Question

错误为System services not available to Activities before onCreate()

下面是我的AsyncTask（它位于单独文件中的另一个类中）的 GetWarehouseSales.java

public class GetWarehouseSales extends Activity {

    private String TAG = GetWarehouseSales.class.getSimpleName();
    private ListView lv;
    private ProgressDialog pDialog;
    private Activity activityContext;
    //URL to get JSON details
    private static String url = "http://192.168.0.1/mycc/retrieve_ws.php";
    ArrayList<HashMap<String,String>> sales_details;

    public GetWarehouseSales(Activity context){
        this.activityContext = context;
        sales_details = new ArrayList<>();

    }

    public void executeGWS(){
        new RetrieveWarehouseSalesTask().execute();
    }


    public class RetrieveWarehouseSalesTask extends AsyncTask<Void,Void,Void>{



        @Override
        protected void onPreExecute(){
            super.onPreExecute();
            pDialog = new ProgressDialog(activityContext);
            pDialog.setMessage("Getting you the best warehouse sales...");
            pDialog.setCancelable(false);
            pDialog.show();
        }

        @Override
        protected Void doInBackground(Void... arg0){
            HttpHandler sh = new HttpHandler();
            //making a request to URL and getting response
            String jsonStr = sh.makeServiceCall(url);
            Log.e(TAG, "Response from url: " + jsonStr);

            if(jsonStr != null){
                try{
                    JSONObject jsonObj = new JSONObject(jsonStr);
                    //Getting JSON Array Node
                    JSONArray sales = jsonObj.getJSONArray("Result");
                    //looping through all results
                    for(int i = 0; i<sales.length();i++){
                        JSONObject s = sales.getJSONObject(i);
                        String title = s.getString("title");
                        String description = s.getString("description");

                        HashMap<String,String> salesDetails = new HashMap<>();

                        //adding each child node to HashMap key =>value
                        salesDetails.put("title",title);
                        salesDetails.put("description",description);

                        //adding to array list
                        sales_details.add(salesDetails);
                    }
                    Log.d("TAG",sales_details.toString());
                }catch(final JSONException e){
                    Log.e(TAG, "JSON parsing error: " + e.getMessage());
                    runOnUiThread(new Runnable() {
                        @Override
                        public void run() {
                            Toast.makeText(getApplicationContext(),"JSON parsing error: " + e.getMessage(),Toast.LENGTH_LONG).show();
                        }
                    });
                }
            }else{
                Log.e(TAG,"Couldn't get json from server");
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        Toast.makeText(getApplicationContext(), "Check logcat", Toast.LENGTH_LONG ).show();
                    }
                });
            }
            return null;
        }

        @Override
        protected void onPostExecute(Void result){
            super.onPostExecute(result);
            if(pDialog.isShowing()){
                pDialog.dismiss();
            }


            //update parsed JSON data into ListView
            ListAdapter adapter = new SimpleAdapter(GetWarehouseSales.this, sales_details,R.layout.item_listview, new String[]{
                    "title","description"}, new int[]{R.id.text,R.id.description});
            lv = (ListView)findViewById(R.id.list_item);

            lv.setAdapter(adapter);
        }
    }


}

MainActivity.java

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState)  ;
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        new GetWarehouseSales(this).executeGWS();


    }

错误指向此行：

ListAdapter adapter = new SimpleAdapter(GetWarehouseSales.this, sales_details,R.layout.item_listview, new String[]{
                    "title","description"}, new int[]{R.id.text,R.id.description});

我正在尝试将数据填充到列表视图中，但无法这样做。

Answer 1

尝试替换

new GetWarehouseSales(this).executeGWS();

到

executeGWS();

一般来说，你是以错误的方式做到的。正如我已在此处NullPointerExeception for ProgressDialog in AsyncTask所述，您应该在AsynkTask中定义MainActivity。

Answer 2

正如Vasily Kabunov指出的那样，不要创建一个新的类实例来调用该方法。当它不是Activity时，也不要延长Activity。

但是一个主要问题是doInBackground应该建立一个结果，然后通过写入活动的一个字段（onPostExecute）安全地传递给sales_details，你这使得这个非线性安全。

因此，请勿访问sales_details中的doInBackground或其他活动字段，请执行以下操作：

public class RetrieveWarehouseSalesTask extends AsyncTask<Void,Void,ArrayList<HashMap<String,String>>>{ //return type set

    @Override
    protected ArrayList<HashMap<String,String>> doInBackground(Void... arg0){
        ArrayList<HashMap<String,String>> result = new ArrayList<>();
        ...
        //adding to array list
        result.add(salesDetails);
        ...
        return result;
    }

    @Override
    protected void onPostExecute(ArrayList<HashMap<String,String>> result){
        super.onPostExecute(result);
        if(pDialog.isShowing()){
            pDialog.dismiss();
        }

        //update parsed JSON data into ListView
        ListAdapter adapter = new SimpleAdapter(GetWarehouseSales.this, result,R.layout.item_listview, new String[]{
                "title","description"}, new int[]{R.id.text,R.id.description});
        lv = (ListView)findViewById(R.id.list_item);

        lv.setAdapter(adapter);
    }
}

无法从AsyncTask填充ListView

2 个答案: