当我按下后退按钮时如何停止再次加载ListView？

Question

我创建了一个包含三个活动的Android应用程序， MainActivity - ＆gt; SongActivity - ＆gt; DownloadActivity 即可。 MainActivity和SongActivity是ListViews，它从mysql数据库中获取项目列表。当我通过按后退按钮从SongActivity导航回MainAcitivty时，ListView再次获取数据，这使得应用程序导航变慢。有解决方案吗？

MainActivity.java

package com.example.telugump3;
import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.ArrayList;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.DefaultHttpClient;
import org.json.JSONArray;
import org.json.JSONObject;

import android.app.Activity;
import android.app.ProgressDialog;
import android.content.Intent;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.View;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.ListView;
import android.widget.TextView;

public class MainActivity extends Activity{
  Activity context;
   HttpPost httppost;
   StringBuffer buffer;
   HttpResponse response;
   HttpClient httpclient;
   ProgressDialog pd;
   CustomAdapter adapter;
   ListView listProduct;
   ArrayList<String> records;
protected void onCreate(Bundle savedInstanceState) {
   //TODO Auto-generated method stub
   super.onCreate(savedInstanceState);
   setContentView(R.layout.activity_main);
   context=this;
   records=new ArrayList<String>();
   listProduct=(ListView)findViewById(R.id.product_list);
   adapter=new CustomAdapter(context, R.layout.list_item,R.id.pro_name, records);
   listProduct.setAdapter(adapter);
   listProduct.setOnItemClickListener(new OnItemClickListener(){
       @Override
       public void onItemClick(AdapterView<?> arg0, View v, int position, long id){

           String sText = ((TextView) v.findViewById(R.id.pro_name)).getText().toString();
           Intent songIntent = new Intent(getApplicationContext(), SongActivity.class);
           songIntent.putExtra("movie_name", sText );
           startActivity(songIntent);

       }

   });
   }
   @Override
   public boolean onCreateOptionsMenu(Menu menu) {
       getMenuInflater().inflate(R.menu.main, menu);
       //create a LayoutTransition object       
      return true;
   }
   public void onStart(){
   super.onStart(); 
   //execute background task
   BackTask bt=new BackTask();
   bt.execute();

   }

  //background process to make a request to server and list product information
   private class BackTask extends AsyncTask<Void,Void,Void>{  
   protected void onPreExecute(){
       super.onPreExecute();
       pd = new ProgressDialog(context);
       pd.setTitle("Retrieving data");
       pd.setMessage("Please wait.");
       pd.setCancelable(true);
       pd.setIndeterminate(true);
       pd.show();       

       }

       protected Void doInBackground(Void...params){ 

       InputStream is=null;
       String result="";
       try{

           records.clear();

       httpclient=new DefaultHttpClient();
          httppost= new HttpPost("http://necrecords.16mb.com/getproducts.php");
       response=httpclient.execute(httppost);         
             HttpEntity entity = response.getEntity();
             is = entity.getContent();

       }catch(Exception e){

       if(pd!=null)
       pd.dismiss();  //close the dialog if error occurs 
       Log.e("ERROR",e.getMessage());

       }

       //convert response to string
       try{
            BufferedReader reader = new BufferedReader(new InputStreamReader(is,"utf-8"),8);
            StringBuilder sb = new StringBuilder();
            String line = null;
            while ((line = reader.readLine()) != null) {
                  sb.append(line + "\n");
            }
            is.close();         
            result=sb.toString();
       }catch(Exception e){
            Log.e("ERROR", "Error converting result "+e.toString());

}

       //parse json data
       try{

       JSONArray jArray =new JSONArray(result);
       for(int i=0;i<jArray.length();i++){
            JSONObject json_data = jArray.getJSONObject(i);
                  String record=json_data.getString("pname")+"__"+json_data.getInt("uprice");
                  records.add(record);


          }


       }
       catch(Exception e){
       Log.e("ERROR", "Error pasting data "+e.toString());

       }

        return null;
       }   


       protected void onPostExecute(Void result){

       if(pd!=null) pd.dismiss(); //close dialog
       adapter.notifyDataSetChanged(); //notify the ListView to get new records

       }

}
} 

SongActivity.java

package com.example.telugump3;
import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.URLEncoder;
import java.util.ArrayList;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.DefaultHttpClient;
import org.json.JSONArray;
import org.json.JSONObject;

import android.app.Activity;
import android.app.ProgressDialog;
import android.content.Intent;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.View;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.AdapterView;
import android.widget.ListView;
import android.widget.TextView;

public class SongActivity extends Activity{
  Activity context;
   HttpPost httppost;
   StringBuffer buffer;
   HttpResponse response;
   HttpClient httpclient;
   ProgressDialog pd;
   CustomAdapter adapter;
   ListView listProduct;
   ArrayList<String> records;
   String mname;
protected void onCreate(Bundle savedInstanceState) {
   //TODO Auto-generated method stub
   super.onCreate(savedInstanceState);
   setContentView(R.layout.song_activity);
   context=this;
   records=new ArrayList<String>();
   listProduct=(ListView)findViewById(R.id.product_list);
   adapter=new CustomAdapter(context, R.layout.newlist_item,R.id.pro_name, records);
   listProduct.setAdapter(adapter);
   TextView songtitle = (TextView)findViewById(R.id.movietitle);
   Intent iin= getIntent();
   Bundle b = iin.getExtras();

   if(b!=null)
   {
       mname =(String) b.getString("movie_name");
       songtitle.setText(mname);

   }

   listProduct.setOnItemClickListener(new OnItemClickListener(){ 

       @Override
       public void onItemClick(AdapterView<?> arg0, View v, int position, long id){

           String sText = ((TextView) v.findViewById(R.id.pro_name)).getText().toString();
           Intent songIntent = new Intent(getApplicationContext(), DownloadActivity.class);
           songIntent.putExtra("song_name", sText );
           startActivity(songIntent);

       }

   });

   }
@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.main, menu);
    //create a LayoutTransition object       
      return true;
}
   public void onStart(){
   super.onStart(); 
   //execute background task
   BackTask bt=new BackTask();
   bt.execute();

   }

  //background process to make a request to server and list product information
   private class BackTask extends AsyncTask<Void,Void,Void>{  
   protected void onPreExecute(){
       super.onPreExecute();
       pd = new ProgressDialog(context);
       pd.setTitle("Retrieving data");
       pd.setMessage("Please wait.");
       pd.setCancelable(true);
       pd.setIndeterminate(true);
       pd.show();       

       }

       protected Void doInBackground(Void...params){ 

       InputStream is=null;
       String result="";
       try{

           records.clear();

           String query = URLEncoder.encode(mname, "utf-8");
       httpclient=new DefaultHttpClient();
          httppost= new HttpPost("http://necrecords.16mb.com/getsongslist.php?password="+query);
       response=httpclient.execute(httppost);         
             HttpEntity entity = response.getEntity();
             is = entity.getContent();

       }catch(Exception e){

       if(pd!=null)
       pd.dismiss();  //close the dialog if error occurs 
       Log.e("ERROR",e.getMessage());

       }

       //convert response to string
       try{
            BufferedReader reader = new BufferedReader(new InputStreamReader(is,"utf-8"),8);
            StringBuilder sb = new StringBuilder();
            String line = null;
            while ((line = reader.readLine()) != null) {
                  sb.append(line + "\n");
            }
            is.close();         
            result=sb.toString();
       }catch(Exception e){
            Log.e("ERROR", "Error converting result "+e.toString());

}

       //parse json data
       try{

       JSONArray jArray =new JSONArray(result);
       for(int i=0;i<jArray.length();i++){
            JSONObject json_data = jArray.getJSONObject(i);
                  String record=json_data.getString("allsongs")+"__"+json_data.getString("test");
                  records.add(record);


          }


       }
       catch(Exception e){
       Log.e("ERROR", "Error pasting data "+e.toString());

       }

        return null;
       }   


       protected void onPostExecute(Void result){

       if(pd!=null) pd.dismiss(); //close dialog
       adapter.notifyDataSetChanged(); //notify the ListView to get new records

       }

}
} 

Answer 1

请添加你的  //执行后台任务

   BackTask bt=new BackTask();
   bt.execute();

<{1>} 代替 onCreate();

如需了解更多信息，请查看

http://developer.android.com/training/basics/activity-lifecycle/index.html

修改

onStart()

Answer 2

当您远程检查数据时，让您的活动响应更好的一种方法是使用onSaveInstanceState（）缓存数据，并将记录列表保存在您可以在活动的onCreate（）期间读取的包中。

如果系统决定从堆栈中删除您的活动，那么它将尝试再次从远程主机加载数据（这可能需要一段时间）。

同样是的，您应该在onCreate（）而不是onStart（）中执行AsyncTask，并确保在从服务器获取数据时从记录列表中删除重复项。