EndlessAdapter.java

package com.conference.app.lib.ui.adapter;

/***
 Copyright (c) 2008-2009 CommonsWare, LLC
 Portions (c) 2009 Google, Inc.

 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.
 */

import java.util.concurrent.atomic.AtomicBoolean;

import android.content.Context;
import android.os.AsyncTask;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ListAdapter;

import com.commonsware.cwac.adapter.AdapterWrapper;

/**
 * Adapter that assists another adapter in appearing endless. For example, this
 * could be used for an adapter being filled by a set of Web service calls,
 * where each call returns a "page" of data.
 * 
 * Subclasses need to be able to return, via getPendingView() a row that can
 * serve as both a placeholder while more data is being appended.
 * 
 * The actual logic for loading new data should be done in appendInBackground().
 * This method, as the name suggests, is run in a background thread. It should
 * return true if there might be more data, false otherwise.
 * 
 * If your situation is such that you will not know if there is more data until
 * you do some work (e.g., make another Web service call), it is up to you to do
 * something useful with that row returned by getPendingView() to let the user
 * know you are out of data, plus return false from that final call to
 * appendInBackground().
 */
abstract public class EndlessAdapter extends AdapterWrapper {
    abstract protected boolean cacheInBackground() throws Exception;

    abstract protected void appendCachedData();

    private View pendingView = null;
    private AtomicBoolean keepOnAppending = new AtomicBoolean(true);
    private Context context;
    private int pendingResource = -1;

    /**
     * Constructor wrapping a supplied ListAdapter
     */
    public EndlessAdapter(ListAdapter wrapped) {
        super(wrapped);
    }

    /**
     * Constructor wrapping a supplied ListAdapter and providing a id for a
     * pending view.
     * 
     * @param context
     * @param wrapped
     * @param pendingResource
     */
    public EndlessAdapter(Context context, ListAdapter wrapped, int pendingResource) {
        super(wrapped);
        this.context = context;
        this.pendingResource = pendingResource;
    }

    public void setKeepOnAppending(final boolean flag) {
        keepOnAppending.set(flag);
    }

    /**
     * How many items are in the data set represented by this Adapter.
     */
    @Override
    public int getCount() {
        if (keepOnAppending.get()) {
            return (super.getCount() + 1); // one more for "pending"
        }

        return (super.getCount());
    }

    /**
     * Masks ViewType so the AdapterView replaces the "Pending" row when new
     * data is loaded.
     */
    public int getItemViewType(int position) {
        if (position == getWrappedAdapter().getCount()) {
            return (IGNORE_ITEM_VIEW_TYPE);
        }

        return (super.getItemViewType(position));
    }

    /**
     * Masks ViewType so the AdapterView replaces the "Pending" row when new
     * data is loaded.
     * 
     * @see #getItemViewType(int)
     */
    public int getViewTypeCount() {
        return (super.getViewTypeCount() + 1);
    }

    /**
     * Get a View that displays the data at the specified position in the data
     * set. In this case, if we are at the end of the list and we are still in
     * append mode, we ask for a pending view and return it, plus kick off the
     * background task to append more data to the wrapped adapter.
     * 
     * @param position
     *            Position of the item whose data we want
     * @param convertView
     *            View to recycle, if not null
     * @param parent
     *            ViewGroup containing the returned View
     */
    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        if (position == super.getCount() && keepOnAppending.get()) {
            if (pendingView == null) {
                pendingView = getPendingView(parent);

                new AppendTask().execute();
            }

            return (pendingView);
        }

        return (super.getView(position, convertView, parent));
    }

    /**
     * Called if cacheInBackground() raises a runtime exception, to allow the UI
     * to deal with the exception on the main application thread.
     * 
     * @param pendingView
     *            View representing the pending row
     * @param e
     *            Exception that was raised by cacheInBackground()
     * @return true if should allow retrying appending new data, false otherwise
     */
    protected boolean onException(View pendingView, Exception e) {
        Log.e("EndlessAdapter", "Exception in cacheInBackground()", e);

        return (false);
    }

    /**
     * A background task that will be run when there is a need to append more
     * data. Mostly, this code delegates to the subclass, to append the data in
     * the background thread and rebind the pending view once that is done.
     */
    class AppendTask extends AsyncTask<Void, Void, Exception> {

        @Override
        protected Exception doInBackground(Void... params) {
            Exception result = null;

            try {
                keepOnAppending.set(cacheInBackground());
            } catch (Exception e) {
                result = e;
            }

            return (result);
        }

        @Override
        protected void onPostExecute(Exception e) {
            if (e == null) {
                appendCachedData();
            } else {
                keepOnAppending.set(onException(pendingView, e));
            }

            pendingView = null;
            notifyDataSetChanged();
        }
    }

    /**
     * Inflates pending view using the pendingResource ID passed into the
     * constructor
     * 
     * @param parent
     * @return inflated pending view, or null if the context passed into the
     *         pending view constructor was null.
     */
    protected View getPendingView(ViewGroup parent) {
        if (context != null) {
            LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            return inflater.inflate(pendingResource, parent, false);
        }

        throw new RuntimeException(
                "You must either override getPendingView() or supply a pending View resource via the constructor");
    }

    /**
     * Getter method for the Context being held by the adapter
     * 
     * @return Context
     */
    protected Context getContext() {
        return (context);
    }
}