无法使用 volley 库实例化片段中的列表

Question

我的问题是，当我将此代码放入 activity 时，它工作正常，但是当我尝试使用片段时，它会产生错误这是显示错误的地方：

public class CustomListAdapter extends BaseAdapter {
private Activity activity;
private LayoutInflater inflater;
private List<News> newItems;
ImageLoader imageLoader = AppController.getInstance().getImageLoader();

public CustomListAdapter(Activity activity, List<News> newItems) {
    this.activity = activity;
    this.newItems = newItems;
}

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

@Override
public Object getItem(int location) {
    return newItems.get(location);
}

@Override
public long getItemId(int position) {
    return position;
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {

    if (inflater == null)
        inflater = (LayoutInflater) activity
                .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    if (convertView == null)
        convertView = inflater.inflate(R.layout.list_row, null);

    if (imageLoader == null)
        imageLoader = AppController.getInstance().getImageLoader();
    NetworkImageView thumbNail = (NetworkImageView) convertView
            .findViewById(R.id.thumbnail);
    TextView title = (TextView) convertView.findViewById(R.id.title);

    // getting new data for the row
    News m = newItems.get(position);

    // thumbnail image
    thumbNail.setImageUrl(m.getImageURL(), imageLoader);

    // title
    title.setText(m.getTitle());

    return convertView;
}

}

public class AppController extends Application {

    public static final String TAG = AppController.class.getSimpleName();

    private RequestQueue mRequestQueue;
    private ImageLoader mImageLoader;

    private static AppController mInstance;

    @Override
    public void onCreate() {
        super.onCreate();
        mInstance = this;
    }

    public static synchronized AppController getInstance() {
        return mInstance;
    }

    public RequestQueue getRequestQueue() {
        if (mRequestQueue == null) {
            mRequestQueue = Volley.newRequestQueue(getApplicationContext());
        }

        return mRequestQueue;
    }

    public ImageLoader getImageLoader() {
        getRequestQueue();
        if (mImageLoader == null) {
            mImageLoader = new ImageLoader(this.mRequestQueue,
                    new LruBitmapCache());
        }
        return this.mImageLoader;
    }

    public <T> void addToRequestQueue(Request<T> req, String tag) {
        // set the default tag if tag is empty
        req.setTag(TextUtils.isEmpty(tag) ? TAG : tag);
        getRequestQueue().add(req);
    }

    public <T> void addToRequestQueue(Request<T> req) {
        req.setTag(TAG);
        getRequestQueue().add(req);
    }

    public void cancelPendingRequests(Object tag) {
        if (mRequestQueue != null) {
            mRequestQueue.cancelAll(tag);
        }
    }
}

这是日志：

05-06 01:36:54.754 23005-23005/pager.fakher.com.mypager E/AndroidRuntime﹕ FATAL EXCEPTION: main java.lang.NullPointerException at pager.fakher.com.mypager.adapter.CustomListAdapter.(CustomListAdapter.java:24) at pager.fakher.com.mypager.fragments.ActualiteFragment.onCreateView(ActualiteFragment.java:62) at android.support.v4.app.Fragment.performCreateView(Fragment.java:1789) at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:955) at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1138) at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:740) at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1501) at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:490) at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:141) at android.support.v4.view.ViewPager.populate(ViewPager.java:1072) at android.support.v4.view.ViewPager.populate(ViewPager.java:918) at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1440) at android.view.View.measure(View.java:15876) at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:728) at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:477) at android.view.View.measure(View.java:15876) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5023) at android.widget.FrameLayout.onMeasure(FrameLayout.java:310) at android.view.View.measure(View.java:15876) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5023) at com.android.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:302) at android.view.View.measure(View.java:15876) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5023) at android.widget.FrameLayout.onMeasure(FrameLayout.java:310) at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2206) at android.view.View.measure(View.java:15876) at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1929) at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1113) at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1293) at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1013) at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5507) at android.view.Choreographer$CallbackRecord.run(Choreographer.java:749) at android.view.Choreographer.doCallbacks(Choreographer.java:562) at android.view.Choreographer.doFrame(Choreographer.java:532) at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:735) at android.os.Handler.handleCallback(Handler.java:730) at android.os.Handler.dispatchMessage(Handler.java:92) at android.os.Looper.loop(Looper.java:137) at android.app.ActivityThread.main(ActivityThread.java:5473) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:525) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:854) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:670) at dalvik.system.NativeStart.main(Native Method)

第 24 行是：

ImageLoader imageLoader = AppController.getInstance().getImageLoader();

Answer 1

我的看法是您忘记像这样在清单中添加该应用程序实现

    <application
    android:name=".package.AppController "
    ...
    />

这就是为什么 getInstance returns 为空。

Answer 2

来自应用程序的 android 指南 class：

Note: There is normally no need to subclass Application. In most situations, static singletons can provide the same functionality in a more modular way. If your singleton needs a global context (for example to register broadcast receivers), include Context.getApplicationContext() as a Context argument when invoking your singleton's getInstance() method.

https://developer.android.com/reference/android/app/Application.html

制作一个普通的单例 class 并在那里接听电话。这是一个简单的例子：

public class Single {
    private static Single mInstance;

    private Single {}

    public static Single getInstance() {

        if (mInstance == null)
            mInstance = new Single();
        return mInstance; 
    }



}

无法使用 volley 库实例化片段中的列表

Cannot instantiate a list in fragment using volley library

android

android-fragments

android-volley