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Populating fragment fields from activity

I have a ViewPager of 3 fragments. All 3 fragments are of the same type, with identical layouts, but they are to hold different (text) information. I am trying to create my activity, where I create the fragments and prepare the text data that I will populate my fragments with. However, I can't seem to work with the fragments from within my activity. All the activity's lifecycle methods are executed before the fragment lifecycle methods. So if I try to update a textview in a fragment from within my activity, it won't work, because the textview is null in the fragment.
I'm going to need to make periodic updates to the fragments, so passing the data as a bundle is not an option. Plus, since I'm passing lots of text, I'm using a StringBuilder object, which is not something I can pass in a bundle (unless I make it Parcelable, which I don't want)
I think I could run a method from within my fragment class that would execute during fragment creation, but that means all 3 fragments would run this method. That's not really the level of control I'm looking for.
Is there a neat way to make this work?
Thanks

Keep references to your fragments, and let them all implement an interface with a common update-method. As an example, let's make it super clear and call the interface Updatable with one method called 'update':
public interface Updatable {
public void update(String text);
}
Now, in your Activity's onCreate, save references to your Updatables there (i.e. when you lookup or instantiate your Fragments).
It should now be trivial to update your Fragments when necessary from the Activity. Needless to say, the fragment implementation of the update code needs to do the actual update of the TextView(s).
If the update implementation is exactly the same for all your Fragments, your could save some lines of code and make a base class which implements Updatable and extends Fragment.

you might able to populating fragment fields during onActivityCreated(Bundle savedInstanceState).
Refer to this site for more information about fragments' life cycle:
http://developer.android.com/guide/components/fragments.html

You should consider using Observer pattern... there is a really great implementation which you can include as gradle dependency called EventBus:
https://github.com/greenrobot/EventBus

You can use Otto Bus to send data to your fragments from your activity.
http://square.github.io/otto/
Create a new bus in you application class
Bus bus = new Bus();
Create an event that contains your data which you'll pass to fragments.
bus.post(new MyDataEvent(data));
Register your fragment in your fragment's onResume() (Do not forget to unregister in your fragment's onPause())
bus.register(this);
And get data with subscribe in your fragment
#Subscribe
public void onDataReceived(MyDataEvent event) {
// TODO Do what ever you want
}
I hope this'll help you.

Android, why shouldn't one Fragment call another directly?

According to Android guidline, http://developer.android.com/training/basics/fragments/communicating.html
One fragment should send data to another one via the hosting Activity. I'm wondering there is a reason for that.
Since in my code, I put one variable to hold pointer to the another fragment, and assign it in onActivityCreated
//this code is in class FragmentType1, assign the pointer to the FragmentType2 for later use
#Override
public void onActivityCreated(final Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
final FragmentManager fm = getFragmentManager();
mOtherFragment = (FragmentType2) fm
.findFragmentById(R.id.container_fragment);
}
and later if I want to set data for FragmentType2, I just call:
public setData(MyData data){
if (mOtherFragment!=null)
mOtherFragment.setData(data);
}
Everything is working fine now. What's wrong with that approach? Tks.

There's at least 2 reasons for it:
To facilitate de-coupling of fragments.
To avoid memory leaks that can happen by storing a reference to one fragment in another.

Another reason would be to maintain the state when the hosting activity is destroyed.
After recreation, you could fetch the state from the hosting activity, since the fragment itself would be unable to save it's state directly.

And last but not least, it is really hard for dynamically added fragments, especially when it comes to nested fragments, to get sure that both fragments are "alive" at the same time. Fragments life cycle is complex to manage properly and using a direct communication presumes you completely control this, which is rarely true.
The best model to my mind is to use an event bus, like Otto or EventBus or RoboGuice's bus.

Is it advisable to manage REST calls in fragments?

If the user e.g. changes device's orientation during the REST (or any other long running async operation), the fragment is detached from the activity.
So if this fragment uses getActivity() anywhere in the code handling the REST response, it will raise a null pointer.
I can protect all the calls of getActivity() with null checks - but there's still the possibility the activity becomes null between the check-line and the use-line. And also there's lot of places where this is done, so the code will become a mess.
setRetainInstance(true) is not usable if I want to change the layout on orientation change. + There's are also some strange effects with that like: Multiple fragments, setRetainInstance(true) and screen rotation
So this leads me to think maybe its generally bad practice to handle rest calls in fragments?
I have seen some practices where the activity contains a non visual fragment to handle the responses. But I can't put this in the fragments, I guess. So I have to use the activity as a mediator and reach the results to the current fragment?
I just thought, it's cleaner to put everything in the fragment. Since I don't need to modify the code elsewhere. And it's then a self contained entity, which I can put somewhere else without problems. But what do I do with these unreliable references to the context? I mean if the fragment is recreated, I really don't care about the detached fragment - it just has to finish whatever it's doing silently and don't disturb my new workflow with exceptions. And of course I don't want to surround everything with try catch(Exception) since I care about the exceptions thrown in other situations.

I'm usually use the fragment as the display aspect and the activity as the logic aspect. just like MVC- the fragment is the view and the activity is the controller.
So all my fragments has this code:
#Override
public void onAttach(Activity activity) {
super.onAttach(activity);
try {
listener = (FragmentListener) activity;
} catch (ClassCastException e) {
throw new ClassCastException(activity.toString() + " must implement FragmentListener");
}
}
and every activity that uses this fragment implement the FragmentListener interface.
Once I got a response from the server I'm just populate (or just calling the right method) the fragment, you can do some null checking here, but It will be rare...

You certainly can do that. You probably want to look at AsyncTaskLoader and the LoaderManager (they specifically cope with the lifecycle issues you are talking about). Also be aware that task loader IDs have to be unique to the activity, so if you have multiple fragments running loaders hosted within the same activity, the loader id's cant all be the same ;-)

on orientation change, the activity will be re-created. To protect this use the below code
android:configChanges="keyboard|keyboardHidden|orientation|screenLayout|uiMode|screenSize|smallestScreenSize" in manifest file ( i.e in activity tag )
and ovrride the method in your MainActivity
#Override
public void onConfigurationChanged(Configuration newConfig) {
// TODO Auto-generated method stub
super.onConfigurationChanged(newConfig);
}

Getting "Called doStart when already started" from LoaderManager. Why?

My code has an Activity, that has a FragmentPagerAdapter that creates 'n' fragments as necessary.
Activity has a loader, and each fragment has its own loader. All loaders have unique ID.
(Activity's loader in fact determines the number of Pages in the adapter)
I keep getting this warning here and there and can't put my finger on what's causing it.
It doesn't seem to be critical, also looking at the LoaderManger's code throwing this warning, but still - warnings are usually signs for bugs..
Had originally used FragmentStatePagerAdapter and then moved to FragmentPagerAdapter, thinking that could somehow be the issue - but obviously it's not.
Posting code would really complicate this and add very little.
Any thoughts?

in your fragment move your initLoader method inside the onActivityCreated method.
#Override
public void onActivityCreated(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onActivityCreated(savedInstanceState);
LoaderManager lm = getLoaderManager();
lm.initLoader(LOADER_ID, null, this);
}

I've just finished a few hours long debugging session with the support library.
TL;DR: DO NOT CALL getLoaderManager in Fragment.onCreate, use onActivityCreated!
(this implies that you can't do initLoader before onActivityCreated)
Fragment.getLoaderManager() will lazily get a LoaderManager instance for you from the activity. However for this to be a valid one the fragment already has to be activated (FragmentManager.makeActive), which implies two things relevant here:
the Fragment has been added
(FragmentManager.addFragment)
the Fragment has an internal identifier (mWho)
(makeActive calls Fragment.setIndex)
The latter is really important because when you call Fragment.getLoaderManager() which in turn asks FragmentActivity.getLoaderManager(who, ...) for a real manager. When called from Fragment.onCreate() the makeActive call didn't happen yet so you'll get back a LoaderManagerImpl with mWho == null which is bad, because the framework will reassign the LoaderManager instance to every fragment which has similar lifecycle.
Because of this reassignment the LoaderManager has already been started by one fragment, but another one will try to start it as well, because the activity didn't know which fragment is asking, neither of them had their identity (mWho) yet.

ASyncTask, hidden fragments, retaining instances, and screen orientation changes

My setup is as follows.
I have a FragmentPagerAdapter called from my Activity which loads two fragments. This is setup within onCreate.
In onResume I call an ASyncTask which loads data from a database, and then calls a callback in my activity onLoadComplete via a load data listener.
#Override
public void onLoadComplete(JSONArray data) {
// TODO Auto-generated method stub
LocalFragment fragmentB = (LocalFragment)getSupportFragmentManager().findFragmentByTag(ListTag);
fragmentB.setList(data);
LMapFragment fragmentA = (LMapFragment)getSupportFragmentManager().findFragmentByTag(MapTag);
GoogleMap our_map = fragmentA.getMap();
fragmentA.plotP(myLocation,data);
}
The fragments are initialized by the Pager, and within each fragments code I set the respective tag e.g in LocalFragment
#Override
public void onAttach(Activity activity) {
// TODO Auto-generated method stub
super.onAttach(activity);
String myTag = getTag();
((PagerTest) activity).setListTag(myTag);
Log.d("what",myTag);
}
This allows me to access the fragment, call a function within it which populates a list or populates a map. It works.
What I am now trying to do is account for screen orientation changes.. If while the ASyncTask is running the orientation is changed, the app crashes.
As suggested here: Hidden Fragments I have been trying to implement a hidden fragment which saves the state of my ASyncTask. So what I have done is set it up so in onResume of my Activity i call a function
static LoadDataFromURL the_data = null;
static JSONArray pub_data = null;
private static final String TAG = "RetainFragment";
public RetainFragment() {}
public static RetainFragment findOrCreateRetainFragment(FragmentManager fm) {
RetainFragment fragment = (RetainFragment) fm.findFragmentByTag(TAG);
if (fragment == null) {
fragment = new RetainFragment();
fm.beginTransaction().add(fragment, TAG).commit(); // add this
}
return fragment;
}
which essentially saves my data.
Basically what this means is that if i rotate my screen i dont call my ASyncTask again.. the screen just rotates.. it works perfectly.
If however I go back to the main menu and then click on the activity again the screen returns blank (but does not crash). My understanding is that the data is retained as an object in the fragment, but on reloading the activity afresh the data needs to be set again.. I.E onLoadComplete needs to be called to populate the list/map..
So i concluded that if initially after the ASyncTask completes i save the returned data in my hidden fragment onRetainInstance, then i could simply call onLoadComplete and pass it..
The problem is, in this situation seemingly the fragment has not been called yet, as such the tags are null, and calling the callbacks within onLoadComplete crashes the app.
I have been banging my head over this for ages.
My ASyncTask is in a seperate class: LoadDataFromURL
What i want to achieve is as follows - a fragmentviewpager whereby on screen rotate the ASyncTask is retained on rotate/attached to the new activity, and if it has completed before it shouldn't be run again..
Could anyone advise.
Many Thanks
EDIT
Having changed the variables in my secret fragment to public variables, everything has seemingly come together.. BUT because im not 100% sure how/when things are called, I dont fully understand WHY it works..
So.. I call findOrCreateRetainFragment and it either creates a new 'secret' fragment or returns the current instance.
If it is returning a current instance, i dont call my async task again. If it is not, I call my asynctask and load the data.
With this setup, when i load the activity and rotate the screen, it rotates as expected woop.
Now, when i go back to the main menu and then click the activity again, it calls the async task.
My understanding is that on rotate the async task is not called again, and the viewpager is somehow saving the fragments.
On the other hand, when i go back my activity is destroyed, as is my secret fragment, and as such when i click on it again it loads the data. THis is essentially what i want..
Have i understood this correctly?
Thanks

There are a few issues here that you're experiencing (I think).
First of all, the reason your callbacks crash is because they're attached to an old Activity that no longer "exists" after a screen orientation and/or Activity push. If you use onAttach() to attach a callback to your fragment, you must use onDetach() to detach that callback when the Fragment is removed from the Activity. Then, whenever you call the callback, check for a null so you don't send data to a dead object.
Basically, the logic you're trying to use here is:
Start Activity.
Check if your Fragment exists. If it does, grab it. Else, create it.
Retrieve the data if it exists. If not, wait for the callback.
Because of the nature of callbacks (depending on your implementation), you will not receive data until the event fires. However, if the Activity is gone and the event has already fired, the callback won't execute. Thus, you have to retrieve the data manually. When using setRetainInstance(), it's helpful to think of it as this entity detatched from your Activity. It will exist as long as you don't pop the current Activity or push a new Activity. However, your current Activity will be destroyed upon screen orientation changes while the Fragment won't. As such, the Fragment shouldn't rely on the existence of the Activity.
A much more elegant solution to the problem that you may want to look in to is implementing the Android Loader API. Loaders are handy tools that are handled by the system that work is roughly the same way but are more in-tune with asynchronously retrieving data. They work effectively the same way. You simply start your loader and the system with either create one if it doesn't exist or re-use one that already exists. It will remain in the system by the LoaderManager upon configuration changes.
EDIT:
To answer your edit, I guess I'll explain what's happening. It's convoluted, so just tell me if anything needs clarification.
Fragments aren't technically speaking part of your currently running Activity. When you create an instance of the Fragment, you have to call beginTransation() and commit() on the FragmentManager. The FragmentManager is a singleton that exists within the realm of your application. The FragmentManager holds on to the instance of the Fragment for you. The FragmentManager then attaches the Fragment to your Activity (see onAttach()). The Fragment then exists within the FragmentManager which is why you never really have to hold a reference to it within your application. You can just call findFragmentByTag/Id() to retrieve it.
Under normal circumstances, when your Activity is being destroyed, the FragmentManager will detach the instance of your Fragment (see onDetach()) and just let it go. The Java garbage collect will detect that no reference to your Fragment exists and will clean it up.
When you call setRetainInstace(), you're telling the FragmentManager to hold on to it. Thus, when your Activity is being destroyed on a configuration change, the FragmentManager will hold on to the reference of your Fragment. Thus when your Activity is rebuilt, you can call findFragmentByTag/Id() to retrieve the last instance. So long as it didn't keep any context of the last Activity, there shouldn't be any problems.
Traditionally, one would use it to keep references to long standing data (as you are) or to keep connection sockets open so a phone flip doesn't delete it.
Your ViewPager has nothing to do with this. How it retrieves the Fragments is completely dependent on how you implement that Adapter that it's attached to. Usually, retained Fragments don't have Views themselves because Views hold Context data of the Activity they were created in. You would just basically want to make it a data bucket to hold on to the data for the Views to pull from when they're being inflated.