在Android的AsyncTask的onPostExecute上显示alertDialog

Question

I have an asyncTask and I want to show an alertDialog when the onPostExecute method is fired. I declared a Context variable and I initialized it inside the OnCreate method like:

mContext = this;

Then in order to show the alertDialog on the onPostExecute method I used the following code:

            AlertDialog.Builder goLogin = new AlertDialog.Builder(mContext);
            goLogin.setMessage("test");
            goLogin.setCancelable(false);
            goLogin.setPositiveButton("ok",
                    new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog, int id) {
                    dialog.cancel();
                }
            });
        AlertDialog alertLogin = goLogin.create();
        alertLogin.show();

But what I get is the following error:

07-10 14:42:09.710: E/AndroidRuntime(12963): java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
07-10 14:42:09.710: E/AndroidRuntime(12963):    at android.content.ContextWrapper.getApplicationInfo(ContextWrapper.java:148)
07-10 14:42:09.710: E/AndroidRuntime(12963):    at android.view.ContextThemeWrapper.getTheme(ContextThemeWrapper.java:99)
07-10 14:42:09.710: E/AndroidRuntime(12963):    at android.app.AlertDialog.resolveDialogTheme(AlertDialog.java:154)
07-10 14:42:09.710: E/AndroidRuntime(12963):    at android.app.AlertDialog$Builder.<init>(AlertDialog.java:379)
07-10 14:42:09.710: E/AndroidRuntime(12963):    at test.test.showAlert(test.java:671)
07-10 14:42:09.710: E/AndroidRuntime(12963):    at test.test.checkResult(test.java:656)
07-10 14:42:09.710: E/AndroidRuntime(12963):    at test.test$login.onPostExecute(test.java:585)
07-10 14:42:09.710: E/AndroidRuntime(12963):    at test.test$login.onPostExecute(test.java:1)
07-10 14:42:09.710: E/AndroidRuntime(12963):    at android.os.AsyncTask.finish(AsyncTask.java:632)
07-10 14:42:09.710: E/AndroidRuntime(12963):    at android.os.AsyncTask.access$600(AsyncTask.java:177)
07-10 14:42:09.710: E/AndroidRuntime(12963):    at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
07-10 14:42:09.710: E/AndroidRuntime(12963):    at android.os.Handler.dispatchMessage(Handler.java:102)
07-10 14:42:09.710: E/AndroidRuntime(12963):    at android.os.Looper.loop(Looper.java:135)
07-10 14:42:09.710: E/AndroidRuntime(12963):    at android.app.ActivityThread.main(ActivityThread.java:5312)
07-10 14:42:09.710: E/AndroidRuntime(12963):    at java.lang.reflect.Method.invoke(Native Method)
07-10 14:42:09.710: E/AndroidRuntime(12963):    at java.lang.reflect.Method.invoke(Method.java:372)
07-10 14:42:09.710: E/AndroidRuntime(12963):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:901)
07-10 14:42:09.710: E/AndroidRuntime(12963):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:696)

What am I doing wrong? Do I need to pass the context in a different way?

EDIT: Whole AsyncTask code:

private class login extends AsyncTask<Void, Void, Void> {


@Override
protected void onPreExecute() {
    super.onPreExecute();
    // Showing progress dialog

}

@SuppressWarnings("deprecation")
@Override
protected Void doInBackground(Void... arg0) {
    // Creating service handler class instance
    ServiceHandler sh = new ServiceHandler();

    // Making a request to url and getting response
    List<NameValuePair> params = new ArrayList<NameValuePair>();

    params.add(new BasicNameValuePair("userName", "test"));
    String jsonStr = sh.makeServiceCall(urlTest, ServiceHandler.POST, params);


    if (jsonStr != null) {
        try {

            JSONObject obj = new JSONObject(jsonStr);

            error = obj.getBoolean("Error");
            if(!error)
            {
                loginResult = true;
                JSONObject array = obj.getJSONObject("Response");
                name = array.getString("Name");
            }
            else
            {
                loginResult = false;
            }

        } catch (JSONException e) {
            e.printStackTrace();
        }
    } else {

    }

    return null;
}

@Override
protected void onPostExecute(Void result) {
    super.onPostExecute(result);
    // Dismiss the progress dialog
    //checkResult();
    AlertDialog.Builder reorder = new AlertDialog.Builder(context);
    reorder.setMessage("error");
    reorder.setCancelable(true);
    reorder.setPositiveButton("ok",
            new DialogInterface.OnClickListener() {
        public void onClick(DialogInterface dialog, int id) {
            dialog.dismiss();
        }
    });

    AlertDialog orderError = reorder.create();
    orderError.show();
}

}

Answer 1

Create constructor of your AsyncTask and do something like this:

private class DemoTask extends AsyncTask<Void, Void, Void> {
         Context context;

    public DemoTask(Context mContext) {
        this.context = mContext;
    }

        @Override
        protected Void doInBackground(Void... params) {
            // TODO Auto-generated method stub
            return null;
        }

        @Override
        protected void onPostExecute(Void result) {
            // TODO Auto-generated method stub
            super.onPostExecute(result);

            AlertDialog.Builder goLogin = new AlertDialog.Builder(context);
            goLogin.setMessage("test");
            goLogin.setCancelable(false);
            goLogin.setPositiveButton("ok", new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog, int id) {
                    dialog.cancel();
                }
            });
            AlertDialog alertLogin = goLogin.create();
            alertLogin.show();
        }

    }

Here mContext is inside onCreate Method:

public class MyActivity extends Activity {

private Context mContext;


@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mContext = this;

        new DemoTask(mContext).execute();
    }

Answer 2

onPostExecute() is called on main thread. mContext has Activity's context.

Regardless of the fact Activity is destroyed or not, onPostExecute() will be called. When Activity is destroyed, the NPE error will occur while creating Alert Dialog Box. Make sure that Activity is on foreground.

update:

code:

@Override
        protected void onPostExecute(Void result) {
            // TODO Auto-generated method stub
            super.onPostExecute(result);

            if(getActivity() == null)
            {
              return; //since activity is in background!
            }

            AlertDialog.Builder goLogin = new AlertDialog.Builder(context);
            goLogin.setMessage("test");
            goLogin.setCancelable(false);
            goLogin.setPositiveButton("ok", new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog, int id) {
                    dialog.cancel();
                }
            });
            AlertDialog alertLogin = goLogin.create();
            alertLogin.show();
        }

Answer 3

Before you get your servis callback in onPostExecute of your AsyncTask, user can send your app to background and your activity can be destroyed. In this case your context will be null and cannot show an alert dialog.

I think it's not a good solution to show an alert dialog on AsyncTask results. You can show a Toast message. Dialogs use activity context and you can get this error.

Also can get BadTokenException in this kind of usage. To avoid these kind of errors check if your activity null and show dialog in a try catch blog.

Answer 4

Remove last two line of code and add the below line :

Instead of this line

AlertDialog alertLogin = goLogin.create();
alertLogin.show();

Use this line:

goLogin.show();