android in practice_Implementing jobs with AsyncTask(MyMovies project)

AsyncTask offers an easy-to-use (but more limited).

You need to perform an asynchronous job that follows a pre-process/process/postprocesspattern, and are looking for a code template that allows you to report progress
to the user or otherwise update the UI in each step.

In the last technique, weretrieved movie thumbnail images by having a simple downloader helper method forka new thread that downloaded the image, and then passed it to a custom handlerobject that updated the image on the list view.

create theAsyncTask classDownloadTask:

public class DownloadTask extends AsyncTask<String,Void,Bitmap> {   private int position;   private ImageView imageView;   private Drawable placeholder;      public DownloadTask(int position,ImageView imageView){   this.position=position;   this.imageView=imageView;   Resources resources=imageView.getContext().getResources();   this.placeholder=resources.getDrawable(android.R.drawable.gallery_thumb);   }   //Called before task runs   protected void onPreExecute(){   imageView.setImageDrawable(placeholder);   }   //task logicprotected Bitmap doInBackground(String... inputUrls) {try{URL url=new URL(inputUrls[0]);return BitmapFactory.decodeStream(url.openStream());}catch(Exception e){e.printStackTrace();return null;}}  //called after task runsprotected void onPostExecute(Bitmap result){int forPosition=(Integer)imageView.getTag();if(forPosition==this.position){this.imageView.setImageBitmap(result);}}}

worker arguments (String), worker progress (Void), and worker result (Bitmap).

trigger the task,modify the class MovieAdapter:

/** * Another perfectly valid approach would be to create a Movie model class. * get rid of the HashMap. *///The MovieAdapter keeps track of selected movies//Adapter separating data from its representation on the screenpublic class MovieAdapter extends ArrayAdapter<String>  {//storing movie state data//this state is transientprivate HashMap<Integer,Boolean> movieCollection=new HashMap<Integer,Boolean>();private String[] movieIconUrls;//List of movie image URLs  private ThreadPoolExecutor executor;//Controls thread pool  public MovieAdapter(Context context) {super(context,R.layout.movie_item,android.R.id.text1,context.getResources().getStringArray(R.array.movies));// TODO Auto-generated constructor stub//Read image URLs into array          movieIconUrls=context.getResources().getStringArray(R.array.movie_thumbs);   }public void toggleMovie(int position){if(!isInCollection(position)){movieCollection.put(position, true);}else{movieCollection.put(position, false);}}public boolean isInCollection(int position){return movieCollection.get(position) == Boolean.TRUE;}@Override//returns the view needed for each item.//cache and reuse item views via convertView for performance reasonspublic View getView(int position,View convertView,ViewGroup parent){    View listItem=super.getView(position, convertView, parent);          CheckedTextView checkMark=(CheckedTextView)listItem.findViewById(android.R.id.text1);          checkMark.setChecked(isInCollection(position));          ImageView imageView =(ImageView)listItem.findViewById(R.id.movie_icon);          imageView.setImageDrawable(null);          imageView.setTag(position);//link position to image view          String imageUrl=movieIconUrls[position];        new DownloadTask(position,imageView).execute(imageUrl);        return listItem;  }}

first, the absence of any explicit interthread communication usinghandlers and messages, and second, a higher code quality achieved by having all coderelated to the download task in a single class.

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