java - 恢复应用程序后上下文为空

Question

我遇到以下错误:

E/AndroidRuntime(658): FATAL EXCEPTION: main E/AndroidRuntime(658): java.lang.NullPointerException E/AndroidRuntime(658): at android.view.LayoutInflater.from(LayoutInflater.java:210) E/AndroidRuntime(658): at com.gauss.sportsapp.messages.RecievedRequestsAdapter.(RecievedRequestsAdapter.java:26) E/AndroidRuntime(658): at com.gauss.sportsapp.messages.MessagesList.setRecievedRequests(MessagesList.java:157) E/AndroidRuntime(658): at com.gauss.sportsapp.messages.MessagesList.onEventsAvailable(MessagesList.java:181) E/AndroidRuntime(658): at com.gauss.sportsapp.eventrequests.GetEventRequestsTask.onPostExecute(GetEventRequestsTask.java:74) E/AndroidRuntime(658): at com.gauss.sportsapp.eventrequests.GetEventRequestsTask.onPostExecute(GetEventRequestsTask.java:1) E/AndroidRuntime(658): at android.os.AsyncTask.finish(AsyncTask.java:602) E/AndroidRuntime(658): at android.os.AsyncTask.access$600(AsyncTask.java:156) E/AndroidRuntime(658): at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:615) E/AndroidRuntime(658): at android.os.Handler.dispatchMessage(Handler.java:99) E/AndroidRuntime(658): at android.os.Looper.loop(Looper.java:154) E/AndroidRuntime(658): at android.app.ActivityThread.main(ActivityThread.java:4945) E/AndroidRuntime(658): at java.lang.reflect.Method.invokeNative(Native Method) E/AndroidRuntime(658): at java.lang.reflect.Method.invoke(Method.java:511) E/AndroidRuntime(658): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784) E/AndroidRuntime(658): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551) E/AndroidRuntime(658): at dalvik.system.NativeStart.main(Native Method)

仅当我通过主页按钮退出应用程序并让它在后台运行一段时间后，才会发生该错误。如果我退出应用程序并立即再次启动它，就不会出现错误。有时，它甚至在我让它静置一段时间后起作用，但更常见的是，当我在适配器的构造函数中设置布局膨胀器时，它会返回上下文的空指针。

我的 fragment 的重要代码位:

public class MessagesList extends Fragment implements EventRequestListener { 

    private ListView lv;
    private GetEventRequestsTask recievedRequestsTask;      
    private List<GetEventRequest> data;
    private RecievedRequestsAdapter adapter;

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onActivityCreated(savedInstanceState);
        recievedRequestsTask = new GetEventRequestsTask(getActivity(), user_id);
        recievedRequestsTask.setEventRequestListener(MessagesList.this);
        recievedRequestsTask.execute(); 
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        return inflater.inflate(R.layout.fragment_messages, container, false);
    }

    //Async Task listener, gets called if(result != null)
    @Override
    public void onEventsAvailable(List<GetEventRequest> result) {
        // TODO Auto-generated method stub
        adapter = new RecievedRequestsAdapter(getActivity(), data);
        lv.setAdapter(adapter);
    }

    @Override
    public void onStop() {
        // TODO Auto-generated method stub
        super.onStop();
        if(recievedRequestsTask != null){
            recievedRequestsTask.cancel(true);
        }
    }
}

适配器:

public class RecievedRequestsAdapter extends BaseAdapter {

    private LayoutInflater inflater;
    private List<GetEventRequest> list; 

    public RecievedRequestsAdapter(Context ctx, List<GetEventRequest> list){
        this.list = list;
        inflater = LayoutInflater.from(ctx); //This is where null pointer happens
    }

    @Override
    public int getCount() {
        // TODO Auto-generated method stub
        return list.size();
    }

    @Override
    public Object getItem(int position) {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public long getItemId(int position) {
        // TODO Auto-generated method stub
        return 0;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        ViewHolder holder;
        if(convertView == null){
            holder = new ViewHolder();
            convertView = inflater.inflate(R.layout.result_list_adapter, parent, false);
            holder.txtTitle = (TextView) convertView.findViewById(R.id.txtTitle);
            holder.txtDesc = (TextView) convertView.findViewById(R.id.txtDescription);      

            convertView.setTag(holder);
        } else {
            holder = (ViewHolder) convertView.getTag();
        }

        //Do stuff, set views, etc

        return convertView;
    }

    private static final class ViewHolder{
        public TextView txtTitle, txtDesc;
     }
}

有人有什么想法吗？测试或调试这一点非常困难，因为它发生得非常随机，并且只有在应用程序长时间闲置在后台之后才会发生。

Answer 1

如果 fragment 未附加到 Activity ，则 fragment 的 getActivity() 返回 null。

您的异步任务监听器onEventsAvailable()未绑定(bind)到 fragment 生命周期，并且当 fragment 分离时它也可以触发。

考虑重新设计您的代码，以便未显示的 fragment 不需要自行更新。