Batista R. Harahap

Android - AsyncTask Is A Beauty - Part 2

Here goes the second part of a short tutorial of how beautiful AsyncTask is, the first tutorial is here. Now that we're moving up to our second activity which is Hacktivate, a brief description about it is that this activity essentially puts all the data retrieved previously and populate a custom layout ListView.

To make reading and understanding easier, here are the codes first before anything else.

  
package com.bango.acerid;

import java.io.IOException;  
import java.util.ArrayList;  
import java.util.List;

import org.json.JSONArray;  
import org.json.JSONException;  
import org.json.JSONObject;  
import org.jsoup.Jsoup;

import android.app.Activity;  
import android.content.Intent;  
import android.graphics.Bitmap;  
import android.os.AsyncTask;  
import android.os.Bundle;  
import android.view.LayoutInflater;  
import android.view.Menu;  
import android.view.MenuInflater;  
import android.view.MenuItem;  
import android.view.View;  
import android.view.ViewGroup;  
import android.widget.AdapterView;  
import android.widget.BaseAdapter;  
import android.widget.ImageView;  
import android.widget.ListView;  
import android.widget.TextView;  
import android.widget.AdapterView.OnItemClickListener;

public class Hacktivate extends Activity {  
    private String jsonStr = "";
    private JSONObject json;
    private List images = new ArrayList();
    private List titles = new ArrayList();
    private List writers = new ArrayList();
    private List dates = new ArrayList();
    private List links = new ArrayList();
    private List body = new ArrayList();
    private List description = new ArrayList();
    private ArticleAdapter adapter = new ArticleAdapter();
    private List l = new ArrayList();

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.hacktivate);

        Bundle the = getIntent().getExtras();
        jsonStr = (the != null) ? the.getString("listData") : "";
        parseJson(jsonStr);

        ListView articleList = (ListView) findViewById(R.id.list);
        articleList.setAdapter(adapter);

        final Intent i = new Intent(this, PostViewer.class);

        articleList.setOnItemClickListener(new OnItemClickListener() {
            public void onItemClick(AdapterView parent, View v, int position,
                    long id) {
                i.putExtra("wvUrl", links.get(position));
                i.putExtra("wvBody", body.get(position));
                Hacktivate.this.startActivity(i);
            }   
        });
    }

    private void parseJson(String j) {
        try {
            json = new JSONObject(j);
            JSONArray rss = json.getJSONArray("rss");

            int max = rss.length();
            for(int i=0; i {
            private ImageView img;

            @Override
            protected ListData doInBackground(ListData... l) {
                int pos = l[0].getPosition();
                View v = l[0].getView();
                Bitmap ret;

                try {
                    String imgStr = images.get(pos);
                    imgMan = new ImageManager(imgStr);

                    img = (ImageView) v.findViewById(R.id.articleImage);

                    if(imgMan.fileExists()) {
                        ret = imgMan.getImage();
                    } else {
                        ret = imgMan.saveImage();
                    }

                    ListData ll = new ListData(pos, v);
                    ll.setBitmap(ret);

                    return ll;
                } catch(Exception e) {
                    e.printStackTrace();
                    return null;
                }
            }

            @Override
            protected void onPostExecute(ListData ret) {
                if(ret != null) {
                    img.setImageBitmap(ret.getBitmap());
                    if(l.get(ret.getPosition()).getBitmap() == null) {
                        l.get(ret.getPosition()).setBitmap(ret.getBitmap());
                    }
                }
            }

        }

        public View getView(int position, View v, ViewGroup vg) {
            LayoutInflater inflater = getLayoutInflater();
            v = inflater.inflate(R.layout.articlerow, vg, false);

            TextView title = (TextView) v.findViewById(R.id.articleTitle);
            TextView desc = (TextView) v.findViewById(R.id.articleDesc);

            title.setText(titles.get(position));
            String d = Jsoup.parse(description.get(position)).text() + "\n\nBy " + writers.get(position) + " on " + dates.get(position);
            desc.setText(d);

            // Lazy load images
            try {
                ListData tmp = l.get(position);

                ImageView img = (ImageView) v.findViewById(R.id.articleImage);
                img.setImageBitmap(tmp.getBitmap());
            } catch(IndexOutOfBoundsException e) {
                ListData lst = new ListData(position, v);
                l.add(lst);
                new DownloadImage().execute(l.get(position));
            }

            return v;
        }

    }

    public void onBackPressed() {
        moveTaskToBack(true);
    }

    public void askDoctor(View v) {
        Intent i = new Intent(this, Ask.class);
        startActivity(i);
    }

    public boolean onCreateOptionsMenu(Menu menu) {
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.menu, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch(item.getItemId()) {
            case R.id.menuIcon:
                return true;
            case R.id.menuRefresh:
                Intent i = new Intent(this, AcerID.class);
                startActivity(i);
                return true;
            case R.id.menuExit:
                moveTaskToBack(true);
                return true;
            default:
                return true;
        }
    }

}

You might noticed that instead of extending ListActivity, I choose to simply extend Activity. My reason is to carve out expected behaviors of a ListActivity like the standardized naming conventions for its ListView and the default row layout.

For this activity, I incorporated an external library called JSOUP. It's a very useful library to deal with HTML or even HTML fragments. Most importantly to correctly decode HTML entities generated by server side languages.

The next interesting technique is to incorporate an SD Card to save downloaded thumbnails to only use bandwidth when required only. I created a separate class called ImageManager.java to do exactly that. Here is the source code.

  
package com.bango.acerid;

import java.io.File;  
import java.io.FileNotFoundException;  
import java.io.FileOutputStream;  
import java.io.IOException;  
import java.io.InputStream;  
import java.net.URL;  
import java.security.MessageDigest;  
import java.security.NoSuchAlgorithmException;

import android.graphics.Bitmap;  
import android.graphics.BitmapFactory;  
import android.graphics.drawable.BitmapDrawable;  
import android.graphics.drawable.Drawable;  
import android.os.Environment;

public class ImageManager {  
    protected final static String STORAGE_PATH = Environment.getExternalStorageDirectory().toString();
    private String imgUrl;
    private String fn;
    private String basepath = STORAGE_PATH + "/AcerID/";

    public ImageManager(String imgUrl) {
        this.setImgUrl(imgUrl);
        fn = md5(imgUrl) + ".jpg";
    }

    protected Drawable getImageFromURL(String url) {
        try {
            InputStream is = (InputStream) new URL(url).getContent();
            Drawable d = Drawable.createFromStream(is, "src name");
            is.close();
            return d;
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }
    }

    public boolean fileExists() {
        return new File(basepath+fn).exists();
    }

    public static String md5(String s) {
        try {
            // Create MD5 Hash
            MessageDigest digest = java.security.MessageDigest
                    .getInstance("MD5");
            digest.update(s.getBytes());
            byte messageDigest[] = digest.digest();

            // Create Hex String
            StringBuffer hexString = new StringBuffer();
            for (int i = 0; i < messageDigest.length; i++)
                hexString.append(Integer.toHexString(0xFF & messageDigest[i]));
            return hexString.toString();

        } catch (NoSuchAlgorithmException e) {
            //e.printStackTrace();
            return s;
        }
    }

    public Bitmap getImage() {
        return BitmapFactory.decodeFile(basepath+fn);
    }

    public Bitmap saveImage() {
        Bitmap b = ((BitmapDrawable) getImageFromURL(imgUrl)).getBitmap();
        FileOutputStream out;

        // Create dir
        File dir = new File(basepath);
        if(!dir.exists()) {
            dir.mkdirs();
        }

        try {
            File output = new File(dir, fn);
            out = new FileOutputStream(output);

            b.compress(Bitmap.CompressFormat.JPEG, 90, out);
            out.flush();
            out.close();
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        return b;
    }

    public void setImgUrl(String imgUrl) {
        this.imgUrl = imgUrl;
    }

    public String getImgUrl() {
        return imgUrl;
    }

}

Since I'm using PhpThumb to generate the thumbnails, it's quite complicated to parse only the filename of the image downloaded. This is why I choose to use an MD5 hash of the whole image URL as its filename and all formats are in JPEG converted by the within the saveImage() method. This class is actually needing more conditions to check whether an external storage is available or not. I'm gonna leave this to your creativity.

To give a better user experience, I lazy loaded all the thumbnails and put in a default image for every thumbnails before it is displayed. The condition checks if that particular is already available or not, if not then it will execute the AsyncTask assigned for this download job.

The interesting part about the AsyncTask in this activity is that it detoured Java's deficiency to pass by reference and instead pass data by value. I made a special class called ListData to contain the data I need to pass. My particular interest is with this line of code:

  
l.get(ret.getPosition()).setBitmap(ret.getBitmap());  

Looks a bit messy but it's an elegant method chaining way to exactly do what I want which is to return and finally set the downloaded image into each rows. Without it, everytime you scroll down and back up vice versa, the image will continuously be downloaded by AsyncTask.

  
            // Lazy load images
            try {
                ListData tmp = l.get(position);

                ImageView img = (ImageView) v.findViewById(R.id.articleImage);
                img.setImageBitmap(tmp.getBitmap());
            } catch(IndexOutOfBoundsException e) {
                ListData lst = new ListData(position, v);
                l.add(lst);
                new DownloadImage().execute(l.get(position));
            }

So this wraps up AsyncTask's beauty and completes the tutorial. The application I made will soon be introduced into Android Market, still have to do a bit more codes. Thanks for tuning in!

27 May 2011 by Batista Harahap on acer | acerid | Android | asynctask | hdpi | java | ldpi | magpierss | mdpi | php | phpthumb | rss | sourceforge | Tutorial | xhdpi
comments powered by Disqus