避免一次又一次从JSON中获取数据

Question

我在MainActivity中解析来自服务器的JSON数据，但是每当我切换到另一个活动，然后再次调用MainActivity.问题来了，它再次访问JSON url，再次从JSON获取数据。

为什么？而我已经从JSON下载了数据

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

尝试使用静态变量

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

然后，在调用AsyncTask之前，检查是否为true或false

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

因此，只有当标志为false时，异步任务才会执行.希望这能有所帮助。

Answer 2

根据你的评论"I am calling MainActivity from SecondActivity using onBackPressed() and also calling MainActivity from ThirdActivity using Intent.“

在SecondActivity onBackPressed()：简单地调用finish()不会重新加载MainActivity，因此它不会重新加载和调用web服务。

另一个选项是有一个标志，保存在shared preferences中。

步骤：

1. 当第一次调用服务时，将标志didCallService = true保存到共享首选项。将json响应也保存在shared preference中作为字符串。
2. 当您从其他2个活动到达onCreate() on MainActivity时，检查来自用户默认值的didCallService是否为真。如果是这样，请不要再次调用该服务。

Answer 3

接受一个boolean变量并将其初始化为true，并将其状态保存为sharedpreference。现在，当MainActivity启动时，检查此boolean是否为真或不使用sharedpreference。如果为真，则获取JSON并将boolean转换为false，并将boolean状态保存为sharedpreference。在此之后，每当调用MainActivity时，由于布尔值为false，将不会获取JSON。