[英]Android ArrayAdapter causes NullPointerException when used in ViewPager
我到处都是草丛,但是我还没有解决问题。
我在活动内使用ViewPager。 ViewPager包含3个片段:一个是普通片段,第二个片段包含自定义ListView,第三个片段是MapFragment(SupportMapFragment)。
我只使用支持片段。
问题是在活动尝试填充设置适配器和地图上标记的片段后,将调用ViewPager适配器(FragmentPagerAdapter)的getItem()方法。
因此,我想在初始化ViewPager之前使用片段的单例模式来初始化片段:
在活动onCreate()中:
setContentView(R.layout.act_discover_small);
profiloFragment = new ProfiloFragment();
mapFragment = new MappaMonumentiFragment();
elencoFragment = new ElencoMonumentiFragment();
initViewPager();
initViewPager()方法为:
private void initViewPager() {
mPagerAdapter = new PagerAdapter(getSupportFragmentManager());
mViewPager = (SmartViewPager) findViewById(R.id.pager);
mViewPager.setAdapter(mPagerAdapter);
mViewPager.setOffscreenPageLimit(3);
mViewPager.setOnPageChangeListener(new OnPageChangeListener() {
@Override
public void onPageScrolled(int arg0, float arg1, int arg2) {
if (!beginSwipe)
elencoFragment.List.finishRefresh();
beginSwipe = true;
}
@Override
public void onPageScrollStateChanged(int arg0) {
}
@Override
public void onPageSelected(int arg0) {
beginSwipe = false;
lockDrawer(arg0);
if (arg0 == 0) {
swipe_left_button.setText("show map");
} else {
swipe_left_button.setText("show list");
}
}
});
}
这是ViewPager适配器
class PagerAdapter extends FragmentPagerAdapter {
public PagerAdapter(FragmentManager fm) {
super(fm);
}
@Override
public int getCount() {
return 3;
}
@Override
public Fragment getItem(int i) {
Log.i(TAG, "getItem viewpager n." + i);
switch (i) {
case 0:
if (profiloFragment == null)
profiloFragment = new ProfiloFragment();
return profiloFragment;
case 1:
if (elencoFragment == null)
elencoFragment = new ElencoMonumentiFragment();
return elencoFragment;
case 2:
if (mapFragment == null)
mapFragment = new MappaMonumentiFragment();
return mapFragment;
}
return null;
}
}
尝试使用此方法中的元素填充第二个片段的ArrayAdapter时,我得到一个NullPointerException(在AsyncTask的onPostExecute()中执行):
public void populateElencoMonumenti(List<SmartPlace> placesList) {
if (placesList != null) {
places = (ArrayList<SmartPlace>) placesList;
//before: adapter.clear(); //NullPointerException at this line
//after:
if (adapter == null) {
adapter = new ElencoMonumentiAdapter(ctx, act, app);
}
adapter.addAll(places);
List.setAdapter(adapter);
} else {
adapter.clear();
List.setAdapter(adapter);
}
}
如果在调用adapter.clear()之前登录(适配器== null),则返回true,因此它为null。
如果我尝试检查它是否为空并重新初始化,则在适配器的构造函数中得到一个NPE:
public ElencoMonumentiAdapter(Context ctx, Discover homAct, SmartApplication app) {
// super(ctx, R.layout.list_row_monument);
super(ctx, R.layout.list_row_monument, new ArrayList<SmartPlace>()); //NPE here
Log.e(TAG, "null ctx? " + (ctx == null));
this.app = app;
this.ctx = ctx;
}
stacktrace是:
12-18 11:13:07.285: E/AndroidRuntime(20865): FATAL EXCEPTION: main
12-18 11:13:07.285: E/AndroidRuntime(20865): Process: com.Thinple.MiWalks.ui, PID: 20865
12-18 11:13:07.285: E/AndroidRuntime(20865): java.lang.NullPointerException
12-18 11:13:07.285: E/AndroidRuntime(20865): at android.widget.ArrayAdapter.init(ArrayAdapter.java:310)
12-18 11:13:07.285: E/AndroidRuntime(20865): at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:153)
12-18 11:13:07.285: E/AndroidRuntime(20865): at com.Thinple.MiWalks.ui.adapters.ElencoMonumentiAdapter.<init>(ElencoMonumentiAdapter.java:41)
12-18 11:13:07.285: E/AndroidRuntime(20865): at com.Thinple.MiWalks.ui.fragments.ElencoMonumentiFragment.populateElencoMonumenti(ElencoMonumentiFragment.java:129)
12-18 11:13:07.285: E/AndroidRuntime(20865): at com.Thinple.MiWalks.ui.Discover.notifyFragments(Discover.java:328)
12-18 11:13:07.285: E/AndroidRuntime(20865): at com.Thinple.MiWalks.ui.Discover.populatePlaces(Discover.java:509)
12-18 11:13:07.285: E/AndroidRuntime(20865): at com.Thinple.MiWalks.threading.tasks.GetPlacesFromDBtask.onPostExecute(GetPlacesFromDBtask.java:53)
12-18 11:13:07.285: E/AndroidRuntime(20865): at com.Thinple.MiWalks.threading.tasks.GetPlacesFromDBtask.onPostExecute(GetPlacesFromDBtask.java:1)
12-18 11:13:07.285: E/AndroidRuntime(20865): at android.os.AsyncTask.finish(AsyncTask.java:632)
12-18 11:13:07.285: E/AndroidRuntime(20865): at android.os.AsyncTask.access$600(AsyncTask.java:177)
12-18 11:13:07.285: E/AndroidRuntime(20865): at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
12-18 11:13:07.285: E/AndroidRuntime(20865): at android.os.Handler.dispatchMessage(Handler.java:102)
12-18 11:13:07.285: E/AndroidRuntime(20865): at android.os.Looper.loop(Looper.java:136)
12-18 11:13:07.285: E/AndroidRuntime(20865): at android.app.ActivityThread.main(ActivityThread.java:5017)
12-18 11:13:07.285: E/AndroidRuntime(20865): at java.lang.reflect.Method.invokeNative(Native Method)
12-18 11:13:07.285: E/AndroidRuntime(20865): at java.lang.reflect.Method.invoke(Method.java:515)
12-18 11:13:07.285: E/AndroidRuntime(20865): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
12-18 11:13:07.285: E/AndroidRuntime(20865): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
12-18 11:13:07.285: E/AndroidRuntime(20865): at dalvik.system.NativeStart.main(Native Method)
我真的找不到解决方案。
预先感谢您的答复。
更改
super(ctx, R.layout.list_row_monument, new ArrayList<SmartPlace>());
new ArrayList<SmartPlace>()
是新的arraysit,但是其空没有填充任何数据
至
super(ctx, R.layout.list_row_monument, list);
列表可能是ArrayList<SmartPlace>()
,该列表传递给适配器类的构造函数
看一下公共构造函数
http://developer.android.com/reference/android/widget/ArrayAdapter.html
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