Question

错误：在BackgroundTask中，我在try块中获取arraylist中的数据，但是当我到达errorListener（）之后，我的arrayList变为null。在LogCat中我发现了这些问题。我如何解决这个问题.. ??

package com.example.rahul.volley_jarray;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;

import java.util.ArrayList;

public class DisplayList extends AppCompatActivity {

    RecyclerView recyclerView;
    RecyclerView.Adapter adapter;
    RecyclerView.LayoutManager layoutManager;
    ArrayList<Contact> arrayList=new ArrayList<>();
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_display_list);

        recyclerView= (RecyclerView) findViewById(R.id.recylerView);
        layoutManager=new LinearLayoutManager(this);
        recyclerView.setLayoutManager(layoutManager);
        recyclerView.setHasFixedSize(true);

        BackgroundTask backgroundTask=new BackgroundTask(DisplayList.this);
        arrayList=backgroundTask.getList();

        Log.d("dispaly Array List",""+arrayList.toString());

        adapter=new RecyclerAdapter(arrayList);
        Log.d("My adapter",arrayList.toString());
        recyclerView.setAdapter(adapter);

    }
}

错误：在这个地方。 BackgroundTask

package com.example.rahul.volley_jarray;

import android.content.Context;
import android.util.Log;
import android.widget.Toast;

import com.android.volley.Request;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.JsonArrayRequest;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.util.ArrayList;

/**
 * Created by rahul on 7/4/2016.
 */
public class BackgroundTask {

    Context context;
    ArrayList<Contact> arrayList=new ArrayList<>();
    String str_url="https://raw.githubusercontent.com/ianbar20/JSON-Volley-Tutorial/master/Example-JSON-Files/Example-Array.JSON";
    String data="";
    public BackgroundTask(Context context)
    {
        this.context=context;
    }
    public ArrayList<Contact> getList()
    {
        final JsonArrayRequest jsonArrayRequest=new JsonArrayRequest(Request.Method.GET, str_url, (String) null, new Response.Listener<JSONArray>() {
            @Override
            public void onResponse(JSONArray response) {

                Log.d("My Json",response.toString());


                    int count = 0;
                    while (count < response.length()) {
                        try {
                            JSONObject jsonObject = response.getJSONObject(count);
                            JSONObject jsonObject1 = jsonObject.getJSONObject("phone");
                            Contact contact = new Contact(jsonObject.getString("name"), jsonObject.getString("email"), jsonObject1.getString("home"));
                            Log.d("contact", "" + contact.toString());
                            arrayList.add(contact);
                            Log.d("arrayList" + count, "" + arrayList.toString());
                            count++;


                        } catch (JSONException e) {
                            e.printStackTrace();
                            Log.d("Mytag", e.toString());
                        }
                        Log.d("arrayList in while" + count, "" + arrayList.toString());

                        Log.d("arrayList2" + count, "" + arrayList.toString());
                        if (arrayList.toString() == null) {
                            Log.d("if first", "null");
                        } else {
                            Log.d("else first", "not null");
                        }

                    }
            }
        },

        new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {

                Toast.makeText(context,"Error...!!!",Toast.LENGTH_SHORT).show();
                error.printStackTrace();
            }
        });

        Log.d("final arrayList",""+arrayList.toString());
        //return arrayList;
        if(arrayList.toString()==null)
        {
            Log.d("if second","null");
        }
        else {
            Log.d("else second","not null");
        }

        Log.d("JsonArray Request",""+jsonArrayRequest.toString());

        MySingleton.getInstance(context).addToRequestQueue(jsonArrayRequest);
//
        return arrayList;
    }
}

// MySinglton类包com.example.rahul.volley_jarray;

import android.content.Context;
import android.util.Log;

import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.toolbox.Volley;

/**
 * Created by rahul on 7/4/2016.
 */
public class MySingleton {

    private static MySingleton mInstance;
    private static Context mCtx;
    private RequestQueue requestQueue;

    private MySingleton(Context context) {
        mCtx = context;
        requestQueue = getRequestQueue();
        Log.d("request queue", "" + requestQueue.toString());
    }

    public RequestQueue getRequestQueue() {
        if (requestQueue == null) {
            requestQueue = Volley.newRequestQueue(mCtx.getApplicationContext());
        }
        return requestQueue;
    }

    public static synchronized MySingleton getInstance(Context context) {
        if (mInstance == null) {
            mInstance = new MySingleton(context);
        }
        Log.d("mInstaces", "" + mInstance);
        return mInstance;
    }

    public <T> void addToRequestQueue(Request<T> request) {
        Log.d("request",""+request.toString());
        requestQueue.add(request);
        Log.d("now request queue",""+requestQueue.toString());
    }
}

Answer 1

Volley执行异步请求。当你检查arrayList时它还没有完成（实际上它还没有在队列中），这就是为什么它是null （实际上应该是空的，因为你已经在字段中初始化它，而不是null）

您可以使用自定义侦听器，将其传递到请求类并将其命名为onResponse。

一些伪代码澄清：

//define this interface in background task and implement it in caller activity
public interface CustomReqFinished(){
    public void onCustomReqFinished(ArrayList list){}
}

//in backgroundtask constructor store the activity you pass as follows
listener = (CustomReqFinished) activity; //set Activity as param instead of context
//in onResponse do this as last step
listener.onCustomReqFinished(arrayList);

编辑按要求澄清。在DisplayList中修改如下：

public class DisplayList extends AppCompatActivity implements CustomReqFinished {

    RecyclerView recyclerView;
    RecyclerView.Adapter adapter;
    RecyclerView.LayoutManager layoutManager;
    ArrayList<Contact> arrayList=new ArrayList<>();
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_display_list);

        recyclerView= (RecyclerView) findViewById(R.id.recylerView);
        layoutManager=new LinearLayoutManager(this);
        recyclerView.setLayoutManager(layoutManager);
        recyclerView.setHasFixedSize(true);

        BackgroundTask backgroundTask=new BackgroundTask(this);
        backgroundTask.getList();
    }

    @Override
    public void onCustomReqFinished(ArrayList<Contact> list) {
        Log.d("dispaly Array List",""+list.toString());

        adapter=new RecyclerAdapter(list);
        recyclerView.setAdapter(adapter);
    }
}

在backgroundTask中修改如下：

public class BackgroundTask {

    public interface CustomReqFinished{
        public void onCustomReqFinished(ArrayList<Contact> list);
    }

    Context context;
    CustomReqFinished listener;
    ArrayList<Contact> arrayList=new ArrayList<>();
    String str_url="https://raw.githubusercontent.com/ianbar20/JSON-Volley-Tutorial/master/Example-JSON-Files/Example-Array.JSON";
    String data="";
    public BackgroundTask(Activity activity)
    {
        this.context = activity;
        this.listener = (CustomReqFinished) activity;

    }
    public void getList()
    {
        final JsonArrayRequest jsonArrayRequest=new JsonArrayRequest(str_url, new Response.Listener<JSONArray>() {
            @Override
            public void onResponse(JSONArray response) {

                Log.d("My Json",response.toString());


                    int count = 0;
                    while (count < response.length()) {
                        try {
                            JSONObject jsonObject = response.getJSONObject(count);
                            JSONObject jsonObject1 = jsonObject.getJSONObject("phone");
                            Contact contact = new Contact(jsonObject.getString("name"), jsonObject.getString("email"), jsonObject1.getString("home"));
                            Log.d("contact", "" + contact.toString());
                            arrayList.add(contact);
                            Log.d("arrayList" + count, "" + arrayList.toString());
                            count++;


                        } catch (JSONException e) {
                            e.printStackTrace();
                        }
                    }

                    listener.onCustomReqFinished(arrayList);
            }
        },

        new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {

                Toast.makeText(context,"Error...!!!",Toast.LENGTH_SHORT).show();
                error.printStackTrace();
            }
        });

        MySingleton.getInstance(context).addToRequestQueue(jsonArrayRequest);
    }
    }

在jsonArrayRequest中的响应之后，arrayList变为null

1 个答案: