从 MySQL 数据库检索数据时出现 Android Studio Spinner 问题

Question

No error in the codes however, spinner data does not appear. When php file is check, data is retrieve successfully on desktop. Please advice on how to trouble shoot this problem. Thank you

MainActivity.java

import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.widget.ArrayAdapter;
import android.widget.Spinner;

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.JSONException;
import org.json.JSONObject;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.ArrayList;

public class MainActivity extends ActionBarActivity {
ArrayList<String> listItems=new ArrayList<>();
ArrayAdapter<String> adapter;
Spinner sp;
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    sp=(Spinner)findViewById(R.id.spinner);
    adapter=new ArrayAdapter<String>      (this,R.layout.spinner_layout,R.id.txt,listItems);
    sp.setAdapter(adapter);

}
public void onStart(){
    super.onStart();
    BackTask bt=new BackTask();
    bt.execute();
}
private class BackTask extends AsyncTask<Void,Void,Void> {
    ArrayList<String> list;
    protected void onPreExecute(){
        super.onPreExecute();
        list=new ArrayList<>();
    }
    protected Void doInBackground(Void...params){
        InputStream is=null;
        String result="";
        try{
            HttpClient httpclient=new DefaultHttpClient();
            HttpPost httppost= new HttpPost("http://10.1.1.57/localhost/getPatientsInfo.php");
            HttpResponse response=httpclient.execute(httppost);
            HttpEntity entity = response.getEntity();
            // Get our response as a String.
            is = entity.getContent();
        }catch(IOException e){
            e.printStackTrace();
        }

        //convert response to string
        try{
            BufferedReader reader = new BufferedReader(new InputStreamReader(is,"utf-8"));
            String line = null;
            while ((line = reader.readLine()) != null) {
                result+=line;
            }
            is.close();
            //result=sb.toString();
        }catch(Exception e){
            e.printStackTrace();
        }
        // parse json data
        try{
            JSONArray jArray =new JSONArray(result);
            for(int i=0;i<jArray.length();i++){
                JSONObject jsonObject=jArray.getJSONObject(i);
                // add interviewee name to arraylist
                list.add(jsonObject.getString("NAME"));
            }
        }
        catch(JSONException e){
            e.printStackTrace();
        }
        return null;
    }
    protected void onPostExecute(Void result){
        listItems.addAll(list);
        adapter.notifyDataSetChanged();
    }
}
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.qi.listview.MainActivity">

<Spinner
    android:id="@+id/spinner"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">
</Spinner>

spinner_layout.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent">

<TextView
    android:id="@+id/txt"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:padding="10dp"
    />

</LinearLayout>

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.qi.listview">

<uses-permission android:name="android.permission.INTERNET" />

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:supportsRtl="true"
    android:theme="@style/AppTheme">
    <activity android:name=".MainActivity">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
</application>

enter image description here

Answer 1

You have error in your async task, you have local in task

ArrayList<String> list; // !?

but you must return in task this List and then assign it.

private class BackTask extends AsyncTask<Void,Void,List<String>> {

and

protected List<String> doInBackground(Void...params){

and in doInBackground:

return list;

and in onPostExecute:

protected void onPostExecute(List<String> result){
    listItems.addAll(result);
    adapter.notifyDataSetChanged();
}

makes sense? ))