避免一次又一次地从JSON中获取数据

Question

I am parsing JSON data from server in MainActivity but whenever I switch to another Activity and then again calling MainActivity... here comes the problem, it again hitting the JSON url, again fetching the data from JSON.

WHY? Whereas I already downloaded data from JSON

public class MainActivity extends Activity {

    ArrayList<Actors> actorsList;

    ActorAdapter adapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        actorsList = new ArrayList<Actors>();
        new JSONAsyncTask().execute("http://microblogging.wingnity.com/JSONParsingTutorial/jsonActors");

        ListView listview = (ListView)findViewById(R.id.list);
        adapter = new ActorAdapter(getApplicationContext(), R.layout.row, actorsList);

        listview.setAdapter(adapter);

        listview.setOnItemClickListener(new OnItemClickListener() {

            @Override
            public void onItemClick(AdapterView<?> arg0, View arg1, int position,
                    long id) {
                // TODO Auto-generated method stub
                Toast.makeText(getApplicationContext(), actorsList.get(position).getName(), Toast.LENGTH_LONG).show();              
            }
        });
    }


    class JSONAsyncTask extends AsyncTask<String, Void, Boolean> {

        ProgressDialog dialog;

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            dialog = new ProgressDialog(MainActivity.this);
            dialog.setMessage("Loading, please wait");
            dialog.setTitle("Connecting server");
            dialog.show();
            dialog.setCancelable(false);
        }

        @Override
        protected Boolean doInBackground(String... urls) {
            try {

                //------------------>>
                HttpGet httppost = new HttpGet(urls[0]);
                HttpClient httpclient = new DefaultHttpClient();
                HttpResponse response = httpclient.execute(httppost);

                // StatusLine stat = response.getStatusLine();
                int status = response.getStatusLine().getStatusCode();

                if (status == 200) {
                    HttpEntity entity = response.getEntity();
                    String data = EntityUtils.toString(entity);


                    JSONObject jsono = new JSONObject(data);
                    JSONArray jarray = jsono.getJSONArray("actors");

                    for (int i = 0; i < jarray.length(); i++) {
                        JSONObject object = jarray.getJSONObject(i);

                        Actors actor = new Actors();

                        actor.setName(object.getString("name"));
                        actor.setDescription(object.getString("description"));
                        actor.setDob(object.getString("dob"));
                        actor.setCountry(object.getString("country"));
                        actor.setHeight(object.getString("height"));
                        actor.setSpouse(object.getString("spouse"));
                        actor.setChildren(object.getString("children"));
                        actor.setImage(object.getString("image"));

                        actorsList.add(actor);
                    }
                    return true;
                }

                //------------------>>

            } catch (ParseException e1) {
                e1.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            } catch (JSONException e) {
                e.printStackTrace();
            }
            return false;
        }

        protected void onPostExecute(Boolean result) {
            dialog.cancel();
            adapter.notifyDataSetChanged();
            if(result == false)
                Toast.makeText(getApplicationContext(), "Unable to fetch data from server", Toast.LENGTH_LONG).show();

        }
    }   
}

Answer 1

Try using a static variable

 static boolean flag=false;
 static ArrayList<Actors> actorsList;

Then before calling the AsyncTask check if flag is true or false

if(!flag){
 actorsList = new ArrayList<Actors>(); //use this inside if statement
 new JSONAsyncTask().execute("http://microblogging.wingnity.com/JSONParsingTutorial/jsonActors");
flag=true;
}

so only if the flag is false the async task is executed... Hope this helps.

Answer 2

According to your comment " I am calling MainActivity from SecondActivity using onBackPressed() and also calling MainActivity from ThirdActivity using Intent. "

At SecondActivity onBackPressed() : simply calling finish() will not reload the MainActivity, hence it will not reload and call web service again.

Another option is to have a flag, saved in shared preferences .

Steps:

1. Save a flag didCallService = true to shared preferences when the service is called first time. Save the json response also in the s hared preference as a string.

2. When you reach onCreate() on MainActivity from other 2 activities check if didCallService from user defaults is true. If so do not call the service again.

Answer 3

Take one boolean variable and initialize it to true and also save it's state to sharedpreference . Now when MainActivity starts, check if this boolean is true or not using sharedpreference . If it is true, fetch JSON and turn boolean to false and also save state of boolean to sharedpreference . Now after this whenever MainActivity is called, because boolean is false, JSON will not be fetched.

Answer 4

you can use "savedInstanceState" varibale for this purpose please check below code

if(savedInstanceState==null)
{
     new JSONAsyncTask().execute("http://microblogging.wingnity.com/JSONParsingTutorial/jsonActors");
}

first time when you start activity its savedInstanceState is null next time t has value so add above condition in your code.

Answer 5

Use singleTask

In your AndroidManifest.xml file add this property to MainActivity ,

<activity ..
      android:launchMode= "singleTask" />

This will reload the instance of MainActivity and onCreate() won't be called, instead new intent will be available to use in onNewIntent() method.