Android Xml 拉解析器错误 - SitesDownloadTask.onPostExecute

Question

我的应用程序中有不同的三个适配器和三个 XML 解析器，用于三个片段在 ViewPager 上滑动。

我的应用程序运行良好，直到我开始在第一个和第二个片段或第二个和第三个片段之间滑动得更快一些。

记录猫消息：

在第一和第二之间滑动时：

FATAL EXCEPTION: main
Process: com.intera.eronetmarket, PID: 3326
java.lang.NullPointerException
    at android.widget.ArrayAdapter.init(ArrayAdapter.java:310)
    at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:153)
    at adapters.KatAdapter.<init>(KatAdapter.java:32)
    at com.intera.eronetmarket.Kategorije$SitesDownloadTask.onPostExecute(Kategorije.java:98)
    at com.intera.eronetmarket.Kategorije$SitesDownloadTask.onPostExecute(Kategorije.java:1)
    at android.os.AsyncTask.finish(AsyncTask.java:632)
    at android.os.AsyncTask.access$600(AsyncTask.java:177)
    at android.os.AsyncTask$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:5017)
    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:779)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
    at dalvik.system.NativeStart.main(Native Method)

在第二和第三之间滑动时：

FATAL EXCEPTION: main
Process: com.intera.eronetmarket, PID: 3359
java.lang.NullPointerException
    at com.intera.eronetmarket.Preporuceno$AppDownloadTask.onPostExecute(Preporuceno.java:85)
    at com.intera.eronetmarket.Preporuceno$AppDownloadTask.onPostExecute(Preporuceno.java:1)
    at android.os.AsyncTask.finish(AsyncTask.java:632)
    at android.os.AsyncTask.access$600(AsyncTask.java:177)
    at android.os.AsyncTask$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:5017)
    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:779)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
    at dalvik.system.NativeStart.main(Native Method)

AppDownlodTask 类所在的第一个片段。所有 3 个片段都相同（每个片段只有适配器不同）：

public class Preporuceno extends Fragment  {

private AppAdapter mAdapter;
private ListView siteList;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
Log.i("mobAppModel", "OnCreate()");    
View rootView = inflater.inflate(R.layout.activity_preporuceno, container, false);


siteList = (ListView) rootView.findViewById(R.id.listView1);   

siteList.setOnItemClickListener(new OnItemClickListener(){

    @Override
    public void onItemClick(AdapterView<?> parent, View v, int pos,long id) {
        String url = mAdapter.getItem(pos).getstoreURL();
        Intent i = new Intent(Intent.ACTION_VIEW);
        i.setData(Uri.parse(url));
        startActivity(i);

    }
});

    if(isNetworkAvailable()){
        Log.i("mobAppModel", "starting download Task");
        AppDownloadTask download = new AppDownloadTask();
        download.execute();
    }else{
        mAdapter = new AppAdapter(getActivity().getApplicationContext(), -1, XMLsourcePullParser.getmobAppModel(getActivity()));
        siteList.setAdapter(mAdapter);
    }

    return rootView;
}

private boolean isNetworkAvailable() {
    ConnectivityManager connectivityManager 
          = (ConnectivityManager) getActivity().getSystemService(Context.CONNECTIVITY_SERVICE);
    NetworkInfo activeNetworkInfo = connectivityManager.getActiveNetworkInfo();
    return activeNetworkInfo != null && activeNetworkInfo.isConnected();
} 
private class AppDownloadTask extends AsyncTask<Void, Void, Void>{
    @Override
    protected Void doInBackground(Void... arg0) {
        //Download the file
        try {
            Downloader.DownloadFromUrl("https://dl.dropboxusercontent.com/s/te0c0s7y7zr79tm/kategorijeXML.xml", getActivity().openFileOutput("XMLsource.xml", Context.MODE_PRIVATE));
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }

        return null;
    } 
    @Override
    protected void onPostExecute(Void result){
        //setup our Adapter and set it to the ListView.

        mAdapter = new AppAdapter(getActivity().getApplicationContext(), -1, XMLsourcePullParser.getmobAppModel(getActivity()));

            siteList.setAdapter(mAdapter);

        Log.i("mobAppModel", "adapter size = "+ mAdapter.getCount());
    }

}

}

我的适配器：

package adapters;

import java.util.List;
import models.KatModel;
import android.annotation.SuppressLint;
import android.content.Context;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.RelativeLayout;
import android.widget.TextView;
import com.intera.eronetmarket.R;
import com.nostra13.universalimageloader.core.DisplayImageOptions;
import com.nostra13.universalimageloader.core.ImageLoader;
import com.nostra13.universalimageloader.core.ImageLoaderConfiguration;


/*
 * Custom Adapter class that is responsible for holding the list of sites after they
 * get parsed out of XML and building row views to display them on the screen.
 */
public class KatAdapter extends ArrayAdapter<KatModel> {

    ImageLoader imageLoader;
    DisplayImageOptions options;


    public KatAdapter(Context ctx, int textViewResourceId, List<KatModel> sites) {

        super(ctx, textViewResourceId, sites);

        //Setup the ImageLoader, we'll use this to display our images
        ImageLoaderConfiguration config = new ImageLoaderConfiguration.Builder(ctx).build();
        imageLoader = ImageLoader.getInstance();
        if (!imageLoader.isInited()) {
            imageLoader.init(config);
        }

        //Setup options for ImageLoader so it will handle caching for us.
        options = new DisplayImageOptions.Builder()
        .cacheInMemory()
        .cacheOnDisc()
        .build();}





    /*
     * (non-Javadoc)
     * @see android.widget.ArrayAdapter#getView(int, android.view.View, android.view.ViewGroup)
     * 
     * This method is responsible for creating row views out of a StackSite object that can be put
     * into our ListView
     */
    @SuppressLint("InflateParams")
    @Override
    public View getView(int pos, View convertView, ViewGroup parent){
        RelativeLayout row = (RelativeLayout)convertView;
        Log.i("StackSites", "getView pos = " + pos);
        if(null == row){
            //No recycled View, we have to inflate one.
            LayoutInflater inflater = (LayoutInflater)parent.getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            row = (RelativeLayout)inflater.inflate(R.layout.row_site, null);
        }

        //Get our View References

        TextView nameTxt = (TextView)row.findViewById(R.id.nameTxt);



        //Initially we want the progress indicator visible, and the image invisible




        //Load the image and use our options so caching is handled.


        //Set the relavent text in our TextViews
        nameTxt.setText(getItem(pos).getcategoryName());




        return row;


    }

}

--

package adapters;

import java.util.List;

import models.mobAppModel;

import android.annotation.SuppressLint;
import android.content.Context;
import android.graphics.Bitmap;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup; 
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.ProgressBar;
import android.widget.RelativeLayout;

import com.intera.eronetmarket.R;
import com.nostra13.universalimageloader.core.DisplayImageOptions;
import com.nostra13.universalimageloader.core.ImageLoader;
import com.nostra13.universalimageloader.core.ImageLoaderConfiguration;
import com.nostra13.universalimageloader.core.assist.FailReason;
import com.nostra13.universalimageloader.core.assist.ImageLoadingListener;

@SuppressLint("InflateParams") public class AppAdapter extends ArrayAdapter<mobAppModel>{
    ImageLoader imageLoader;
    DisplayImageOptions options;
    public AppAdapter(Context ctx,int textViewResourceId, List<mobAppModel> appModel){
        super(ctx,textViewResourceId,appModel);

        ImageLoaderConfiguration config = new ImageLoaderConfiguration.Builder(ctx).build();
        imageLoader = ImageLoader.getInstance();
        if (!imageLoader.isInited()) {
            imageLoader.init(config);
         }



        options= new DisplayImageOptions.Builder()
        .cacheInMemory()
        .cacheOnDisc()
        .build();
    }

    public View getView(int pos, View convertView, ViewGroup parent){
        RelativeLayout row = (RelativeLayout)convertView;
        Log.i("mobAppModels", "getView pos = " + pos);
        if(null == row){
            //No recycled View, we have to inflate one.
            LayoutInflater inflater = (LayoutInflater)parent.getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            row = (RelativeLayout)inflater.inflate(R.layout.row_app, null);
        }

        //Get our View References
        final ImageView iconImg = (ImageView)row.findViewById(R.id.iconUrl);
        final ProgressBar indicator = (ProgressBar)row.findViewById(R.id.progress);

        //Initially we want the progress indicator visible, and the image invisible
        indicator.setVisibility(View.VISIBLE);
        iconImg.setVisibility(View.INVISIBLE);

        //Setup a listener we can use to swtich from the loading indicator to the Image once it's ready
        ImageLoadingListener listener = new ImageLoadingListener(){



            @Override
            public void onLoadingStarted(String arg0, View arg1) {
                // TODO Auto-generated method stub

            }

            @Override
            public void onLoadingCancelled(String arg0, View arg1) {
                // TODO Auto-generated method stub

            }

            @Override
            public void onLoadingComplete(String arg0, View arg1, Bitmap arg2) {
                indicator.setVisibility(View.INVISIBLE);
                iconImg.setVisibility(View.VISIBLE);
            }

            @Override
            public void onLoadingFailed(String arg0, View arg1, FailReason arg2) {
                // TODO Auto-generated method stub

            }

        };

        //Load the image and use our options so caching is handled.
        imageLoader.displayImage(getItem(pos).getimageUrl(),iconImg,options,listener);

        //Set the relavent text in our TextViews




        return row;


    }

}

--

package adapters;

import java.util.List;
import models.mobAppModel;
import android.annotation.SuppressLint;
import android.content.Context;
import android.graphics.Bitmap;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.ProgressBar;
import android.widget.RelativeLayout;
import android.widget.TextView;
import android.widget.RatingBar;
import com.intera.eronetmarket.R;
import com.nostra13.universalimageloader.core.DisplayImageOptions;
import com.nostra13.universalimageloader.core.ImageLoader;
import com.nostra13.universalimageloader.core.ImageLoaderConfiguration;
import com.nostra13.universalimageloader.core.assist.FailReason;
import com.nostra13.universalimageloader.core.assist.ImageLoadingListener;

public class AppAdapterNajpop extends ArrayAdapter<mobAppModel>{
    ImageLoader imageLoader;
    DisplayImageOptions options;
    public AppAdapterNajpop(Context ctx,int textViewResourceId, List<mobAppModel> appModel){
        super(ctx,textViewResourceId,appModel);

        ImageLoaderConfiguration config = new ImageLoaderConfiguration.Builder(ctx).build();
        imageLoader = ImageLoader.getInstance();
        if (!imageLoader.isInited()) {
            imageLoader.init(config);
         }


        options= new DisplayImageOptions.Builder()
        .cacheInMemory()
        .cacheOnDisc()
        .build();
    }

    @SuppressLint("InflateParams")
    public View getView(int pos, View convertView, ViewGroup parent){
        RelativeLayout row = (RelativeLayout)convertView;
        Log.i("mobAppModels", "getView pos = " + pos);
        if(null == row){
            //No recycled View, we have to inflate one.
            LayoutInflater inflater = (LayoutInflater)parent.getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            row = (RelativeLayout)inflater.inflate(R.layout.row_app_najpopularnije, null);
        }

        //Get our View References
        final ImageView iconImg = (ImageView)row.findViewById(R.id.iconUrl);
        TextView appHeadline = (TextView)row.findViewById(R.id.textView1);
        TextView developer = (TextView)row.findViewById(R.id.textView2);
        RatingBar ratingpoints =(RatingBar)row.findViewById(R.id.ratingBar);
        final ProgressBar indicator = (ProgressBar)row.findViewById(R.id.progress);

        //Initially we want the progress indicator visible, and the image invisible
        indicator.setVisibility(View.VISIBLE);
        iconImg.setVisibility(View.INVISIBLE);

        //Setup a listener we can use to swtich from the loading indicator to the Image once it's ready
        ImageLoadingListener listener = new ImageLoadingListener(){



            @Override
            public void onLoadingStarted(String arg0, View arg1) {
                // TODO Auto-generated method stub

            }

            @Override
            public void onLoadingCancelled(String arg0, View arg1) {
                // TODO Auto-generated method stub

            }

            @Override
            public void onLoadingComplete(String arg0, View arg1, Bitmap arg2) {
                indicator.setVisibility(View.INVISIBLE);
                iconImg.setVisibility(View.VISIBLE);
            }

            @Override
            public void onLoadingFailed(String arg0, View arg1, FailReason arg2) {
                // TODO Auto-generated method stub

            }

        };

        //Load the image and use our options so caching is handled.
        imageLoader.displayImage(getItem(pos).geticonUrl(),iconImg,options,listener);

        //Set the relavent text in our TextViews
        appHeadline.setText(getItem(pos).getappHeadline());
        developer.setText(getItem(pos).getdeveloper());
        ratingpoints.setRating(Float.parseFloat(getItem(pos).getratingPoints()));


        return row;


    }

}

当我滑得较慢时，一切正常。怎么了？ :\

Answer 1

当你从片段 A 快速翻转到 B 时，A 可能随时被破坏，而你无法控制。在片段销毁过程完成之前，它也会从活动中分离出来。从这一刻开始，任何调用 go getActivity() 都可能返回null。

所以您的问题是您的片段没有对活动的引用，但异步任务仍在后台线程中运行，导致空指针异常。

我建议你这样做：

Activity attached = getActivity();
if (attached != null) {
    mAdapter = new AppAdapter(attached.getApplicationContext(), -1, XMLsourcePullParser.getmobAppModel(getActivity()));
    siteList.setAdapter(mAdapter);
}

有关此问题的更多信息，请参见：http://developer.android.com/guide/components/fragments.html#Lifecycle。我想强调一下 Caution 会话：如果您在 Fragment 中需要一个 Context 对象，您可以调用 getActivity()。但是，请注意仅当片段附加到活动时才调用 getActivity()。当 Fragment 尚未附加，或者在其生命周期结束时被分离时，getActivity() 将返回 null。