Question

我的问题是，当我将此代码放入一个活动时，它工作正常，但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：致命异常：主要显示java.lang.NullPointerException at pager.fakher.com.mypager.adapter.CustomListAdapter。（CustomListAdapter.java:24） at pager.fakher.com.mypager.fragments.ActualiteFragment.onCreateView（ActualiteFragment.java:62）在android.support.v4.app.Fragment.performCreateView（Fragment.java:1789）在android.support.v4.app.FragmentManagerImpl.moveToState（FragmentManager.java:955）在android.support.v4.app.FragmentManagerImpl.moveToState（FragmentManager.java:1138）在android.support.v4.app.BackStackRecord.run（BackStackRecord.java:740）在android.support.v4.app.FragmentManagerImpl.execPendingActions（FragmentManager.java:1501）在android.support.v4.app.FragmentManagerImpl.executePendingTransactions（FragmentManager.java:490）在android.support.v4.app.FragmentPagerAdapter.finishUpdate（FragmentPagerAdapter.java:141）在android.support.v4.view.ViewPager.populate（ViewPager.java:1072）在android.support.v4.view.ViewPager.populate（ViewPager.java:918）在android.support.v4.view.ViewPager.onMeasure（ViewPager.java:1440）在android.view.View.measure（View.java:15876）在android.widget.RelativeLayout.measureChildHorizontal（RelativeLayout.java:728）在android.widget.RelativeLayout.onMeasure（RelativeLayout.java:477）在android.view.View.measure（View.java:15876）在android.view.ViewGroup.measureChildWithMargins（ViewGroup.java:5023）在android.widget.FrameLayout.onMeasure（FrameLayout.java:310）在android.view.View.measure（View.java:15876）在android.view.ViewGroup.measureChildWithMargins（ViewGroup.java:5023）在com.android.internal.widget.ActionBarOverlayLayout.onMeasure（ActionBarOverlayLayout.java:302）在android.view.View.measure（View.java:15876）在android.view.ViewGroup.measureChildWithMargins（ViewGroup.java:5023）在android.widget.FrameLayout.onMeasure（FrameLayout.java:310）在com.android.internal.policy.impl.PhoneWindow $ DecorView.onMeasure（PhoneWindow.java:2206）在android.view.View.measure（View.java:15876）在android.view.ViewRootImpl.performMeasure（ViewRootImpl.java:1929）在android.view.ViewRootImpl.measureHierarchy（ViewRootImpl.java:1113）在android.view.ViewRootImpl.performTraversals（ViewRootImpl.java:1293）在android.view.ViewRootImpl.doTraversal（ViewRootImpl.java:1013）在android.view.ViewRootImpl $ TraversalRunnable.run（ViewRootImpl.java:5507）在android.view.Choreographer $ CallbackRecord.run（Choreographer.java:749）在android.view.Choreographer.doCallbacks（Choreographer.java:562）在android.view.Choreographer.doFrame（Choreographer.java:532）在android.view.Choreographer $ FrameDisplayEventReceiver.run（Choreographer.java:735）在android.os.Handler.handleCallback（Handler.java:730）在android.os.Handler.dispatchMessage（Handler.java:92）在android.os.Looper.loop（Looper.java:137）在android.app.ActivityThread.main（ActivityThread.java:5473） at java.lang.reflect.Method.invokeNative（Native Method）在java.lang.reflect.Method.invoke（Method.java:525）在com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:854）在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:670）在dalvik.system.NativeStart.main（本地方法）

第24行是：

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

Answer 1

我的看法是你忘了在清单中添加该应用程序实现，如下所示

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

这就是getInstance返回null的原因。

Answer 2

从Application类的android指南：

注意：通常不需要子类Application。在大多数情况下，静态单例可以以更模块化的方式提供相同的功能。如果您的单例需要全局上下文（例如注册广播接收器），则在调用单例的getInstance（）方法时将Context.getApplicationContext（）包含为Context参数。

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

制作一个普通的单身人士课程并在那里接听电话。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; 
    }



}

无法使用齐射库

2 个答案: