Zagorax Zagorax - 6 months ago 12
Android Question

IllegalStateException: The application's PagerAdapter changed the adapter's content without calling PagerAdapter#notifyDataSetChanged

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.

Answer

I had an hard time making my ViewPager working. At the end, it seems 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 trhows the IllegalStateException because it notices a discrepancy between the expected item count and the actual one.

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