Getting reference to nested fragment from FragmentTabHost

Question

In my application, I use an Activity which holds one Fragment with FragmentTabHost and hence all its tabs are nested Fragments.

Inside an Activity which holds a Fragment with its nested Fragment, we may get a reference to attached one using onAttachedFragment().

But how to get a reference to nested Fragment from FragmentTabHost?

Knotty

Answer 1

Well, exploring the source code of FragmentTabHost I've found that when it adds a fragment tab, it assignes a tag of TabSpec to nested Fragment.

So to get the reference to this Fragment we should call

getChildFragmentManager().findFragmentByTag(tabSpecTag)

Knotty

can you please post the code where exactly do you do the mapping because i always get NullPointerException on my reference – Coefficient

@Coefficient , I added this code in my fragment onActivityCreated() method there I get reference to this fragment – Beetlebrowed

You call getChildFragmentManager() on which object ? – Monsoon

I call it in parent Fragment. – Knotty

If I call this fragmentTabHost.addTab(fragmentTabHost.newTabSpec(TAG) .setIndicator(view), MyFragment.class, null); then right after do getSupportFragmentManager().findFragmentByTag(TAG); I get null. I'm doing this in the Activity since I need a reference to the fragment there. Not sure how to get it to work. – Permalloy

@Permalloy Seems like you invoke improper fragmentManager try getChildFragmentManager() instead of getSupportFragmentManager() – Knotty

Answer 2

I was trying this for a while, but I was getting null returned from the FragmentManager because I was trying to access the manager in onCreateView() immediately after adding.

Here is a good explanation on what happened

It's also important to note that Fragment tabs that have not yet been selected don't exist yet in the FragmentManager, and so will return null as well. I got around this by calling mTabHost.setCurrentTab(index) before trying get to the Fragment with the FragmentManager. It's not very clean, but it works.

Gossipry

Well, I get a reference to the nested Fragment in onViewCreated() at the beginning of parent Fragment's lifecycle, checking if savedInstanceState bundle is null - in this case I know what Fragment should appear by default and use the specific tag, in other case I get an int value from this bundle which represents checked tab which I saved there before and use it to make casting to specific fragment class. Also I assign nested tab reference with OnTabChangeListener using TabSpec's tag passed to onTabChanged() method. – Knotty

Answer 3

Above solutions are also working but I have one more easy solution,

 @Override
public void onTabChanged(final String tabId) {

    new Handler().postDelayed(new Runnable() {
        @Override
        public void run() {

            mFragment = getChildFragmentManager().findFragmentByTag("Tagname");
        }
    },1000);
}

Here you have to implement FragmentTabHost.onTabChangeListener We have kept a second delay in fetching fragment from the childFragmentManager.

Note : You need to cast mFragment which fragment you have used.

Briny

One thing, I see as a drawback at least, is that here you make a potential memory leak, allocating memory for anonymous Handler object. After being invoked it keeps the reference to the outer class about a second. So if you quit your activity just after changing a tab you will still have a dangling reference to it in memory. You should think twice if you invoke this code inside Activity class. Moreover, if user decides to piddle with your tabs and would swipe them randomly then you swamp your message queue with Runnables and there is no way to remove them from Handler – Knotty

Agree with you but I would like to get the proper location from where to call this method mFragment = getChildFragmentManager().findFragmentByTag("Tagname"); ...This is the code where I found solution. Thanks for your deep review :) – Briny

Answer 4

I found a solution that I like a little better because it doesn't involving executing code with a delay (which is always iffy given android hardware fragmentation and different processor speeds).

In your onTabChanged() method, before you try to find the fragment, call executePendingTransactions() on the fragment manager associated with your tabHost. It seems there are some places in the FragmentTabHost source code where they should be calling executePendingTransactions() but fail to do so.

This works every time the tab changes with one exception... the first tab that is selected still comes back null... In my specific case, I was able to handle this exception differently anyway, by putting some code in onResume.

Hope this helps.

Multiracial