: selectionArgs = DatabaseUtils.appendSelectionArgs(selectionArgs, new String[] { uri.getLastPathSegment() }); break; default: throw new IllegalArgumentException("Unknown URI " + uri); } SQLiteDatabase db = mDataBaseHelper.getReadableDatabase(); Cursor c = db.query(CityTable.TABLE_NAME, null, selection, selectionArgs, null, null, null); c.setNotificationUri(getContext().getContentResolver(), uri); return c; } @Override public String getType(Uri uri) { switch (mUriMatcher.match(uri)) { case MAIN: return CONTENT_TYPE; case MAIN_ID: return CONTENT_ITEM_TYPE; default: throw new IllegalArgumentException("Unknown URI " + uri); } } @Override public Uri insert(Uri uri, ContentValues values) { if (mUriMatcher.match(uri) != MAIN) { throw new IllegalArgumentException("Unknown URI " + uri); } SQLiteDatabase db = mDataBaseHelper.getWritableDatabase(); long rowId = db.insert(CityTable.TABLE_NAME, null, values); if (rowId > 0) { Uri noteUri = ContentUris.withAppendedId( CityTable.CONTENT_BASE_URI, rowId); getContext().getContentResolver().notifyChange(noteUri, null); return noteUri; } throw new SQLException("Failed to insert row into " + uri); } @Override public int delete(Uri uri, String whereClause, String[] whereArgs) { SQLiteDatabase db = mDataBaseHelper.getWritableDatabase(); String finalWhere; int count = 0; switch (mUriMatcher.match(uri)) { case MAIN: count = db.delete(CityTable.TABLE_NAME, whereClause, whereArgs); break; case MAIN_ID: finalWhere = DatabaseUtils.concatenateWhere(CityTable._ID + " = " + ContentUris.parseId(uri), whereClause); count = db.delete(CityTable.TABLE_NAME, finalWhere, whereArgs); break; default: throw new SQLException("Failed to insert row into " + uri); } getContext().getContentResolver().notifyChange(uri, null); return count; } @Override public int update(Uri uri, ContentValues values, String whereClause, String[] whereArgs) { SQLiteDatabase db = mDataBaseHelper.getWritableDatabase(); int count = 0; String finalWhere; switch (mUriMatcher.match(uri)) { case MAIN: count = db.update(CityTable.TABLE_NAME, values, whereClause, whereArgs); break; case MAIN_ID: finalWhere = DatabaseUtils.concatenateWhere(CityTable._ID + " = " + ContentUris.parseId(uri), whereClause); count = db.update(CityTable.TABLE_NAME, values, finalWhere, whereArgs); break; default: throw new SQLException("Failed to insert row into " + uri); } getContext().getContentResolver().notifyChange(uri, null); return count; } }
在insert,delete,update方法的最后,调用getContext().getContentResolver().notifyChange(uri, null);来通知内容的改变。
接着,fragment实现 LoaderManager.LoaderCallbacks<Cursor> ,并重写回调。
@Override public Loader<Cursor> onCreateLoader(int id, Bundle args) { CursorLoader cl = new CursorLoader(getActivity(), CityTable.CONTENT_URI, null, null, null, null); cl.setUpdateThrottle(1000); // update at most every 2 seconds. return cl; } @Override public void onLoadFinished(Loader<Cursor> loader, Cursor data) { mAdapter.swapCursor(data); if (isResumed()) { setListShown(true); } else { setListShownNoAnimation(true); } } @Override public void onLoaderReset(Loader<Cursor> loader) { mAdapter.swapCursor(null); }
cl.setUpdateThrottle(1000);指定了,数据发生变化时,过1秒再刷新内容。
接下来,在菜单中添加插入/删除数据的操作:
@Override public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) { menu.add(Menu.NONE, POPULATE_ID, 0, "Populate") .setShowAsAction(MenuItem.SHOW_AS_ACTION_IF_ROOM); menu.add(Menu.NONE, CLEAR_ID, 0, "Clear") .setShowAsAction(MenuItem.SHOW_AS_ACTION_IF_ROOM); } @Override public boolean onOptionsItemSelected(MenuItem item) { final ContentResolver cr = getActivity().getContentResolver(); switch (item.getItemId()) { case POPULATE_ID: if (mPopulatingTask != null) { mPopulatingTask.cancel(false); } mPopulatingTask = new AsyncTask<Void, Void, Void>() { @Override protected Void doInBackground(Void... params) { for (int i = 0; i < cities.length; i++) { if (isCancelled()) { break; } ContentValues values = new ContentValues(); values.put(CityTable.NAME, cities[i]); cr.insert(CityTable.CONTENT_URI, values); try { Thread.sleep(250); } catch (InterruptedException e) { } } return null; } }; mPopulatingTask.executeOnExecutor( AsyncTask.THREAD_POOL_EXECUTOR, (Void[])null); return true; case CLEAR_ID: if (mPopulatingTask != null) { mPopulatingTask.cancel(false); mPopulatingTask = null; } AsyncTask<Void, Void, Void> task = new AsyncTask<Void, Void, Void>() { @Override protected Void doInBackground(Void... params) { cr.delete(CityTable.CONTENT_URI, null, null); return null; } }; task.execute((Void[])null); return true; default: return super.onOptionsItemSelected(item); } }之后,在manifest文件中增加ContentProvider的声明
<provider android:name="com.example.cityloader.CityProvider" android:authorities="com.example.cityloader.provider" ></provider>最后,在Activity中显示Fragment
MainActivity.java:
public class MainActivity extends ActionBarActivity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); FragmentManager fm = getFragmentManager(); if (fm.findFragmentById(android.R.id.content) == null) { CityListFragment list = new CityListFragment(); fm.beginTransaction().add(android.R.id.content, list).commit(); } } }CityListFragment.java完整代码:
public class CityListFragment extends ListFragment implements LoaderManager.LoaderCallbacks<Cursor> { static final int POPULATE_ID = Menu.FIRST; static final int CLEAR_ID = Menu.FIRST + 1; SimpleCursorAdapter mAdapter; AsyncTask<Void, Void, Void> mPopulatingTask; String[] cities = { "北京", "上海", "广州", "深圳" }; @Override public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) { menu.add(Menu.NONE, POPULATE_ID, 0, "Populate").setShowAsAction( MenuItem.SHOW_AS_ACTION_IF_ROOM); menu.add(Menu.NONE, CLEAR_ID, 0, "Clear").setShowAsAction( MenuItem.SHOW_AS_ACTION_IF_ROOM); } @Override public boolean onOptionsItemSelected(MenuItem item) { final ContentResolver cr = getActivity().getContentResolver(); switch (item.getItemId()) { case POPULATE_ID: if (mPopulatingTask != null) { mPopulatingTask.cancel(false); } mPopulatingTask = new AsyncTask<Void, Void, Void>() { @Override protected Void doInBackground(Void... params) { for (int i = 0; i < cities.length; i++) { if (isCancelled()) { break; } ContentValues values = new ContentValues(); values.put(CityTable.NAME, cities[i]); cr.insert(CityTable.CONTENT_URI, values); try { Thread.sleep(250); } catch (InterruptedException e) { } } return null; } }; mPopulatingTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, (Void[]) null); return true; case CLEAR_ID: if (mPopulatingTask != null) { mPopulatingTask.cancel(false); mPopulatingTask = null; } AsyncTask<Void, Void, Void> task = new AsyncTask<Void, Void, Void>() { @Override protected Void doInBackground(Void... params) { cr.delete(CityTable.CONTENT_URI, null, null); return null; } }; task.execute((Void[]) null); return true; default: return super.onOptionsItemSelected(item); } } @Override public void onActivityCreated(Bundle savedInstanceState) { super.onActivityCreated(savedInstanceState); setEmptyText("No data."); setHasOptionsMenu(true); mAdapter = new SimpleCursorAdapter(getActivity(), android.R.layout.simple_list_item_1, null, new String[] { CityTable.NAME }, new int[] { android.R.id.text1 }, 0); setListAdapter(mAdapter); setListShown(false); getLoaderManager().initLoader(0, null, this); } @Override public Loader<Cursor> onCreateLoader(int id, Bundle args) { CursorLoader cl = new CursorLoader(getActivity(), CityTable.CONTENT_URI, null, null, null, null); cl.setUpdateThrottle(1000); // update at most every 2 seconds. return cl; } @Override public void onLoadFinished(Loader<Cursor> loader, Cursor data) { mAdapter.swapCursor(data); if (isResumed()) { setListShown(true); } else { setListShownNoAnimation(true); } } @Override public void onLoaderReset(Loader<Cursor> loader) { mAdapter.swapCursor(null); } }