Android Honeycomb: How to change Fragments in a FrameLayout, without re-creating them?

Question

is it possible to switch between Fragments without re-creating them all the time? If so, how?

In the documentation I found an example of how to replace Fragments.

// Create new fragment and transaction
Fragment newFragment = new ExampleFragment();
FragmentTransaction transaction = getFragmentManager().beginTransaction();

// Replace whatever is in the fragment_container view with this fragment,
// and add the transaction to the back stack
transaction.replace(R.id.fragment_container, newFragment);
transaction.addToBackStack(null);

// Commit the transaction
transaction.commit();

But I don't want to create my Fragments from the scratch every time I need them.

I also found this example of hiding/showing Fragments:

// The content view embeds two fragments; now retrieve them and attach
// their "hide" button.
FragmentManager fm = getFragmentManager();
addShowHideListener(R.id.frag1hide, fm.findFragmentById(R.id.fragment1));
addShowHideListener(R.id.frag2hide, fm.findFragmentById(R.id.fragment2));

But how would I create a fragment with an ID outside an XML file?

I think this might be related to this question, but there isn't an answer. :/

Thank you very much in advance, jellyfish

Edit:

That's how I'm doing it now:

Fragment shown = fragmentManager.findFragmentByTag(shownFragment);

//...

FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
if (shown != null) fragmentTransaction.hide(shown);

//switch statetement for menu selection, just one example:

SettingsFragment set = (SettingsFragment) fragmentManager.findFragmentByTag(SET);
Toast.makeText(this, "Settings:" + set, Toast.LENGTH_LONG).show();
if (set == null)
{
        set = new SettingsFragment();
        fragmentTransaction.add(R.id.framelayout_content, set, SET);
}
else fragmentTransaction.show(set);
shownFragment = SET;
fragmentTransaction.commit();

If I call up the settings, then something else, and then go back to settings, the toast gives me "null" first and "Settings:SettingsFragment{40ef..." second.

However, if I replace fragmentTransaction.add(R.id.framelayout_content, set, SET); with fragmentTransaction.replace(R.id.framelayout_content, set, SET); I keep getting "null", "null", "null"... so it doesn't seem to find the Fragment by tag.

Edit2:

Adding fragmentTransaction.addToBackStack(null); did the trick. :) This saves the whole hiding/memorizing which fragment is shown part so I suppose it's the most elegant solution for this.

I found this tutorial quite helpful on the topic.

Edit3:

Looking at my code I realized I could get rid of some parts, so I changed it to:

FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
if (shown != null) fragmentTransaction.hide(shown);
Settings set = (Settings) fragmentManager.findFragmentByTag(SET);
if (set == null) set = new Settings();

fragmentTransaction.replace(R.id.framelayout_content, set, SET);
fragmentTransaction.addToBackStack(null);
fragmentTransaction.commit();

However, this invoked an IllegalStateException: Fragment already added, much the same like here. Is there an easy way to prevent this? Otherwise I think I might switch back to the hide/show bit.

Answer 1

It could depend on what you are trying to avoid being re-created.

// Replace whatever is in the fragment_container view with this fragment,
// and add the transaction to the back stack
transaction.replace(R.id.fragment_container, newFragment);
transaction.addToBackStack(null);

In your example example when you hit the back button from your newFragment the previous fragment will be shown (you'll get an onCreateView, onActivityCreated but no onCreate) so that fragment isn't being re-created as such. As for you newFragment you can still keep it around if you plan to use it again updating any internal state as required in say onCreate or onActivityCreated.

EDIT:

If you simply have a menu list with each entry invoking a different fragment in a right pane then adding to the back stack is not what you want. For this you might get away with calling add(...) on each fragment up-front and simply hide/show each fragment as required (I've not tested this). Otherwise I would suggest holding a reference to each fragment, call replace(...) on selecting a different menu item ensuring that you don't add to the back stack.

Answer 2

To avoid the

IllegalStateException: Fragment already added

I found a workaround that is working fine for me: use remove(AFrag) and add(BFrag) in your transaction, instead of replace().

It looks like it is a bug: 4th comment in the accepted answer.