Android Loader机制

╰半橙微兮° 2022-07-13 08:52 155阅读 0赞

[ Android应用Loaders全面详解及源码浅析][Android_Loaders]

[加载器][Link 1]

# LoaderCursor #

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 OnQueryTextListener, OnCloseListener, LoaderManager.LoaderCallbacks<Cursor> { 
    
            // This is the Adapter being used to display the list's data.
            SimpleCursorAdapter mAdapter;
    
            // The SearchView for doing filtering.
            SearchView mSearchView;
    
            // 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);
            }
    
            public static class MySearchView extends SearchView { 
                public MySearchView(Context context) {
                    super(context);
                }
    
                // The normal SearchView doesn't clear its search text when
                // collapsed, so we will do this for it.
                @Override
                public void onActionViewCollapsed() {
                    setQuery("", false);
                    super.onActionViewCollapsed();
                }
            }
    
            @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);
                mSearchView = new MySearchView(getActivity());
                mSearchView.setOnQueryTextListener(this);
                mSearchView.setOnCloseListener(this);
                mSearchView.setIconifiedByDefault(true);
                item.setActionView(mSearchView);
            }
    
            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 boolean onClose() {
                if (!TextUtils.isEmpty(mSearchView.getQuery())) {
                    mSearchView.setQuery(null, true);
                }
                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);
            }
        }
    
    }

# LoaderThrottle #

public class LoaderThrottle extends Activity { 
        // Debugging.
        static final String TAG = "LoaderThrottle";
    
        /** * The authority we use to get to our sample provider. */
        public static final String AUTHORITY = "com.example.android.apis.app.LoaderThrottle";
    
        /** * Definition of the contract for the main table of our provider. */
        public static final class MainTable implements BaseColumns { 
    
            // This class cannot be instantiated
            private MainTable() {}
    
            /** * The table name offered by this provider */
            public static final String TABLE_NAME = "main";
    
            /** * The content:// style URL for this table */
            public static final Uri CONTENT_URI =  Uri.parse("content://" + AUTHORITY + "/main");
    
            /** * The content URI base for a single row of data. Callers must * append a numeric row id to this Uri to retrieve a row */
            public static final Uri CONTENT_ID_URI_BASE
                    = Uri.parse("content://" + AUTHORITY + "/main/");
    
            /** * The MIME type of {@link #CONTENT_URI}. */
            public static final String CONTENT_TYPE
                    = "vnd.android.cursor.dir/vnd.example.api-demos-throttle";
    
            /** * The MIME type of a {@link #CONTENT_URI} sub-directory of a single row. */
            public static final String CONTENT_ITEM_TYPE
                    = "vnd.android.cursor.item/vnd.example.api-demos-throttle";
            /** * The default sort order for this table */
            public static final String DEFAULT_SORT_ORDER = "data COLLATE LOCALIZED ASC";
    
            /** * Column name for the single column holding our data. * <P>Type: TEXT</P> */
            public static final String COLUMN_NAME_DATA = "data";
        }
    
        /** * This class helps open, create, and upgrade the database file. */
       static class DatabaseHelper extends SQLiteOpenHelper {
    
           private static final String DATABASE_NAME = "loader_throttle.db";
           private static final int DATABASE_VERSION = 2;
    
           DatabaseHelper(Context context) {
    
               // calls the super constructor, requesting the default cursor factory.
               super(context, DATABASE_NAME, null, DATABASE_VERSION);
           }
    
           /** * * Creates the underlying database with table name and column names taken from the * NotePad class. */
           @Override
           public void onCreate(SQLiteDatabase db) {
               db.execSQL("CREATE TABLE " + MainTable.TABLE_NAME + " ("
                       + MainTable._ID + " INTEGER PRIMARY KEY,"
                       + MainTable.COLUMN_NAME_DATA + " TEXT"
                       + ");");
           }
    
           /** * * Demonstrates that the provider must consider what happens when the * underlying datastore is changed. In this sample, the database is upgraded the database * by destroying the existing data. * A real application should upgrade the database in place. */
           @Override
           public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    
               // Logs that the database is being upgraded
               Log.w(TAG, "Upgrading database from version " + oldVersion + " to "
                       + newVersion + ", which will destroy all old data");
    
               // Kills the table and existing data
               db.execSQL("DROP TABLE IF EXISTS notes");
    
               // Recreates the database with a new version
               onCreate(db);
           }
       }
    
        /** * A very simple implementation of a content provider. */
        public static class SimpleProvider extends ContentProvider { 
            // A projection map used to select columns from the database
            private final HashMap<String, String> mNotesProjectionMap;
            // Uri matcher to decode incoming URIs.
            private final UriMatcher mUriMatcher;
    
            // The incoming URI matches the main table URI pattern
            private static final int MAIN = 1;
            // The incoming URI matches the main table row ID URI pattern
            private static final int MAIN_ID = 2;
    
            // Handle to a new DatabaseHelper.
            private DatabaseHelper mOpenHelper;
    
            /** * Global provider initialization. */
            public SimpleProvider() {
                // Create and initialize URI matcher.
                mUriMatcher = new UriMatcher(UriMatcher.NO_MATCH);
                mUriMatcher.addURI(AUTHORITY, MainTable.TABLE_NAME, MAIN);
                mUriMatcher.addURI(AUTHORITY, MainTable.TABLE_NAME + "/#", MAIN_ID);
    
                // Create and initialize projection map for all columns. This is
                // simply an identity mapping.
                mNotesProjectionMap = new HashMap<String, String>();
                mNotesProjectionMap.put(MainTable._ID, MainTable._ID);
                mNotesProjectionMap.put(MainTable.COLUMN_NAME_DATA, MainTable.COLUMN_NAME_DATA);
            }
    
            /** * Perform provider creation. */
            @Override
            public boolean onCreate() {
                mOpenHelper = new DatabaseHelper(getContext());
                // Assumes that any failures will be reported by a thrown exception.
                return true;
            }
    
            /** * Handle incoming queries. */
            @Override
            public Cursor query(Uri uri, String[] projection, String selection,
                    String[] selectionArgs, String sortOrder) {
    
                // Constructs a new query builder and sets its table name
                SQLiteQueryBuilder qb = new SQLiteQueryBuilder();
                qb.setTables(MainTable.TABLE_NAME);
    
                switch (mUriMatcher.match(uri)) {
                    case MAIN:
                        // If the incoming URI is for main table.
                        qb.setProjectionMap(mNotesProjectionMap);
                        break;
    
                    case MAIN_ID:
                        // The incoming URI is for a single row.
                        qb.setProjectionMap(mNotesProjectionMap);
                        qb.appendWhere(MainTable._ID + "=?");
                        selectionArgs = DatabaseUtils.appendSelectionArgs(selectionArgs,
                                new String[] { uri.getLastPathSegment() });
                        break;
    
                    default:
                        throw new IllegalArgumentException("Unknown URI " + uri);
                }
    
    
                if (TextUtils.isEmpty(sortOrder)) {
                    sortOrder = MainTable.DEFAULT_SORT_ORDER;
                }
    
                SQLiteDatabase db = mOpenHelper.getReadableDatabase();
    
                Cursor c = qb.query(db, projection, selection, selectionArgs,
                        null /* no group */, null /* no filter */, sortOrder);
    
                c.setNotificationUri(getContext().getContentResolver(), uri);
                return c;
            }
    
            /** * Return the MIME type for an known URI in the provider. */
            @Override
            public String getType(Uri uri) {
                switch (mUriMatcher.match(uri)) {
                    case MAIN:
                        return MainTable.CONTENT_TYPE;
                    case MAIN_ID:
                        return MainTable.CONTENT_ITEM_TYPE;
                    default:
                        throw new IllegalArgumentException("Unknown URI " + uri);
                }
            }
    
            /** * Handler inserting new data. */
            @Override
            public Uri insert(Uri uri, ContentValues initialValues) {
                if (mUriMatcher.match(uri) != MAIN) {
                    // Can only insert into to main URI.
                    throw new IllegalArgumentException("Unknown URI " + uri);
                }
    
                ContentValues values;
    
                if (initialValues != null) {
                    values = new ContentValues(initialValues);
                } else {
                    values = new ContentValues();
                }
    
                if (values.containsKey(MainTable.COLUMN_NAME_DATA) == false) {
                    values.put(MainTable.COLUMN_NAME_DATA, "");
                }
    
                SQLiteDatabase db = mOpenHelper.getWritableDatabase();
    
                long rowId = db.insert(MainTable.TABLE_NAME, null, values);
    
                // If the insert succeeded, the row ID exists.
                if (rowId > 0) {
                    Uri noteUri = ContentUris.withAppendedId(MainTable.CONTENT_ID_URI_BASE, rowId);
                    getContext().getContentResolver().notifyChange(noteUri, null);
                    return noteUri;
                }
    
                throw new SQLException("Failed to insert row into " + uri);
            }
    
            /** * Handle deleting data. */
            @Override
            public int delete(Uri uri, String where, String[] whereArgs) {
                SQLiteDatabase db = mOpenHelper.getWritableDatabase();
                String finalWhere;
    
                int count;
    
                switch (mUriMatcher.match(uri)) {
                    case MAIN:
                        // If URI is main table, delete uses incoming where clause and args.
                        count = db.delete(MainTable.TABLE_NAME, where, whereArgs);
                        break;
    
                        // If the incoming URI matches a single note ID, does the delete based on the
                        // incoming data, but modifies the where clause to restrict it to the
                        // particular note ID.
                    case MAIN_ID:
                        // If URI is for a particular row ID, delete is based on incoming
                        // data but modified to restrict to the given ID.
                        finalWhere = DatabaseUtils.concatenateWhere(
                                MainTable._ID + " = " + ContentUris.parseId(uri), where);
                        count = db.delete(MainTable.TABLE_NAME, finalWhere, whereArgs);
                        break;
    
                    default:
                        throw new IllegalArgumentException("Unknown URI " + uri);
                }
    
                getContext().getContentResolver().notifyChange(uri, null);
    
                return count;
            }
    
            /** * Handle updating data. */
            @Override
            public int update(Uri uri, ContentValues values, String where, String[] whereArgs) {
                SQLiteDatabase db = mOpenHelper.getWritableDatabase();
                int count;
                String finalWhere;
    
                switch (mUriMatcher.match(uri)) {
                    case MAIN:
                        // If URI is main table, update uses incoming where clause and args.
                        count = db.update(MainTable.TABLE_NAME, values, where, whereArgs);
                        break;
    
                    case MAIN_ID:
                        // If URI is for a particular row ID, update is based on incoming
                        // data but modified to restrict to the given ID.
                        finalWhere = DatabaseUtils.concatenateWhere(
                                MainTable._ID + " = " + ContentUris.parseId(uri), where);
                        count = db.update(MainTable.TABLE_NAME, values, finalWhere, whereArgs);
                        break;
    
                    default:
                        throw new IllegalArgumentException("Unknown URI " + uri);
                }
    
                getContext().getContentResolver().notifyChange(uri, null);
    
                return count;
            }
        }
    
        @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) {
                ThrottledLoaderListFragment list = new ThrottledLoaderListFragment();
                fm.beginTransaction().add(android.R.id.content, list).commit();
            }
        }
    
        public static class ThrottledLoaderListFragment extends ListFragment implements LoaderManager.LoaderCallbacks<Cursor> { 
    
            // Menu identifiers
            static final int POPULATE_ID = Menu.FIRST;
            static final int CLEAR_ID = Menu.FIRST+1;
    
            // 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;
    
            // Task we have running to populate the database.
            AsyncTask<Void, Void, Void> mPopulatingTask;
    
            @Override public void onActivityCreated(Bundle savedInstanceState) {
                super.onActivityCreated(savedInstanceState);
    
                setEmptyText("No data. Select 'Populate' to fill with data from Z to A at a rate of 4 per second.");
                setHasOptionsMenu(true);
    
                // Create an empty adapter we will use to display the loaded data.
                mAdapter = new SimpleCursorAdapter(getActivity(),
                        android.R.layout.simple_list_item_1, null,
                        new String[] { MainTable.COLUMN_NAME_DATA },
                        new int[] { android.R.id.text1 }, 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) {
                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 (char c='Z'; c>='A'; c--) {
                                    if (isCancelled()) {
                                        break;
                                    }
                                    StringBuilder builder = new StringBuilder("Data ");
                                    builder.append(c);
                                    ContentValues values = new ContentValues();
                                    values.put(MainTable.COLUMN_NAME_DATA, builder.toString());
                                    cr.insert(MainTable.CONTENT_URI, values);
                                    // Wait a bit between each insert.
                                    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(MainTable.CONTENT_URI, null, null);
                                return null;
                            }
                        };
                        task.execute((Void[])null);
                        return true;
    
                    default:
                        return super.onOptionsItemSelected(item);
                }
            }
    
            @Override public void onListItemClick(ListView l, View v, int position, long id) {
                // Insert desired behavior here.
                Log.i(TAG, "Item clicked: " + id);
            }
    
            // These are the rows that we will retrieve.
            static final String[] PROJECTION = new String[] {
                MainTable._ID,
                MainTable.COLUMN_NAME_DATA,
            };
    
            public Loader<Cursor> onCreateLoader(int id, Bundle args) {
                CursorLoader cl = new CursorLoader(getActivity(), MainTable.CONTENT_URI,
                        PROJECTION, null, null, null);
                cl.setUpdateThrottle(2000); // update at most every 2 seconds.
                return cl;
            }
    
            public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
                mAdapter.swapCursor(data);
    
                // The list should now be shown.
                if (isResumed()) {
                    setListShown(true);
                } else {
                    setListShownNoAnimation(true);
                }
            }
    
            public void onLoaderReset(Loader<Cursor> loader) {
                mAdapter.swapCursor(null);
            }
        }
    }

[Android_Loaders]: http://blog.csdn.net/yanbober/article/details/48861457
[Link 1]: https://developer.android.google.cn/guide/components/loaders.html