What to determine a Fragment restore upon Activity re-create?

问题

For Views having IDs, they will be auto-saved when calling super.onSaveInstanceState(outState);.

For Fragments added to an Activity, what are the cases that it will be re-created when its Activity is re-created (e.g. screen rotation), and what are the cases it will not? What to determine? What is the rule?

So far, i have tried the following cases. But trial-n-error does not mean any rules or solutions.

Cases when Fragments are re-stored:

1. Normal case: FragmentTransaction.add() to the layout with an ID.
2. Fragment without a UI: FragmentTransaction.add() to a tag only

Cases when Fragments are NOT re-stored:

1. When super.onSaveInstanceState(outState); is skipped.
2. Restored Fragments in a UI without a matching ID.

What is the general rule? Anything i missed in the documentation?

Thanks in advance.

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Edit:

To my understanding, and experiments, ALL dynamically (programmatically) added Fragments are saved upon their Activity calling super.onSaveInstanceState(outState).

• ALL dynamically (programmatically) added Fragments includes,
  • Fragments with only a tag (no UI),
  • Fragments attached to a View (with UI),
  • and Fragments with both an UI and a tag.
  • (is there any types else?)

Regarding restoring a Fragment with an UI into a layout that with no matching ID, the Fragment is indeed re-created. It just cannot be shown visually in the layout, with the following warning message:

04-08 11:41:22.445: W/PhoneWindow(9853): Previously focused view reported id 2131165226 during save, but can't be found during restore.

Once we are back into an UI with its matching ID, it will be restored correctly.

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i am still looking forward to some reliable references and your opinions!

回答1:

I had the same problem, you can see it here: After screen rotation, findFragmentById() returns a fragment, even if there's no such ID inside the layout

The Android Developer Documentation at http://developer.android.com/training/basics/fragments/communicating.html quotes this:

When a configuration change causes the activity hosting these fragments to restart, its new instance may use a different layout that doesn't include the same fragments as the previous layout. In this case all of the previous fragments will still be instantiated and running in the new instance. However, any that are no longer associated with a tag in the view hierarchy will not have their content view created and will return false from isInLayout(). (The code here also shows how you can determine if a fragment placed in a container is no longer running in a layout with that container and avoid creating its view hierarchy in that case.)

That means, we have to check screen orientation than believing in null pointer checks.

来源：https://stackoverflow.com/questions/15555964/what-to-determine-a-fragment-restore-upon-activity-re-create