findViewById（int）方法未定义

Question

I'm new to Android development and I'm trying to code a little app which allows me to grab an external JSON file and parse it. I got this to work, however it wont work if I try to execute it in the background as an AsyncTask . Eclipse gives me the error

The method findViewById(int) is undefined for the type LongOperation

in this line:

TextView txtView1 = (TextView)findViewById(R.id.TextView01);

Here is my code:

public class Main extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        new LongOperation().execute();
    }
}


class LongOperation extends AsyncTask<String, Void, String> {
    private final Context LongOperation = null;


    @Override
    protected String doInBackground(String... params) {
        try {
            URL json = new URL("http://www.corps-marchia.de/jsontest.php");
            URLConnection tc = json.openConnection();
            BufferedReader in = new BufferedReader(new InputStreamReader(tc.getInputStream()));

            String line;
            while ((line = in.readLine()) != null) {
                    JSONArray ja = new JSONArray(line);
                    JSONObject jo = (JSONObject) ja.get(0);
                    TextView txtView1 = (TextView)findViewById(R.id.TextView01);
                    txtView1.setText(jo.getString("text") + " - " + jo.getString("secondtest"));
            }
        } catch (MalformedURLException e) {
            Toast.makeText(this.LongOperation, "Malformed URL Exception: " + e, Toast.LENGTH_LONG).show();
        } catch (IOException e) {
            Toast.makeText(this.LongOperation, "IO Exception: " + e, Toast.LENGTH_LONG).show();
        } catch (JSONException e) {
            Toast.makeText(this.LongOperation, "JSON Exception: " + e, Toast.LENGTH_LONG).show();
        }
        return null;
    }

    @Override
    protected void onPostExecute(String result) {
    }
    protected void onPreExecute() {
    }

    @Override
    protected void onProgressUpdate(Void... values) {
        ProgressDialog pd = new ProgressDialog(LongOperation);
        pd.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
        pd.setMessage("Working...");
        pd.setIndeterminate(true);
        pd.setCancelable(false);
    }    

}

Any ideas on how to fix this?

Answer 1

Here is what you should do to make it work as you want. Use onPostExecude()

public class Main extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        new LongOperation(this).execute();
    }
}


class LongOperation extends AsyncTask<String, Void, String> {
    private Main longOperationContext = null;

    public LongOperation(Main context) {
        longOperationContext = context;
        Log.v("LongOper", "Konstuktor");
    }

    @Override
    protected String doInBackground(String... params) {
        Log.v("doInBackground", "inside");
        StringBuilder sb = new StringBuilder();

        try {
            URL json = new URL("http://www.corps-marchia.de/jsontest.php");
            URLConnection tc = json.openConnection();
            BufferedReader in = new BufferedReader(new InputStreamReader(tc.getInputStream()));

            String line;
            while ((line = in.readLine()) != null) {
                    JSONArray ja = new JSONArray(line);
                    JSONObject jo = (JSONObject) ja.get(0);
                    Log.v("line = ", "jo.getString() ="+jo.getString("text"));
                    sb.append(jo.getString("text") + " - " + jo.getString("secondtest")).append("\n");
            }
        } catch (MalformedURLException e) {
            e.printStackTrace();
            Log.v("Error", "URL exc");
        } catch (IOException e) {
            e.printStackTrace();
            Log.v("ERROR", "IOEXECPTOIn");
        } catch (JSONException e) {
            e.printStackTrace();
            Log.v("Error", "JsonException");
        }
        String result = sb.toString();
        return result;
    }

    @Override
    protected void onPostExecute(String result) {
        Log.v("onPostExe", "result = "+result);
      TextView txtView1 = (TextView)longOperationContext.findViewById(R.id.textView01);
      txtView1.setText(result);


    }
    protected void onPreExecute() {
    }

    @Override
    protected void onProgressUpdate(Void... values) {
        ProgressDialog pd = new ProgressDialog(longOperationContext);
        pd.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
        pd.setMessage("Working...");
        pd.setIndeterminate(true);
        pd.setCancelable(false);
    }    

}

Answer 2

You are trying to do something which won't work. First of all you are inside of a class that extends AsyncTask so you won't have that method available as it is a method of the class Activity .

The second problem is that you are trying to do UI stuff in a method that is not synchronized with the UI thread. That is nothing you would want to do.

Process your JSON response in the doInBackground method and pass the result to the onPostExecute method where you will be able to handle UI stuff as it is synchronized with the UI thread.

The current setup you have will not make it easier for you to handle what you are trying to do anyway. You could make your LongOperation class a private class of your Activity class and define the TextView as a instance member. Grab it off the layout using findViewById inside of your OnCreate and modify (set text or whatever) inside the onPostExecute method of your AsyncTask .

I hope it is somewhat clear what I meant.

Answer 3

The implementation of AsyncTask in one of the other answers is flawed. The progress dialog is being created every time within publishProgress , and the reference to the dialog is not visible outside the method. Here is my attempt:

public class Main extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        new LongOperation().execute();
    }
    class LongOperation extends AsyncTask<String, Void, String> {
        ProgressDialog pd = null;
        TextView tv = null;

        @Override
        protected void onPreExecute(){
            tv = Main.this.findViewById(R.id.textvewid);
            pd = new ProgressDialog(Main.this);
            pd.setMessage("Working...");
            // setup rest of progress dialog
        }
        @Override
        protected String doInBackground(String... params) {
            //perform existing background task
            return result;
        }
        @Override
        protected void onPostExecute(String result){
            pd.dismiss();
            tv.setText(result);
        }
    }
}

Answer 4

findViewById is method in Activity class. You should pass instance of your activity to your LongOperation when you create it. Then use that instance to call findViewById.