fixes NPE ActionBar.TabListener

[gabrielittner]

This is a fix for Issue #391. The explanation is in the commit message.

For testing you can overwrite the FragmentTabs class in the fragment sample with the following code:

public class FragmentTabs extends SherlockFragmentActivity {

    ActionBar actionBar;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        setTheme(SampleList.THEME); //Used for theme switching in samples
        super.onCreate(savedInstanceState);

        actionBar = getSupportActionBar();
        actionBar.addTab(actionBar.newTab().setText("Simple")
            .setTabListener(new TabListener<FragmentStackSupport.CountingFragment>
            (this, "simple", FragmentStackSupport.CountingFragment.class)));
        actionBar.addTab(actionBar.newTab().setText("Contacts")
            .setTabListener(new TabListener<CursorLoaderListFragment>
            (this, "contacts", LoaderCursorSupport.CursorLoaderListFragment.class)));
        actionBar.addTab(actionBar.newTab().setText("Custom")
            .setTabListener(new TabListener<AppListFragment>
            (this, "custom", LoaderCustomSupport.AppListFragment.class)));
        actionBar.addTab(actionBar.newTab().setText("Throttle")
            .setTabListener(new TabListener<ThrottledLoaderListFragment>
            (this, "throttle", LoaderThrottleSupport.ThrottledLoaderListFragment.class)));
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
    }

public static class TabListener<T extends Fragment> implements ActionBar.TabListener {
    private Fragment mFragment;
    private final Activity mActivity;
    private final String mTag;
    private final Class<T> mClass;

    public TabListener(Activity activity, String tag, Class<T> clz) {
        mActivity = activity;
        mTag = tag;
        mClass = clz;
    }

    public void onTabSelected(Tab tab, FragmentTransaction ft) {
        if (ft == null) {
             Log.i("tabselected", "fragmenttransaction == null");
             return;
        }
        if (mFragment == null) {
            mFragment = Fragment.instantiate(mActivity, mClass.getName());
            ft.add(android.R.id.content, mFragment, mTag);
        } else {
            ft.attach(mFragment);
        }
    }

    public void onTabUnselected(Tab tab, FragmentTransaction ft) {
        if (ft == null) {
            Log.i("tabunselected", "fragmenttransaction == null");
                return;
        }
        if (mFragment != null) {
            ft.detach(mFragment);
        }
    }

    public void onTabReselected(Tab tab, FragmentTransaction ft) {
    }
}
}

[JakeWharton]

Can you add an instanceof check to the if statement as well? If someone is using tabs inside of a regular SherlockActivity you'll get a ClassCastException on the activity start.

[SimoneCasagranda]

Hi to all :)
I don't really like this solution because if you rotate your device the onTabUnselected isn't called and after rotation you'll find two "lastVisibleFragment".

I have solved using directly the FragmentManager. For example for onTabSelected:

public void onTabSelected(Tab tab, FragmentTransaction ft) {
if (ft == null) {
Log.i(TAG, "FragmentTransaction == null during Tab selection");
return;
}
Fragment fragment = mFragmentManager.findFragmentByTag(mTag);
if(fragment == null) {
ft.add(android.R.id.content, Fragment.instantiate(mActivity, mClass.getName()), mTag);
} else {
ft.attach(fragment);
}
}

Is this a valid solution?

Thanks a lot (and thanks Jake for your great work!),
Simone