包含来自异步任务的数据的 Viewpager

Question

所以我正在制作一个从服务器提取故事的应用程序。故事分为几个部分。我想将每个故事部分放在不同的 ViewPager 页面上，但到目前为止我所做的尝试要么使应用程序崩溃，要么只给我一个空白屏幕。

这是我的活动代码：

package com.firegaming.myfirstapp;

import android.app.ProgressDialog;
import android.os.AsyncTask;
import android.os.Parcelable;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentStatePagerAdapter;
import android.support.v4.view.ViewPager;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import org.apache.http.NameValuePair;
import org.apache.http.message.BasicNameValuePair;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;


public class ContinueStoryActivity extends FragmentActivity {


private ProgressDialog pDialog;
JSONParser jsonParser = new JSONParser();
private static int partCounter;
private static String storyID;
ArrayList<String> storyPartList = new ArrayList<>();
FragmentStatePagerAdapter adapter;
ViewPager pager;
JSONArray parts = null;

private static final String GET_STORY_URL = "http://firegaming.host56.com/get_story.php";
private static final String TAG_SUCCESS = "success";
private static final String TAG_MESSAGE = "message";
private static final String TAG_PARTS = "parts";
private static final String TAG_PARTCOUNT = "partCount";
private static final String TAG_STORYID = "storyID";
private static final String TAG_STORYTEXT = "storyText";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_continue_story);


    pager = (ViewPager) findViewById(R.id.pager);
    adapter = new MyPagerAdapter(getSupportFragmentManager());
    pager.setAdapter(adapter);

    new GetStory().execute();

}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_continue_story, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}

public class MyPagerAdapter extends FragmentStatePagerAdapter {

    public MyPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int pos) {
        return StoryFragment.newInstance(storyPartList.get(pos).toString());
    }

    @Override
    public int getCount() {
        return storyPartList.size();
    }

    public int getItemPosition(Object object){
        return POSITION_NONE;
    }

    @Override
    public void restoreState(Parcelable arg0, ClassLoader arg1) {
        //do nothing here! no call to super.restoreState(arg0, arg1);
    }
}



    class GetStory extends AsyncTask<String, String, String>{

    @Override
    protected void onPreExecute(){
        super.onPreExecute();
        pDialog = new ProgressDialog(ContinueStoryActivity.this);
        pDialog.setMessage("Getting Story...");
        pDialog.setIndeterminate(false);
        pDialog.setCancelable(true);
        pDialog.show();
    }

    @Override
    protected String doInBackground(String... args){
        int success;
        try{

            List<NameValuePair> params = new ArrayList<NameValuePair>();
            params.add(new BasicNameValuePair("username", MainActivity.USERNAME));

            Log.d("Request!", "Starting!");

            JSONObject json = jsonParser.makeHttpRequest(GET_STORY_URL, "GET", params);

            Log.d("Story Attempt", json.toString());
            success = json.getInt(TAG_SUCCESS);
            if (success == 1){

                parts = json.getJSONArray(TAG_PARTS);
                partCounter = Integer.parseInt(json.getString(TAG_PARTCOUNT));
                storyID = json.getString(TAG_STORYID);
                return json.getString(TAG_SUCCESS);

            } else {

                Log.d("Story Get Failure!", json.getString(TAG_MESSAGE));
                return json.getString(TAG_MESSAGE);
            }

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

        return null;
    }

    @Override
    protected void onPostExecute(String file_url) {

        pDialog.dismiss();

        if (file_url != null) {
            try {
                for (int i = 0; i < parts.length(); i++) {
                    JSONObject c = parts.getJSONObject(i);
                    int id = c.getInt(TAG_STORYID);
                    String storyText = c.getString(TAG_STORYTEXT);
                    storyPartList.add(id-1, storyText);
                }
            } catch (JSONException ex) {
                ex.printStackTrace();
            }

            adapter.notifyDataSetChanged();

        }

    }

}


}

我做错了什么？

编辑：当没有像建议的那样返回 null 时，应用程序再次崩溃。这些是应用程序崩溃时 logcat 中出现的所有错误。

01-03 12:29:19.298  29861-29861/com.firegaming.myfirstapp E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.firegaming.myfirstapp, PID: 29861
java.lang.NullPointerException
        at com.firegaming.myfirstapp.ContinueStoryActivity$MyPagerAdapter.getItem(ContinueStoryActivity.java:88)
        at android.support.v4.app.FragmentStatePagerAdapter.instantiateItem(FragmentStatePagerAdapter.java:105)
        at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:837)
        at android.support.v4.view.ViewPager.populate(ViewPager.java:987)
        at android.support.v4.view.ViewPager.populate(ViewPager.java:919)
        at android.support.v4.view.ViewPager.setAdapter(ViewPager.java:447)
        at com.firegaming.myfirstapp.ContinueStoryActivity$GetStory.onPostExecute(ContinueStoryActivity.java:163)
        at com.firegaming.myfirstapp.ContinueStoryActivity$GetStory.onPostExecute(ContinueStoryActivity.java:99)
        at android.os.AsyncTask.finish(AsyncTask.java:632)
        at android.os.AsyncTask.access0(AsyncTask.java:177)
        at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:136)
        at android.app.ActivityThread.main(ActivityThread.java:5586)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1268)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1084)
        at dalvik.system.NativeStart.main(Native Method)

Answer 1

假设您的网络服务一切正常并且您正在正确解析数据，请将以下代码添加到您的 MyPagerAdapter class:

@Override
public void restoreState(Parcelable arg0, ClassLoader arg1) {
        //do nothing here! no call to super.restoreState(arg0, arg1);
}

首先尝试这个，看看它是否有效......也请 post 你的崩溃 logcat。

您还需要确保三件事：

确保 android.support.v4.app.Fragment 和 android.app.Fragment class 没有一起使用。仅使用 v4 支持库中的所有内容。
确保getCount()returns实际片段数对应你的getItem(int index)方法：在getCount()中使用storyPartList.size() .
正如 cYrixmorten 所建议的那样，将 storyPartList 设为 ArrayList 并调用 storyPartList.add(...) 而不是 storyPartList.put(...)。

Answer 2

我有一个建议列表：

我怀疑 storyPartList 是一个 hasmap，其 id 指示为传入数据。例如，如果传入的 id 是 [0,2,3]，则 getItem(1) 将失败。如果让 storyPartList 简单地成为一个数组列表（顾名思义）会发生什么？

适配器的getCount同样由传入数据决定：

@Override
public int getCount() {
    return partCounter;
}

而是让计数器取决于 storyPartList:

的实际大小

@Override
public int getCount() {
    return storyPartList.size();
}

现在，如果寻呼机数据完全由添加到 storyPartList 的数据量决定，我建议在 onCreate() 中创建一次寻呼机，而不是每次添加时都重新创建一个故事：

ViewPager pager;
FragmentStatePagerAdapter adapter;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_continue_story);
    new GetStory().execute();

    pager = (ViewPager) findViewById(R.id.pager);
    adapter = new MyPagerAdapter(getSupportFragmentManager())
    pager.setAdapter(adapter);
}

在 onPostExecute 中，现在只需执行：

adapter.notifyDataSetChanged();

这将使适配器根据 storyPartList 的内容刷新其内容。要使适配器正确更新，您可能必须将其添加到 MyPagerAdapter:

public int getItemPosition(Object object){
     return POSITION_NONE;
}

建议汇总

将 storyPartList 设为 List 而不是 Map
让getCount return storyPartList.size()
在 onCreate 中创建分页器，并在添加故事时调用 notifyDataSetChanged
最后在适配器中实现 getItemPosition 使其正确更新

包含来自异步任务的数据的 Viewpager

Viewpager with data from asynctask

java

android

android-asynctask

android-viewpager