Question

我在Viewpager中使用AsyncTask，到目前为止一切正常。当我运行我的应用程序时，一切正常，并通过从一列滑动到另一列来显示数据。但是当您启动应用程序时，第一列中的数据不会出现。我已尝试在onCreate中插入以下代码：

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    url = "1";
    layout = R.id.lista;
    empty = R.id.empty;
    DownloadJSON newTask = new DownloadJSON(url,layout,empty);
    newTask.execute();
    ...

它不起作用。该应用已关闭。如何在应用启动时加载此数据？

我的代码：

public class MainActivity extends Activity {

ViewPager vp;
private vpAdapter myAdapter;
ListViewAdapter adapter;
ListView listview;
String url;
int layout;
int empty;
ArrayList<HashMap<String, String>> arraylist;

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    ctx = this;
    setContentView(R.layout.activity_main);

 vp = (ViewPager) findViewById(R.id.pager);
    myAdapter = new vpAdapter();
    vp.setAdapter(myAdapter);


vp.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
        @Override
        public void onPageScrolled(int i, float v, int i2) {

        }

        @Override
        public void onPageSelected(int i) {
                                 //CASE 0 = FIRST PAGE
            switch (i) {
            case 0:
                url = "1";
                layout = R.id.lista;
                empty = R.id.empty;
                DownloadJSON newTask = new DownloadJSON(url,layout,empty);
                newTask.execute();
                break;
            default:
                break;
            case 1:
                url = "2";
                layout = R.id.lista2;
                empty = R.id.empty2;
                DownloadJSON newTask2 = new DownloadJSON(url,layout,empty);
                newTask2.execute();
                break;
            case 2:
                url = "3";
                layout = R.id.lista3;
                empty = R.id.empty3;
                DownloadJSON newTask3 = new DownloadJSON(url,layout,empty);
                newTask3.execute();
                break;
            case 3:
                url = "4";
                layout = R.id.lista4;
                empty = R.id.empty4;
                DownloadJSON newTask4 = new DownloadJSON(url,layout,empty);
                newTask4.execute();
                break;
            case 4:
                url = "5";
                layout = R.id.lista5;
                empty = R.id.empty5;
                DownloadJSON newTask5 = new DownloadJSON(url,layout,empty);
                newTask5.execute();
                break;
            case 5:
                url = "6";
                layout = R.id.lista6;
                empty = R.id.empty6;
                DownloadJSON newTask6 = new DownloadJSON(url,layout,empty);
                newTask6.execute();
                break;
            }   
        }

        @Override
        public void onPageScrollStateChanged(int arg0) {
            // TODO Auto-generated method stub

        }
    });

    private class DownloadJSON extends AsyncTask<Void, Void, Void> {
    String givemeurl;
    int givemelayout;
    int givemeempty;

    public DownloadJSON(String url, int layout, int empty) {
        this.givemeurl = url;
        this.givemelayout = layout;
        this.givemeempty = empty;
    }


    @Override
    protected void onPreExecute() {
        super.onPreExecute();

        ListView listview = (ListView) findViewById(layout);
        if(listview.getCount()==0){
            mProgressDialog = new ProgressDialog(MainActivity.this);
            mProgressDialog.setMessage("Cargando...");
            mProgressDialog.setIndeterminate(false);
            mProgressDialog.show();
        }else{
            cancel(true);
            Log.e("CANCELADO","CANCELADO");
        }

    }

    @Override
    protected Void doInBackground(Void... params) {
        // Create an array
        arraylist = new ArrayList<HashMap<String, String>>();
        // Retrieve JSON Objects from the given URL address
        jsonobject = JSONfunctions
                .getJSONfromURL("URL");

        if(jsonobject != null){         
            try {
                // Locate the array name in JSON
                jsonarray = jsonobject.getJSONArray("productos");

                for (int i = 0; i < jsonarray.length(); i++) {
                    HashMap<String, String> map = new HashMap<String, String>();
                    jsonobject = jsonarray.getJSONObject(i);
                    // Retrive JSON Objects
                    map.put("imagen", jsonobject.getString("imagen"));
                    map.put("nombre", jsonobject.getString("nombre"));
                    map.put("total", jsonobject.getString("total"));
                    map.put("empresa", jsonobject.getString("empresa"));
                    map.put("unidades", jsonobject.getString("unidades"));
                    map.put("precionuevo", jsonobject.getString("precionuevo")+" €");
                    map.put("precioantiguo", jsonobject.getString("precioantiguo")+" €");
                    map.put("descripcion", jsonobject.getString("descripcion"));
                    map.put("direccion", jsonobject.getString("direccion"));
                    map.put("telefono", jsonobject.getString("telefono"));
                    map.put("latitud", jsonobject.getString("latitud"));
                    map.put("longitud", jsonobject.getString("longitud"));
                    map.put("codeqr", jsonobject.getString("codeqr"));
                    arraylist.add(map);
                }

            } catch (JSONException e) {
                Log.e("Error", e.getMessage());
                //e.printStackTrace();
            } catch (Exception e) {
                Log.e("Error", e.getMessage());         
            }
        }else{
            //Log.e("Response","No data");
        }

        return null;
    }

    @Override
    protected void onPostExecute(Void args) {
        listview = (ListView) findViewById(givemelayout);
        listview.setEmptyView(findViewById(givemeempty));
        adapter = new ListViewAdapter(MainActivity.this, arraylist);
        listview.setAdapter(adapter);
        mProgressDialog.dismiss();
        //Log.e("",""+listview.getCount());
    }
}

Answer 1

为了获得更好的性能，您应该在显示视图寻呼机之前下载所有数据。目前，只要用户滑动视图寻呼机，您就会一次又一次地下载相同的数据。并且最初看不到第一页上的任何数据是预期的结果，因为您只在onPageChangeListener中下载数据。我的建议是：

创建一个虚拟活动并使其成为启动器。它将用于下载数据。您最初也可以进行一些配置。
在该活动的onCreate方法中，使用AsyncTask下载您的应用所需的所有必要数据。
onPostExecute的{{1}}方法开始您的主要目的。
并完成虚拟活动以将其从堆栈中移除，以便当用户单击时，您的虚拟活动将不会被看到。

您可以将获得的数据传递给主要意图：

使用putExtra
或者您可以将它们存储在Application类中，这是唯一不会被Android操作系统销毁的类，因此在那里存储数据是安全的。

使用viewpager AsyncTask启动应用程序并加载数据

1 个答案: