我有一个从 ListFragment 扩展的课程。我需要显示一个进度对话框，因为从服务器加载数据需要一些时间。我正在使用一个名为MenuItemData的私有类，并从 AsyncTask 扩展该类。在 onPreExecute 方法中，我有这一行，
pDialog = ProgressDialog.show(getActivity(), "Loading...", "Please wait...", false); //line 1
和 onPostExecute 方法，
pDialog.dismiss(); //line 2
现在，当我运行该程序时，它会停止并提供 NullPointerException 。
但是当我评论这两行时，程序运行正常。 我认为问题是getActivity（）返回null。
MenuItemData类的代码
private class MenuItemData extends AsyncTask<String, String, String> {
@Override
protected void onPostExecute(String s) {
super.onPostExecute(s);
try {
if ((json_.getString(KEY_SUCCESS) != null)) {
int success = Integer.parseInt(json_.getString(KEY_SUCCESS));
if (success == 1) {
jsonArray = json_.getJSONObject("data").getJSONArray("items");
itemCount = jsonArray.length();
for (int i = 0; i < itemCount; i++) {
jsonObject = jsonArray.getJSONObject(i);
menuItems = new Menu_Items();
menuItems.setName(jsonObject.getString("name"));
menuItems.setDescription(jsonObject.getString("description"));
menuItems.setPrice("$"+jsonObject.getString("price"));
menu_itemsArrayList.add(menuItems);
}
}
}
} catch (JSONException e) {
Log.e("Menu Items2", e.toString());
}
pDialog.dismiss();
}
@Override
protected void onPreExecute() {
pDialog = ProgressDialog.show(getActivity(), "Loading...", "Please wait...", false);
super.onPreExecute();
}
@Override
protected String doInBackground(String... params) {
json_ = userFunctions.getMenuItems("Appetizers");
return null;
}
}
logcat的
02-05 11:25:24.245 28619-28619/com.burgerhot.main E/AndroidRuntime﹕ FATAL EXCEPTION: main
java.lang.NullPointerException
at android.app.AlertDialog.resolveDialogTheme(AlertDialog.java:142)
at android.app.AlertDialog.<init>(AlertDialog.java:98)
at android.app.ProgressDialog.<init>(ProgressDialog.java:77)
at android.app.ProgressDialog.show(ProgressDialog.java:110)
at android.app.ProgressDialog.show(ProgressDialog.java:99)
at com.burgerhot.main.MenuCategory1$MenuItemData.onPreExecute(MenuCategory1.java:105)
at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:561)
at android.os.AsyncTask.execute(AsyncTask.java:511)
at com.burgerhot.main.MenuCategory1.setDataToMenu(MenuCategory1.java:61)
at com.burgerhot.main.MenuCategory1.<init>(MenuCategory1.java:46)
at com.burgerhot.main.MyAdapter.getItem(Menu.java:47)
at com.burgerhot.main.MyAdapter.getItem(Menu.java:36)
at android.support.v4.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:97)
at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:832)
at android.support.v4.view.ViewPager.populate(ViewPager.java:982)
at android.support.v4.view.ViewPager.populate(ViewPager.java:914)
at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1436)
at android.view.View.measure(View.java:12911)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4805)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:297)
at android.view.View.measure(View.java:12911)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:828)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:557)
at android.view.View.measure(View.java:12911)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4805)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:297)
at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2097)
at android.view.View.measure(View.java:12911)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1064)
at android.view.ViewRootImpl.handleMessage(ViewRootImpl.java:2446)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4448)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:823)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:590)
at dalvik.system.NativeStart.main(Native Method)
MenuCategory1类
public class MenuCategory1 extends ListFragment {
private UserFunctions userFunctions;
private static String KEY_SUCCESS = "success";
private JSONObject json_, jsonObject;
private JSONArray jsonArray;
int itemCount = 0;
private ProgressDialog pDialog;
private Menu_Items menuItems;
final private Context context=getActivity();
private ArrayList<Menu_Items> menu_itemsArrayList;
CustomAdapterForMenu myCustomAdapter;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
public MenuCategory1() {
userFunctions = new UserFunctions();
setDataToMenu();
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
myCustomAdapter = new CustomAdapterForMenu(getActivity(), R.layout.menu_category1, menu_itemsArrayList);
/** Setting the list adapter for the ListFragment */
this.setListAdapter(myCustomAdapter);
return super.onCreateView(inflater, container, savedInstanceState);
}
private void setDataToMenu() {
menu_itemsArrayList = new ArrayList<Menu_Items>();
new MenuItemData().execute();
}
//private class goes here (mentioned above)
} 那么无论如何要解决这个问题？ （显示进度对话框）
感谢。
答案 0 :(得分：2)
试试这个..
调用如下代码
new MenuItemData(getActivity()).execute();
并在AsyncTask内使用构造函数
Context context;
public MenuItemData(Context context) {
this.context = context;
}
@Override
protected void onPreExecute() {
super.onPreExecute();
pDialog = ProgressDialog.show(context, "Loading...", "Please wait...", false);
}
答案 1 :(得分：1)
问题在于你的构造函数
public MenuCategory1() {
userFunctions = new UserFunctions();
setDataToMenu();
}
在构造函数中，getActivity（）返回null，因为Android没有“初始化”片段所需的后端内容。
尝试在onViewCreated（）上移动setDataToMenu（），这应该可以解决您的问题，因为getActivity（）应该立即返回活动。
答案 2 :(得分：0)
定义一个变量来存储上下文并使其成为最终。使用AsyncTask中的final变量。
final Context activity = getActivity();
答案 3 :(得分：0)
首先调用你的super.onPreExecute（）并使用构造函数来获取活动参考
Activity act;
MenuItemData(Activity a){
act = a;
}
@Override
protected void onPreExecute() {
super.onPreExecute();
pDialog = ProgressDialog.show(act, "Loading...", "Please wait...", false);
}
从列表片段
中调用它Activity act = this;
new MenuItemData(act).execute();
答案 4 :(得分：0)
将对异步方法的调用移至onactivitycreated。因为在从构造函数中调用它时，getactivity将返回null。