I'm using the ViewPager
example with ActionBar
tabs taken from the Android documentation here.
Unfortunately, as soon as I call the addTab
method, the application crashes with the following exception:
IllegalStateException: The application's PagerAdapter changed the adapter's content without calling PagerAdapter#notifyDataSetChanged! Expected adapter item count 0, found 1.
This is the FragmentPagerAdapter
code:
public static class TabsAdapter extends FragmentPagerAdapter
implements ActionBar.TabListener, ViewPager.OnPageChangeListener {
private final Context mContext;
private final ActionBar mActionBar;
private final ViewPager mViewPager;
private final ArrayList<TabInfo> mTabs = new ArrayList<TabInfo>();
static final class TabInfo {
private final Class<?> clss;
private final Bundle args;
TabInfo(Class<?> _class, Bundle _args) {
clss = _class;
args = _args;
}
}
public TabsAdapter(Activity activity, ViewPager pager) {
super(activity.getFragmentManager());
mContext = activity;
mActionBar = activity.getActionBar();
mViewPager = pager;
mViewPager.setAdapter(this);
mViewPager.setOnPageChangeListener(this);
}
public void addTab(ActionBar.Tab tab, Class<?> clss, Bundle args) {
TabInfo info = new TabInfo(clss, args);
tab.setTag(info);
tab.setTabListener(this);
mTabs.add(info);
mActionBar.addTab(tab);
notifyDataSetChanged();
}
@Override
public int getCount() {
return mTabs.size();
}
@Override
public Fragment getItem(int position) {
TabInfo info = mTabs.get(position);
return Fragment.instantiate(mContext, info.clss.getName(), info.args);
}
@Override
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
}
@Override
public void onPageSelected(int position) {
mActionBar.setSelectedNavigationItem(position);
}
@Override
public void onPageScrollStateChanged(int state) {
}
@Override
public void onTabSelected(Tab tab, FragmentTransaction ft) {
Object tag = tab.getTag();
for (int i=0; i<mTabs.size(); i++) {
if (mTabs.get(i) == tag) {
mViewPager.setCurrentItem(i);
}
}
}
@Override
public void onTabUnselected(Tab tab, FragmentTransaction ft) {
}
@Override
public void onTabReselected(Tab tab, FragmentTransaction ft) {
}
}
}
I'm not modifying my adapter in any other part of my code and I'm calling the addTab
method from the main thread, the addTab
method ends with a call to notifyDataSetChanged
. As the documentation recommends to do:
PagerAdapter supports data set changes. Data set changes must occur on the main thread and must end with a call to notifyDataSetChanged() similar to AdapterView adapters derived from BaseAdapter.
I had a hard time making my ViewPager
working. At the end, it seems that the example in the documentation is wrong.
The addTab
method should be as follows:
public void addTab(Tab tab, Class<?> clss, Bundle args) {
TabInfo info = new TabInfo(clss, args);
tab.setTag(info);
tab.setTabListener(this);
mTabs.add(info);
notifyDataSetChanged();
mActionBar.addTab(tab);
}
Notice the order of the last three operations. In the original example, notifyDataSetChanged
was called after the mActionBar.addTab
function.
Unfortunately, as soon as you call the addTab
on the ActionBar
, the first tab you add is automatically selected. Because of this, the onTabSelected
event is fired and while trying to retrieve the page, it throws the IllegalStateException
because it notices a discrepancy between the expected item count and the actual one.