Question

我使用Activity通过AsyncTask从json加载数据并在Fragment类中显示。显示内容包括：ImageView，TextView，TextView以及当用户点击某个项目时会重定向GridView中相应的项目

在ManufactureFragment中包括：

POJO ManufacturerItem
LoadAllManufacturers扩展AsyncTask以从json
适配器：从json调用加载数据

我的片段：

public class ManufacturerFragment extends Fragment {

ProgressDialog pDialog;
JSONParser jsonParser = new JSONParser();


// initiliaze adapter variable
MyAdapter adapter; 
JSONArray manufacturers = null;

// manufacturers JSON url
private static final String URL_MANUFACTURERS = "http://MYURL";

public ManufacturerFragment() {
    // Required empty public constructor
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    View view = inflater.inflate(R.layout.gridview_manufacturer, container, false);

    GridView gridView = (GridView) view.findViewById(R.id.gridview);
    // gridView.setAdapter(new MyAdapter(getActivity(), manufacturersList));
    adapter = new MyAdapter(getActivity(), manufacturersList);
    gridView.setAdapter(adapter);

    gridView.setOnItemClickListener(new android.widget.AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> arg0, View view, int arg2, long arg3) {
            // on selecting a single manufacturer
            // CategoryCarActivity will be launched to show category car inside the manufacturer
            Intent i = new Intent(getActivity(), CategoryCarActivity.class);

            // send manufacturer id to activity to get list of cars under that manufacturer
            String manufacturer_id = ((TextView) view.findViewById(R.id.manufacturer_id)).getText().toString();
            i.putExtra("manufacturer_id", manufacturer_id);

            startActivity(i);
        }
    });

    return view;
}

@Override
public void onActivityCreated(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onActivityCreated(savedInstanceState);

    new LoadAllManufacturers().execute();
}


class LoadAllManufacturers extends AsyncTask<String, String, String> {
    /**
     * Before starting background thread Show Progress Dialog
     * */
    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        pDialog = new ProgressDialog(getActivity());
        pDialog.setMessage("Loading manufacturers. Please wait...");
        pDialog.setIndeterminate(false);
        pDialog.setCancelable(false);
        pDialog.show();
    }

    /**
     * After completing background task Dismiss the progress dialog
     * **/
    @Override
    protected void onPostExecute(String s) {
        super.onPostExecute(s);

        adapter.notifyDataSetChanged();
        // dismiss the dialog after getting all manufacturers
        if (pDialog.isShowing())
            pDialog.dismiss();
    }
}

private class MyAdapter extends BaseAdapter
{
    LayoutInflater inflater;
    Context context;

    public MyAdapter(Context context, ArrayList<HashMap<String, String>> arrayList)
    {
        this.context = context;
        this.manufacturersList = arrayList;
        this.inflater = LayoutInflater.from(context);
    }

    @Override
    public int getCount() {
        if (manufacturersList != null)
            return manufacturersList.size();
        return 0;
    }

    @Override
    public Object getItem(int i)
    {
        if (manufacturersList != null)
            return manufacturersList.get(i);
        return null;
    }

    @Override
    public long getItemId(int i)
    {
        if (manufacturersList != null)
            return manufacturersList.get(i).hashCode();
        return 0;
    }

    @Override
    public View getView(int i, View view, ViewGroup viewGroup)
    {
        View v = view;
        ImageView picture;
        TextView name;

        if(v == null)
        {
            v = inflater.inflate(R.layout.gridview_item, viewGroup, false);
            v.setTag(R.id.picture, v.findViewById(R.id.picture));
            v.setTag(R.id.text, v.findViewById(R.id.text));
        }

        picture = (ImageView)v.getTag(R.id.picture);
        name = (TextView)v.getTag(R.id.text);   

        picture.setImageResource(item.drawableId);
        name.setText(item.name);

        return v;
    }
   }
 }

我的list_item_manufacturers.xml

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

<!-- Manufacturer_id / Hidden by default -->
<TextView
    android:id="@+id/manufacturer_id"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:visibility="gone" />

<!-- Manufacturer_name -->
<TextView
    android:id="@+id/manufacturer_name"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:textSize="16dip"
    android:textColor="#000000"
    android:paddingTop="15dip"
    android:paddingBottom="15dip"
    android:paddingLeft="10dip"
    android:textStyle="bold"/>

<!-- Logo image -->
<ImageView
    android:id="@+id/iconName"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentRight="true"
    android:background="#000000"
    android:padding="1dp" />
 </RelativeLayout>

我的gridview_manufacturer.xml

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<GridView
    android:id="@+id/gridview"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:verticalSpacing="0dp"
    android:horizontalSpacing="0dp"
    android:stretchMode="columnWidth"
    android:numColumns="2" />
</FrameLayout>

当我调试和构建时，我看到json加载成功，但是然后它在行发生异常： logcat的：

E/AndroidRuntime: FATAL EXCEPTION: AsyncTask #1
              Process: com.totoroads.android.app, PID: 3723
              java.lang.RuntimeException: An error occurred while executing doInBackground()
                  at android.os.AsyncTask$3.done(AsyncTask.java:309)
                  at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:354)
                  at java.util.concurrent.FutureTask.setException(FutureTask.java:223)
                  at java.util.concurrent.FutureTask.run(FutureTask.java:242)
                  at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:234)
                  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)
                  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)
                  at java.lang.Thread.run(Thread.java:818)
               Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.util.ArrayList.add(java.lang.Object)' on a null object reference
                  at com.totoroads.android.app.FeedBackFragment$LoadAllManufacturers.doInBackground(ManufacturerFragment.java:136)
                  at com.totoroads.android.app.FeedBackFragment$LoadAllManufacturers.doInBackground(ManufacturerFragment.java:89)
                  at android.os.AsyncTask$2.call(AsyncTask.java:295)
                  at java.util.concurrent.FutureTask.run(FutureTask.java:237)
                  at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:234) 
                  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113) 
                  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588) 
                  at java.lang.Thread.run(Thread.java:818)

Logcat中的最终异常：

E/AndroidRuntime: FATAL EXCEPTION: main
              Process: com.totoroads.android.app, PID: 5730
              java.lang.ClassCastException: java.util.HashMap cannot be cast to com.totoroads.android.app.ManufacturerFragment$MyAdapter$ManufacturerItem
                  at com.totoroads.android.app.ManufacturerFragment$MyAdapter.getView(FeedBackFragment.java:226)
                  at android.widget.AbsListView.obtainView(AbsListView.java:2346)
                  at android.widget.GridView.onMeasure(GridView.java:1065)
                  at android.view.View.measure(View.java:18788)
                  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)

在线：

ManufacturerFragment.MyAdapter.ManufacturerItem item = (ManufacturerFragment.MyAdapter.ManufacturerItem)getItem(i);

有些错误我无法检测到它，如何修复此异常

非常感谢!!

Answer 1

在致电ArrayList之前，您没有初始化AsyncTask。在执行AsyncTask中的onActivityCreated()之前添加此行：

manufacturersList = new ArrayList<>();

编辑1： 将适配器分配给变量：

adapter = new MyAdapter(getActivity(), manufacturersList);
gridView.setAdapter(adapter);

因为现在您正在使用更改通知空适配器。

编辑2： 您的代码永远不会输入onPostExecute，因为您没有覆盖AsyncTask的{{1}}方法。您的onPostExecute初始化为AsyncTask，因此AsyncTask<String, String, String>的参数应为onPostExecute，而不是String。

Void

编辑3： 这是您可以从@Override protected void onPostExecute(String s) { super.onPostExecute(s); adapter.notifyDataSetChanged(); // dismiss the dialog after getting all manufacturers if (pDialog.isShowing()) pDialog.dismiss(); }方法开始的一个工作示例：

getView

Answer 2

我只编写代码中的更改以及它上面和下面的行。

更改

您的ArrayList是null。 Ininialize。
为适配器创建变量，因为您需要调用 notifyDataSetChanged上的onPostExecute {/ 1}}。

// manufacturers JSON url
private static final String URL_MANUFACTURERS = "http://MYURL";

MyAdapter adapter;
ArrayList<HashMap<String, String>> manufacturersList = new ArrayList<>();

public ManufacturerFragment() {
    // Required empty public constructor
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    ...

    // initialize the adapter
    adapter = new MyAdapter(getActivity(), manufacturersList)
    GridView gridView = (GridView) view.findViewById(R.id.gridview);
    gridView.setAdapter(adapter);

    ...
}


class LoadAllManufacturers extends AsyncTask<String, String, String> {

    /**
     * After completing background task Dismiss the progress dialog
     * **/
    protected void onPostExecute(Void args) {

        // notify the adapter
        adapter.notifyDataSetChanged();

        // dismiss the dialog after getting all manufacturers
        ...
    }
}

片段在android

2 个答案: