org.gnucash.android.db.DatabaseCursorLoader.java Source code

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Here is the source code for org.gnucash.android.db.DatabaseCursorLoader.java

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/*
 * Copyright (c) 2012 - 2014 Ngewi Fet <ngewif@gmail.com>
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.gnucash.android.db;

import android.content.Context;
import android.database.Cursor;
import android.support.v4.content.AsyncTaskLoader;

/**
 * Abstract base class for asynchronously loads records from a database and manages the cursor.
 * In order to use this class, you must subclass it and implement the
 * {@link #loadInBackground()} method to load the particular records from the database.
 * Ideally, the database has {@link DatabaseAdapter} which is used for managing access to the 
 * records from the database
 * @author Ngewi Fet <ngewif@gmail.com>
 * @see DatabaseAdapter
 */
public abstract class DatabaseCursorLoader extends AsyncTaskLoader<Cursor> {
    /**
     * Cursor which will hold the loaded data set.
     * The cursor will be returned from the {@link #loadInBackground()} method
     */
    private Cursor mCursor = null;

    /**
     * {@link DatabaseAdapter} which will be used to load the records from the database
     */
    protected DatabaseAdapter mDatabaseAdapter = null;

    /**
     * A content observer which monitors the cursor and provides notifications when
     * the dataset backing the cursor changes. You need to register the oberserver on
     * your cursor using {@link #registerContentObserver(Cursor)}
     */
    protected final ForceLoadContentObserver mObserver;

    /**
     * Constructor
     * Initializes the content observer
     * @param context Application context
     */
    public DatabaseCursorLoader(Context context) {
        super(context);
        mObserver = new ForceLoadContentObserver();
    }

    /**
     * Asynchronously loads the results from the database. 
     */
    public abstract Cursor loadInBackground();

    /**
     * Registers the content observer for the cursor. 
     * @param cursor {@link Cursor} whose content is to be observed for changes
     */
    protected void registerContentObserver(Cursor cursor) {
        cursor.registerContentObserver(mObserver);
    }

    @Override
    public void deliverResult(Cursor data) {
        if (isReset()) {
            if (data != null) {
                onReleaseResources(data);
            }
            return;
        }

        Cursor oldCursor = mCursor;
        mCursor = data;

        if (isStarted()) {
            super.deliverResult(data);
        }

        if (oldCursor != null && oldCursor != data && !oldCursor.isClosed()) {
            onReleaseResources(oldCursor);
        }
    }

    @Override
    protected void onStartLoading() {
        if (mCursor != null) {
            deliverResult(mCursor);
        }

        if (takeContentChanged() || mCursor == null) {
            // If the data has changed since the last time it was loaded
            // or is not currently available, start a load.
            forceLoad();
        }
    }

    @Override
    protected void onStopLoading() {
        cancelLoad();
    }

    @Override
    public void onCanceled(Cursor data) {
        super.onCanceled(data);
        onReleaseResources(data);
    }

    /**
      * Handles a request to completely reset the Loader.
      */
    @Override
    protected void onReset() {
        super.onReset();

        onStopLoading();

        // At this point we can release the resources associated with 'mCursor'
        // if needed.
        if (mCursor != null && !mCursor.isClosed()) {
            onReleaseResources(mCursor);
        }
        mCursor = null;
    }

    /**
      * Helper function to take care of releasing resources associated
      * with an actively loaded data set.
      * @param c {@link Cursor} to be released
      */
    protected void onReleaseResources(Cursor c) {
        if (c != null)
            c.close();

        if (mDatabaseAdapter != null) {
            mDatabaseAdapter.close();
        }
    }
}