Android-AsyncTask-ListView适配器

Question

In short. i have one asynctask, JSONReadTask that requests & a list of radio presenters. From this list an image reference is taken and passed to another Asyntask, ImageRetrival.

i'm having difficulty working with the asynchronous of android whilst downloading images to a listview.

I intially had the adapter settings with ListView.setAdapter() at the end of the JSONReadTask so once all the images were retrived, the list could then be populated. however when setting the adapter for the listview, i kept getting NullPointerException of the data source of the adapter.

What i require, and cannot get my head around is how to set the adapter to the list view, then somehow use notifyDataSetChanged() to update the list view as each imageRetrival PostExecutes.

public class Presenters extends Activity {
    private String jsonResult;
    private String url = "http://www.example.co.uk/Radio/api.php?q=Presenters";
    public List<HashMap<String, Bitmap>> ImageList;
    private ListView listView;
    private ExtendedSimpleAdapter exad;

//private Bitmap[] images;
//private int counter;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_presenters);
    // Show the Up button in the action bar.
    setupActionBar();
    //counter = 0;

    listView = (ListView) findViewById(R.id.listViewPres);
    accessWebService();

}

/**
 * Set up the {@link android.app.ActionBar}, if the API is available.
 */
@TargetApi(Build.VERSION_CODES.HONEYCOMB)
private void setupActionBar() {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
        getActionBar().setDisplayHomeAsUpEnabled(true);
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.presenters, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
    case android.R.id.home:
        // This ID represents the Home or Up button. In the case of this
        // activity, the Up button is shown. Use NavUtils to allow users
        // to navigate up one level in the application structure. For
        // more details, see the Navigation pattern on Android Design:
        //
        // http://developer.android.com/design/patterns/navigation.html#up-vs-back
        //
        NavUtils.navigateUpFromSameTask(this);
        return true;
    }
    return super.onOptionsItemSelected(item);
}

private class ImageRetrival extends AsyncTask<String, Void, Bitmap> {

    private Bitmap btmp;
    String id;
    private String ImageUrl = "http://www.example.co.uk/RPUser/"; //Image Directory

    // ///////////
    // GET IMAGE//
    // ///////////

    protected Bitmap doInBackground(String... params) {
        // download image to bitmap
        try {
            Log.d("URLparam", params[0]);
            String u = ImageUrl + params[0];
            URL url = new URL(u);
            Log.d("URL:", "" + url);
            // split arguments
            // if(overView request)->continue;
            // if(SpecificRequest)->explode(url, 'i')-> id;
            /*
             * if(url.toString().contains("q=") &&
             * url.toString().contains("i=")){ String[] urlArray =
             * url.toString().split("i=", 2); id = urlArray[1];
             * Log.d("Url.Split>id", ""+id); }
             */
            id = url.toString();
            InputStream stream;
            BufferedInputStream bis;

            URLConnection ConUrl = url.openConnection();

            HttpURLConnection htpcon = (HttpURLConnection) ConUrl;
            htpcon.setRequestMethod("GET");
            htpcon.setDoInput(true);
            htpcon.connect();

            if (htpcon.getResponseCode() != HttpURLConnection.HTTP_OK) {
                Log.d("HTTP-ERROR", "" + htpcon.getResponseCode());
            }
            stream = htpcon.getInputStream();
            bis = new BufferedInputStream(stream);
            Log.d("Stream" + url, "" + stream);
            btmp = BitmapFactory.decodeStream(bis);

            Log.d("Bitmap-Code", "" + btmp);
            stream.close();
        } catch (IOException e) {
            Log.d("Catch", "Issue Here");
            e.printStackTrace();
        }

        return btmp;
    }

    protected void onPostExecute(Bitmap result) {

         HashMap<String, Bitmap> table = new HashMap<String, Bitmap>();
         table.put("Image", result); 
         Log.d("OnPost", ""+result);

         ImageList.add(table); //Line 154 which returns NullPointer

         exad = new ExtendedSimpleAdapter(Presenters.this,
                    ImageList, R.layout.listview_item_row_presenters, new String[]
                            {"Image", "Name", "About"}, new int[] {R.id.imgIcon, R.id.txtTitle,
                            R.id.txtAbout});

        Log.d("PostExecute", "" + result);
        listView.setAdapter(exad);
    }

}

//
// Start JSON CODE
//

private class JsonReadTask extends AsyncTask<String, Void, String> {

    @Override
    protected String doInBackground(String... params) {

        HttpClient httpclient = new DefaultHttpClient();
        HttpPost httppost = new HttpPost(params[0]);
        Log.d("JSON", "" + params[0]);
        try {

            HttpResponse responce = httpclient.execute(httppost);

            jsonResult = inputStreamToString(
                    responce.getEntity().getContent()).toString();
            Log.d("JSON", "" + jsonResult);
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }

    private StringBuilder inputStreamToString(InputStream is) {
        String rLine = "";
        StringBuilder answer = new StringBuilder();
        BufferedReader rd = new BufferedReader(new InputStreamReader(is));

        try {
            while ((rLine = rd.readLine()) != null) {
                answer.append(rLine);
            }
        }

        catch (IOException e) {
            // e.printStackTrace();
            Toast.makeText(getApplicationContext(),
                    "Error..." + e.toString(), Toast.LENGTH_LONG).show();
        }
        return answer;
    }

    @Override
    protected void onPostExecute(String result) {
        ListDrwaer();
    }
} // end Async Task extension

public void accessWebService() {
    JsonReadTask task = new JsonReadTask();
    // passes values for the urls string array
    task.execute(new String[] { url });
}

// build hash set for list view
public void ListDrwaer() {
    List<HashMap<String, String>> presenterlist = new ArrayList<HashMap<String, String>>();

    try {
        JSONObject jsonResponse = new JSONObject(jsonResult);
        JSONArray jsonMainNode = jsonResponse.getJSONArray("Presenters");

        for (int i = 0; i < jsonMainNode.length(); i++) {
            JSONObject jsonChildNode = jsonMainNode.getJSONObject(i);
            String name = jsonChildNode.getString("FirstName");
            String lastName = jsonChildNode.getString("LastName");
            String fullName = name + " " + lastName;
            String about = jsonChildNode.getString("About");
            String image = jsonChildNode.getString("Image");

            ImageRetrival getImage = new ImageRetrival();
            getImage.execute(image);
            Log.d("Progress", "Got Image " + image);

            HashMap<String, String> hm = new HashMap<String, String>();
            hm.put("Name", fullName);
            hm.put("About", about);
            hm.put("Image", image);
            presenterlist.add(hm);
        }
    } catch (JSONException e) {
        Toast.makeText(getApplicationContext(),
                "Error - JSON Retrival - HashMap" + e.toString(),
                Toast.LENGTH_SHORT).show();
    }

    /*SimpleAdapter simpleAdapter = new SimpleAdapter(this, presenterlist,
            R.layout.listview_item_row_presenters, new String[] { "Image",
                    "Name", "About" }, new int[] { R.id.imgIcon,
                    R.id.txtTitle, R.id.txtAbout });*/
    if(ImageList == null){
        Log.d("Report", "ImageListNULL");
    }
    else{
        Log.d("Report", ""+ImageList);
    }

    listView.setAdapter(exad);//Currently throws nullPointerException of HashMap<String, Bitmap>
    //listView.getAdapter().notifyDataSetChanged();//want to know how to utilise this
}
//
// End JSON Code

}

The Extended Adapter:

public class ExtendedSimpleAdapter extends SimpleAdapter{
    List<HashMap<String, Bitmap>> map;
    String[] from;
    int layout;
    int[] to;
    Context context;
    LayoutInflater mInflater;
    public ExtendedSimpleAdapter(Context context, List<HashMap<String, Bitmap>> data, int resource, String[] from, int[] to) {
        super(context, data, resource, from, to);
        layout = resource;
        map = data;
        this.from = from;
        this.to = to;
        this.context = context;
    }

@Override
public int getCount(){
    return map.size();
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    mInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    return this.createViewFromResource(position, convertView, parent, layout);
}

private View createViewFromResource(int position, View convertView,
        ViewGroup parent, int resource) {
    View v;
    if (convertView == null) {
        v = mInflater.inflate(resource, parent, false);
    } else {
        v = convertView;
    }

    this.bindView(position, v);

    return v;
}


private void bindView(int position, View view) {
    final Map dataSet = map.get(position);
    if (dataSet == null) {
        return;
    }

    final ViewBinder binder = super.getViewBinder();
    final int count = to.length;

    for (int i = 0; i < count; i++) {
        final View v = view.findViewById(to[i]);
        if (v != null) {
            final Object data = dataSet.get(from[i]);
            String text = data == null ? "" : data.toString();
            if (text == null) {
                text = "";
            }

            boolean bound = false;
            if (binder != null) {
                bound = binder.setViewValue(v, data, text);
            }

            if (!bound) {
                if (v instanceof Checkable) {
                    if (data instanceof Boolean) {
                        ((Checkable) v).setChecked((Boolean) data);
                    } else if (v instanceof TextView) {
                        // Note: keep the instanceof TextView check at the bottom of these
                        // ifs since a lot of views are TextViews (e.g. CheckBoxes).
                        setViewText((TextView) v, text);
                    } else {
                        throw new IllegalStateException(v.getClass().getName() +
                                " should be bound to a Boolean, not a " +
                                (data == null ? "<unknown type>" : data.getClass()));
                    }
                } else if (v instanceof TextView) {
                    // Note: keep the instanceof TextView check at the bottom of these
                    // ifs since a lot of views are TextViews (e.g. CheckBoxes).
                    setViewText((TextView) v, text);
                } else if (v instanceof ImageView) {
                    if (data instanceof Integer) {
                        setViewImage((ImageView) v, (Integer) data);                           
                    } else if (data instanceof Bitmap){
                        setViewImage((ImageView) v, (Bitmap)data);
                    } else {
                        setViewImage((ImageView) v, text);
                    }
                } else {
                    throw new IllegalStateException(v.getClass().getName() + " is not a " +
                            " view that can be bounds by this SimpleAdapter");
                }
            }
        }
    }
} 
private void setViewImage(ImageView v, Bitmap bmp){
    v.setImageBitmap(bmp);
} 
}

Thank you in advance

LogCat Output:

05-27 13:56:19.010: E/AndroidRuntime(2861): FATAL EXCEPTION: main
05-27 13:56:19.010: E/AndroidRuntime(2861): java.lang.NullPointerException
05-27 13:56:19.010: E/AndroidRuntime(2861):     at com.example.dev_app001.Presenters$ImageRetrival.onPostExecute(Presenters.java:154)
05-27 13:56:19.010: E/AndroidRuntime(2861):     at com.example.dev_app001.Presenters$ImageRetrival.onPostExecute(Presenters.java:1)
05-27 13:56:19.010: E/AndroidRuntime(2861):     at android.os.AsyncTask.finish(AsyncTask.java:631)
05-27 13:56:19.010: E/AndroidRuntime(2861):     at android.os.AsyncTask.access$600(AsyncTask.java:177)
05-27 13:56:19.010: E/AndroidRuntime(2861):     at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
05-27 13:56:19.010: E/AndroidRuntime(2861):     at android.os.Handler.dispatchMessage(Handler.java:99)
05-27 13:56:19.010: E/AndroidRuntime(2861):     at android.os.Looper.loop(Looper.java:213)
05-27 13:56:19.010: E/AndroidRuntime(2861):     at android.app.ActivityThread.main(ActivityThread.java:5225)
05-27 13:56:19.010: E/AndroidRuntime(2861):     at java.lang.reflect.Method.invokeNative(Native Method)
05-27 13:56:19.010: E/AndroidRuntime(2861):     at java.lang.reflect.Method.invoke(Method.java:525)
05-27 13:56:19.010: E/AndroidRuntime(2861):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:741)
05-27 13:56:19.010: E/AndroidRuntime(2861):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:557)
05-27 13:56:19.010: E/AndroidRuntime(2861):     at dalvik.system.NativeStart.main(Native Method)

Answer 1

You have 2 tasks JsonReadTask and ImageRetrival

You run JsonReadTask first, then onPostExecute you run ImageRetrival

The problem is you instantiate exad only later ImageRetrival .

So calling listView.setAdapter(exad); when ImageRetrival is not finished will create problems since the tasks run asynchronously.

I would say remove listView.setAdapter(exad); from JsonReadTask since you call it already once ImageRetrival finished executing.

You should avoid variable with class scope they can cause bugs and such null pointers.

try to keep things as private as possible.