Android：访问内部AsyncTask类的变量，但在片段的onCreateView中将其获取为null

Question

I have the following codes which get the value of LDR sensor from the cloud. The variable which I am trying to access is ldrVal but when I am trying to access that value outside that inner class I am getting it as 0 when I am printing it in a Toast . However, when I printed it in a Runnable in the inner AsyncTask class, I got the correct non-zero ldr value.

public class AutomatedControlsFragment extends Fragment {
        public int ldrVal=0;

    @Override
        public void onActivityCreated(Bundle savedInstanceState) {
            super.onActivityCreated(savedInstanceState);
            new UbidotsConnection().execute();
        }

        public class UbidotsConnection extends AsyncTask{
            private final String API_KEY = "XXXXXXXXXXXXXXXXX";
            private final String UBIDOTS_ID_FOR_LIGHT1 = "XXXXXXXXXXXXX";
            Value[] ldrValues;

            @Override
            protected Object doInBackground(Object[] params){
                ApiClient apiClient = new ApiClient(API_KEY);
                light= apiClient.getVariable(UBIDOTS_ID_FOR_LIGHT1);
                ldrValues= light.getValues();

                ldrVal= (int) ldrValues[0].getValue();
                //Toast.makeText(getActivity().getApplicationContext(), "ldr value: "+ldrVal,Toast.LENGTH_LONG).show();

                //toast giving correct value
                getActivity().runOnUiThread(new Runnable() {
                    public void run() {
                        Toast.makeText(getActivity().getApplicationContext(), "ldr value: "+ldrVal, Toast.LENGTH_SHORT).show();
                    }
                });

                return null;
            }
    @Override
    protected void onPostExecute(Object o) {
        ldrVal= (int) ldrValues[0].getValue();
    }
        }

     @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

            View view = inflater.inflate(R.layout.fragment_automated, container, false);

            //toast giving 0
            Toast.makeText(getActivity().getApplicationContext(), "ldr value: "+ldrVal,Toast.LENGTH_LONG).show();

      return view; 
    }
}

The Toast on the onCreateView shows ldr value: 0 while the Toast in the AsyncTask shows ldr value: 90 . I need to access the variable ldrVal in onCreateView with its latest value.

Answer 1

I think the value is showing correctly. I do not understand what do you want exactly. However, the values stored in lrdVal is fetched in the doInBackground method of your AsyncTask and then its initialized inside the AsyncTask . So the Toast is showing the non-zero value while inside your onCreateView function you are not getting the value, as the value is not set yet.

You need to check the Fragment lifecycle here, which clearly shows that the onCreateView is called before onActivityCreated . You are calling the new UbidotsConnection().execute(); method inside onActivityCreated which is initializing the value based on the code segment you have shared. So I think the behaviour is correct based on your implementation.

I'm not sure if either of them is possible though. I have to access ldrVal in onCreateView in order to perform some operations on it based if it exceeds a certain value.

You cannot get the updated value of ldrVal inside your onCreateView . Take the actions necessary inside your doInBackgroud method or inside the onPostExecute function of your AsyncTask where your updated value will be available for ldrVal .

I fetched ldrVal in onPostExecute but I still got it as 0 when I displayed variable in a Toast in onCreateView. Here is what I did: @Override protected void onPostExecute(Object o) { ldrVal= (int) ldrValues[0].getValue(); }

Instead of passing a generic Object o , you might consider passing the array instead, in your onPostExecute function.

public class UbidotsConnection extends AsyncTask<Void, Void, Value[]> {
    private final String API_KEY = "XXXXXXXXXXXXXXXXX";
    private final String UBIDOTS_ID_FOR_LIGHT1 = "XXXXXXXXXXXXX";

    @Override
    protected Value[] doInBackground(Void... params) {
        ApiClient apiClient = new ApiClient(API_KEY);
        light = apiClient.getVariable(UBIDOTS_ID_FOR_LIGHT1);
        Value[] ldrValues = light.getValues();

        ldrVal = (int) ldrValues[0].getValue();
        //Toast.makeText(getActivity().getApplicationContext(), "ldr value: "+ldrVal,Toast.LENGTH_LONG).show();

        //toast giving correct value
        getActivity().runOnUiThread(new Runnable() {
            public void run() {
                Toast.makeText(getActivity().getApplicationContext(), "ldr value: " + ldrVal, Toast.LENGTH_SHORT).show();
            }
        });

        return ldrValues;
    }

    @Override
    protected void onPostExecute(Value[] values) {
        ldrVal = (int) values[0].getValue();
    }
}