How to correctly start activity from PostExecute in Android?

Question

I have an AsyncTask, that fills a custom List with parsed data from Internet.

In PostExecute I fill that List and get it ready to transfer it to a new Activity.

I do it this way:

@Override
protected void onPostExecute(List<VideoDataDescription> result) 
{
    super.onPostExecute(result);
    MainActivity.progressDialog.dismiss();

    context.startActivity(new Intent(context, ResultsQueryActivity.class));


}

where context

    private Context context;

In LogCat after executing this code I get a Java.lang.NullPointerException. Is this possible and correct to start an Activity as I do it?

UPD I have added

    private Context mContext;


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

to initialize context and call

YoutubeAndYahooParser youtubeAndYahooParser = new YoutubeAndYahooParser(ResultsQueryActivity.this);
                    youtubeAndYahooParser.execute("my string to pass in asynctak");

After this in PostExecute

Intent intent = new Intent(mContext, ResultsQueryActivity.class);
 intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
 mContext.startActivity(intent);

I added new flag because of I have got in LogCat the next:

*Calling startActivity() from outside of an Activity context requires the FLAG_ACTIVITY_NEW_TASK flag. Is this really what you want?*

Am I right?

how have you initialized `context`? – Vladimir Feb 02 '12 at 18:47 — Vladimir, Feb 02 '12 at 18:47
Seems to be, no. How to do initialize context? – Eugene Shmorgun Feb 02 '12 at 19:04 — Eugene Shmorgun, Feb 02 '12 at 19:04

dhaag23 · Accepted Answer · 2012-02-02T23:10:33.010

44

You should pass in the application context rather than a context from the local activity. I.e. use context.getApplicationContext() and save that in a local variable in your AsyncTask subsclass.

The code might looks something like this:

public class MyAsyncTask extends AsyncTask {

    Context context;
    private MyAsyncTask(Context context) {
        this.context = context.getApplicationContext();
    }

    @Override
    protected Object doInBackground(Object... params) {
        ...
    }

    @Override
    protected void onPostExecute(List<VideoDataDescription> result) {
        super.onPostExecute(result);
        MainActivity.progressDialog.dismiss();

        context.startActivity(new Intent(context, ResultsQueryActivity.class));
    }
}

you'd call it like this:

   new MyAsyncTask(context).execute();

edited Feb 02 '12 at 23:10

answered Feb 02 '12 at 19:03

dhaag23

6,106
37
35

Please, can you add come code how to do it? I updated topicfor details. – Eugene Shmorgun Feb 02 '12 at 20:52
1

On `new MyAsyncTask(context).execute();` where does context come from? – dotty Jul 02 '12 at 11:50
12

you also need to set the flag activity new task - else you will get a runtime exception: Intent i = new Intent(context, MyActivity.class); i.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK); context.startActivity(i); – crazy horse Jul 20 '12 at 08:40
1

A reference related with the @crazyhorse's comment: http://stackoverflow.com/q/3689581/842697 – Brais Gabin Jun 13 '13 at 12:15
1

@dotty YourClassName.this should be instead of context. – Saba Jamalian Jun 04 '14 at 21:19
I had to save the value of the context out of the AsyncTask, and pass it as a parameter, as it crashed when I was trying to get the value inside. – PayToPwn Mar 05 '16 at 15:47

score 18 · Answer 2 · answered Mar 16 '13 at 12:25

18

I tried this just now ... it works in PostExecute Method!!!

Intent intent_name = new Intent();
intent_name.setClass(getApplicationContext(),DestinationClassName.class);
startActivity(intent_name);

answered Mar 16 '13 at 12:25

score 0 · Answer 3 · answered Jun 26 '18 at 19:39

But its better if you start a new Intent Based on the response(result) obtained from the previous activities.

This will eliminate the possibility of the error response from invoking the new intent.

Example if the previous activity was supposed to return Succesfully... or Welcome to allow the new intent to start, the i could check it out in this way

  protected void onPostExecute(String result) {
       if (result.equals("Succesfully...")){
           context.startActivity(new Intent(context, Login_Activity.class));
           Toast.makeText(context, result, Toast.LENGTH_LONG).show();


       }else  if (result.contains("Welcome")){
           context.startActivity(new Intent(context, MainActivity.class));
       }else {
           Toast.makeText(context,result,Toast.LENGTH_LONG).show();
       }

    }

How to correctly start activity from PostExecute in Android?

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