Saving ListData Objects with onSavedInstanceState

Issue

I have an app that has one main Activity that swaps out numerous Fragment's. Well it doesn't matter what Fragment you are on, after low memory kills the Activity and you try to return to the app, it boots you back to the "start" Fragment that the Activity first calls. (Note: Almost all of these are actually ListFragment's)

So here are my questions:

Should I be using onSaveInstanceState() in EACH Fragment? And if so, am I saving the Data in the Fragment OR the Fragment itself? Or do you use onSaveInstanceState() only once in the Main Activity. (If this is even the course to take)

Note: I have setRetainInstance(true) but I don't think I am handling that correctly, if that is the solution. These are all put as the last line of onActivityCreated().

Solution

The answer depends a lot on how you are managing fragments.

I'll assume you are not using the Fragment backstack, and that you have called setRetainInstance(true) on EACH fragment.

You need to use a tag when you attach the fragments.

In Activity#onSaveInstanceState() you need to remember which fragments are visible.

In Activity#onCreate you need to find the existing Fragments by tag for each fragment, then create new instances of any Fragments you can't find. Now you can use the information from the saved instance state to make the appropriate Fragments visible (show or add or replace as necessary depending on how your code manages the fragments.)

Edit in response to questions/comments:

• activty.getFragmentManager().findFragmentByTag(tag); finds an existing fragment
• in a Fragment transaction: add(fragment, tag), replace(id, fragment, tag), etc. lets you specify the tag. You can also put it in a layout file using the attribute
  • class=".myFrag$tag"

The actual fragment object including its contents still exist when you use setRetainInstance.

Note: If you don't want to use tags, you may also use the fragment manager's putFragment/getFragment methods to put the fragment into the instance state bundle.

Finally you can simply let the fragment save itself by calling FragmentManager's saveFragmentInstanceState but I've had trouble using this correctly.

Answered By - Dale Wilson