无法将值从doInBackground赋值给主类

Question

I have a android class where I initialise my variables:

public class myClass extends Activity {
String itemIdString;
...
}

then I invoke AssyncTask in my onCreate method

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.next_item_caller);

            Log.d("Before " +itemName);
            new getItemNumber().execute();
            Log.d("After" +itemName);
            ...
     return "success";
    }

In AssyncTask I parse JSON object and assign values to variables:

 class getItemNumber extends AsyncTask<String, String, String> {
    protected void onPreExecute() {
        super.onPreExecute();
        Log.d("getItemNumber", "On pre-execute");
    }

protected String doInBackground(String... args) {
//parse here
itemIdString = c.getString(TAG_ITEM_ID);
Log.d(TAG, itemIdString);
}
 protected void onPostExecute(String result) {
        super.onPostExecute(result);
        Log.d("getItemNumber", "on Post-Execute");
    }

My Log in AssyncTask prints values as supposed to, but in onCreate before and after values are null? How should I assign those strings so that I can access them from other methods as well? Thanks

Answer 1

AsyncTask 's doInBackground runs on another thread.

onCreate runs on the UI thread.

Therefore when you call AsyncTask.execute() you don't wait for it to finish and move on with your next line which is

Log.d("After" +itemName);

To detect when doInBackground has finished, you have a convenience callback method onPostExecute :

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

    @Override
    protected Void doInBackground(Void... params) {
        return null;
    }

    @Override
    protected void onPostExecute(Void aVoid) {
        super.onPostExecute(aVoid);
        // doInBackground finished, use your variable now
    }

}

Answer 2

Return the string value from the doInBackgroind method and then assign it to itemIdString in the onPostExecute method which runs on the main thread

class getItemNumber extends AsyncTask<String, String, String> {
    protected void onPreExecute() {
        super.onPreExecute();
        Log.d("getItemNumber", "On pre-execute");
    }

protected String doInBackground(String... args) {
    //parse here
    return c.getString(TAG_ITEM_ID);
}

 protected void onPostExecute(String result) 
   super.onPostExecute(result);
    //assign itemIdString here
   itemIdString = result;
}

Answer 3

There is also another way to do this is that you can check status of your AsyncTask as follows:

in onCreate() , you can check the status of AysncTask like this one:

MyAsyncTask myAsyncTask = new MyAsyncTask();
myAsyncTask.execute();

if(myAsyncTask.getStatus() == AsyncTask.Status.FINISHED){
        // MyAsyncTask completed. assign your variable now    
}