Can't show AlertDialog in DoInBackground

问题

I am trying to use a AlertDialog in Android which will notify the users that they are runing in offline mode after checking the internet connection.

I have used the following codes:

protected Void doInBackground(Void... params) {
            if (this.isOnline()) {
                    new GetJson().execute();
            } else {

        AlertDialog.Builder builder1 = new AlertDialog.Builder(homeFragment);
        builder1.setMessage("INTERNET CONNECTION NOT AVAILABLE. Now you are viewing the news in Offline Mode.");
        builder1.setCancelable(true);

        AlertDialog alert1 = builder1.create();
        alert1.show();

        try {
            saveFile = new SaveIntoFile(fileName);
            jsonStr = saveFile.read();
            // Log.d(TAG,"offline data reading from a file");
            if (!jsonStr.equals(null))
                new GetDatas().execute();
            else {

            }
        } catch (Exception e) {
            e.printStackTrace();

        }
    }
    return null;

}

But I am getting the error on adding the codes for AlertDialog. The app works fine without the codes for AlertDialog. What can be the mistakes with this code and how can i correct it to work well??

回答1:

doInBackground() runs on a separate thread, other than the main thread. You can use UI elements only on main thread. Try using runOnUiThread() method inside doInBackground() to show the dialog.

回答2:

here AlertDialog will not work, you can use System.out.println("some text"); if you are using alert just for testing purpose. otherwise you have to set into a parameter and what ever text you want to put in alertbox and just use it in main thread.

if you have any confusion please let me know .

回答3:

In Android you can do UI operations only in main thread. So you can show dialog in onPostExecute method. Example:

    class anyClassName extends AsyncTask<String,String,String>{

    /* uses worker thread */
    protected String doInBackground(Void... params) {
                if (this.isOnline()) {
                        new GetJson().execute();
                } else { return null }
// ... rest of your code
return ""
    }

    /* uses main thread*/
    protected void onPostExecute(String result){
    if(result == null ){
    AlertDialog.Builder builder1 = new AlertDialog.Builder(homeFragment);
            builder1.setMessage("INTERNET CONNECTION NOT AVAILABLE. Now you are viewing the news in Offline Mode.");
            builder1.setCancelable(true);

            AlertDialog alert1 = builder1.create();
            alert1.show();
    }
    }
    }

回答4:

 class myAsync extends AsyncTask<String, String, String>{

    @Override
    protected String doInBackground(String... arg0) {
        // TODO Auto-generated method stub          
        if(yourWorkIsDone){
            return new String("done");
        }else{
            return new String("Error message");
        }

        //BUT, if you'd like to check and update user on the error and keep trying then this way
        // use just one of these two examples
        if(yourWorkIsDone){
            // definately is done but hey it depends on you
            publishProgress(new String("done")); 
        }else{
            // this is when you have error and wona let the user know about and keep trying to hit it right
            publishProgress(new String("error message")); 
        }
    }

    @Override
    protected void onPostExecute(String result) {
        // TODO Auto-generated method stub
        super.onPostExecute(result);
        // here your task is done
        // you can put myAlertDialogToShow(Context context,String message) depending on ur choice
    }

    @Override
    protected void onPreExecute() {
        // TODO Auto-generated method stub
        super.onPreExecute();
    }

    @Override
    protected void onProgressUpdate(String... values) {
        // TODO Auto-generated method stub
        super.onProgressUpdate(values);
        // here your task is running but you are just informing user, runs on the ui..
        // you can put myAlertDialogToShow(Context context,String message) depending on ur choice
    }

}

void myAlertDialogToShow(Context context,String message){
    AlertDialog.Builder builder1 = new AlertDialog.Builder(context);
    builder1.setMessage(message);
    builder1.setCancelable(true);
    AlertDialog alert1 = builder1.create();
    alert1.show();

}

ayt..check for silly errors and spellings.. im tired

来源：https://stackoverflow.com/questions/27499292/cant-show-alertdialog-in-doinbackground