Activity中的片段在setView的setAdapter中获取上下文null

时间：2014-06-24 19:53:56

标签： android listview android-fragments android-asynctask fragment

这是我3个月来一直存在的一个问题，实际上，自从我开始开发这个应用程序以来。我已经阅读了这个网站上的数百个帖子，在其他网站上，我已经阅读过教程，但我还没有找到解决方案。 我认为这个问题必须与我拥有应用程序结构的方式相关。

我正在开发一个显示多个部分的应用程序，每个部分都是一个片段，主要活动使用ViewPager一次显示一个片段（如fb app或twitter app）。每个部分都会显示包含不同信息的列表视图：新闻，视频，分类广告等。一旦显示某个部分，让我们说新闻部分，它会显示包含文章/新闻的列表视图。问题是当我尝试显示列表视图时，因为在用于显示列表视图的适配器中，我在行setAdapter中给出了NullPointerException。

我追踪了给我异常的行，实际上是context声明。尽管我的所有片段中的上下文都是空的（我不知道为什么），但我能够显示列表视图，但在其他一些片段中，应用程序崩溃并提供NPE。我通过行Context context=getActivity()得到了上下文，但是当我测试它时，它给了我null。

这是我成功展示了listview的片段之一

public class HomeActivity extends Fragment{
   static ListView newsList;
   static Item item=null;
   Context context=getActivity();
   ItemAdapter itemAdapter;
   List<Item> news=new ArrayList<Item>();
   static int i=0;
   static Activity activity;
   static String HOME_FRAGMENT="NEWS";
   public static final int TIME_ENTRY_REQUEST_CODE=1;
   FragmentManager fm;


public View onCreateView(LayoutInflater inflater,ViewGroup container,Bundle savedInstanceState){
   super.onCreateView(inflater, container, savedInstanceState);
   View view=inflater.inflate(R.layout.home_activity, container,false);
   newsList=(ListView)view.findViewById(R.id.newslist);
   setRetainInstance(true); 
   return view;
}

public void onAttach(Activity activity){
    super.onAttach(activity);
    HomeActivity.activity=activity;
}

public void onSaveInstanceState(Bundle outState){
    super.onSaveInstanceState(outState);
}

public void displayNews(final List<Item> news){
    try {        
         if(news==null || news.size()==0){
                //Retrieve again the data list
         }
         Log.d("HomeActivity","total news: "+news.size());
     } catch (Exception e) {
         e.printStackTrace();
     }
    if(context==null){
         Log.d("HomeActivity","context null!");
    }
    itemAdapter=new ItemAdapter(context,(ArrayList<Item>) news);
    newsList.setAdapter(itemAdapter);
}

}

更新：这是我获得NPE的另一个片段，

public class VideosActivity extends Fragment{

ListView listvideo;
ItemAdapter itemAdapter;
Context context;
Activity activity;
public View onCreateView(LayoutInflater inflater,ViewGroup container,Bundle savedInstanceState){
       super.onCreateView(inflater, container, savedInstanceState);
       View view=inflater.inflate(R.layout.videos_activity, container,false);
       listvideo=(ListView)view.findViewById(R.id.videoslist);
       Log.d("videos activity","onCreateView ready!!");
               context=getActivity();
       setRetainInstance(true); 
       return view;
    }


public void onAttach(Activity activity){
    super.onAttach(activity);
    this.activity=activity;
}

public void displayVideos(final List<Item> videos){
    try {        
         if(videos==null || videos.size()==0){
                //Retrieve again the data list

         }
     } catch (Exception e) {
         e.printStackTrace();
     }
    for(int i=0;i<videos.size();i++){

    }
    itemAdapter=new ItemAdapter(activity,(ArrayList<Item>) videos);//I also used context in this line, but I still get the NPE
    listvideo.setAdapter(itemAdapter); //Here's the line that gives me the NPE
}

}

我正在解析XML中的所有信息，并且我通过AsyncTask管理所有内容 这是我的适配器 

public class ItemAdapter extends BaseAdapter implements OnItemClickListener{
    Item item=new Item();
    private ArrayList<Item> news=new ArrayList<Item>();
    private static final int NO_PICTURE_VIEW=0;
    private static final int PICTURE_VIEW=1;

    public ItemAdapter(Context context,ArrayList<Item> items) {
    super();
    news.addAll(items);
    }
    public int getCount() {
    // TODO Auto-generated method stub
    return news.size();
}

    public Item getItem(int index) {
    // TODO Auto-generated method stub
    return getItem(index);
}

    public long getItemId(int index) {
    // TODO Auto-generated method stub
    return index;
}

    public int getViewTypeCount() {
         return 2;
}

    public int getItemViewType(int position) {
    Item article=news.get(position);      
        if(article.getImage()!=null)      
            return PICTURE_VIEW;
        else
                return NO_PICTURE_VIEW;
}
@Override
    public View getView(int index, View view, ViewGroup parent) {
    item=news.get(index);
    int type=getItemViewType(index);
    if(view==null){
            LayoutInflater inflater=LayoutInflater.from(parent.getContext());
        if(type==NO_PICTURE_VIEW){
        view=inflater.inflate(R.layout.item_no_picture_list,parent,false);
                TextView titleView=(TextView)view.findViewById(R.id.titleNoPicture);
        titleView.setText(item.getTitle());
        }
        else{
        view=inflater.inflate(R.layout.item_picture_list,parent,false);
        TextView titleView=(TextView)view.findViewById(R.id.titleArticle);
        ImageView image=(ImageView)view.findViewById(R.id.imageItem);
        titleView.setText(item.getTitle());
        image.setImageBitmap(item.getImage());
    }
}

return view;
}
}

这是主要活动 

public class MainActivity extends FragmentActivity{
    private String tabs[]={"News","Sports","Valley Life","Videos","Classifieds","Directory","Weather"};
ViewPager viewPager=null;
FragmentManager fragmentManager=getSupportFragmentManager();
android.support.v4.app.FragmentTransaction ft=fragmentManager.beginTransaction();
    AdapterView adapterView;
static ParseManager parser=new ParseManager();
static List<Item> news=new ArrayList<Item>();
ArrayList<Item> sports=new ArrayList<Item>();
ArrayList<Item> valleyLife=new ArrayList<Item>();
public static final int TIME_ENTRY_REQUEST_CODE=1;
HomeActivity home;
JSONManager jmanager=new JSONManager();
WeatherManager weather=new WeatherManager();

    protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    viewPager=(ViewPager)findViewById(R.id.pager);
    viewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener(){
                public void onPageSelected(int position)          
                getActionBar().setSelectedNavigationItem(position);
            }
    });
    viewPager.setAdapter(new AdapterView(fragmentManager));
    viewPager.setOffscreenPageLimit(tabs.length);
    adapterView=new AdapterView(getSupportFragmentManager());
    /*initialization of the action bar: color, icon & title.*/
    final android.app.ActionBar actionbar=getActionBar();
    ColorDrawable color=new ColorDrawable(Color.parseColor("#cd853f"));
    actionbar.setBackgroundDrawable(color);
    actionbar.setTitle("Title");
    ActionBar.TabListener tabListener=new ActionBar.TabListener() {
        public void onTabUnselected(Tab tab, FragmentTransaction ft) {

        }   


        @Override
        public void onTabSelected(Tab tab, FragmentTransaction ft) {
            viewPager.setCurrentItem(tab.getPosition());
            ft=getFragmentManager().beginTransaction();
            //ft.replace(layouts[tab.getPosition()], (adapterView.getItem(tab.getPosition()));
            ft.commit();

        }

        @Override
        public void onTabReselected(Tab tab, FragmentTransaction ft) {  

        }
    };
    /*Displaying Tabs for the app */
    for(int i=0;i<tabs.length;i++)  
        actionbar.addTab(actionbar.newTab().setText(tabs[i]).setTabListener(tabListener));
    actionbar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
            viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
                @Override
                public void onPageSelected(int position) {
                actionbar.setSelectedNavigationItem(position);
            }

                @Override
                public void onPageScrolled(int arg0, float arg1, int arg2) {
            }
                @Override
                public void onPageScrollStateChanged(int arg0) {
            }
        });
        parser.execute();
        jmanager.execute();
        weather.execute();
        getApplicationContext();
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

}

这是我的logcat 

FATAL EXCEPTION: main
java.lang.NullPointerException
at com.example.test23.VideosActivity.displayVideos(VideosActivity.java:54)
at com.example.test23.ParseManager.onPostExecute(ParseManager.java:52)
at com.example.test23.ParseManager.onPostExecute(ParseManager.java:1)
at android.os.AsyncTask.finish(AsyncTask.java:631)
at android.os.AsyncTask.access$600(AsyncTask.java:177)
at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:5031)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:792)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:555)
at dalvik.system.NativeStart.main(Native Method)

更新：这是video_activity.xml 

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

        <ListView 
            android:id="@+id/videoslist"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content" >
        </ListView>
     </LinearLayout>

我只是想知道我的应用程序出了什么问题以及为什么我的上下文无效，所以我可以继续我的应用程序的下一步，也许是某些东西结构，它的编程方式，我不知道。任何帮助表示赞赏，如果您需要更多代码，请告诉我。

2 个答案:

答案 0 :(得分：1)

确保从此行中删除“= getActivity（）”：

Context context=getActivity();

所以它只是读取：

Context context;

然后改变这个：

public View onCreateView(LayoutInflater inflater,ViewGroup container,Bundle        savedInstanceState){
   super.onCreateView(inflater, container, savedInstanceState);
   View view=inflater.inflate(R.layout.home_activity, container,false);
   newsList=(ListView)view.findViewById(R.id.newslist);
   setRetainInstance(true); 
   return view;
}

到此：

public View onCreateView(LayoutInflater inflater,ViewGroup container,Bundle        savedInstanceState){
   super.onCreateView(inflater, container, savedInstanceState);
   View view=inflater.inflate(R.layout.home_activity, container,false);
   newsList=(ListView)view.findViewById(R.id.newslist);
   context = getActivity();  // <--- add this line here
   setRetainInstance(true); 
   return view;
}

答案 1 :(得分：0)

您只能在调用getActivity()后使用onAttach设置上下文，所以

context = getActivity()

在onCreateView

中
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