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How can I save activity state (B) when moving to previous activity (A) and how to restore it's previous state when moving back (to activity B)?

SCENARIO

Content of activity B can be changed by user. After finishing work, performed changes can be persisted (e. g. saved to local DB). Before persisting, user may want to navigate back to previous activity. At this point, "work session" should be somehow saved temporary in memory. After returning back to activity B "work session" should be restored, so user can continue in work.

APPROACHES

Standard methods onCreate(Bundle) and onSaveInstanceState(Bundle) are used to restore/save activity state when device configuration changes (e. g. on rotate). As onSaveInstanceState(Bundle) is not part of activity's life-cycle, this method does not get called on "back-press" when activity is destroyed (by default finish() is called). The same applies for onRestoreInstanceState(Bundle). Obviously these methods alone are not sufficient to restore activity state after lefting an activity.

Transfering bundle state between activities

One way I can think of is to override onBackPressed() in such way it will start an Intent to previous activity A with bundle state Intent#putExtras(Bundle) of current activity B. When moving back, starting an Intent to activity B, this bundle state will be delivered back Intent#putExtras(Bundle) to restore the state of activity B. However this also require to override onCreate(Bundle) and onSaveInstanceState(Bundle) in A so bundle state of B would not be lost on configuration change before navigating back to B.

Not finishing an activity on back-press + non-default launchMode

Another way (more cleaner) is to override onBackPressed() so that it would start Intent to previous activity A without calling finish() of current activity B so activity B will hang in memory in paused state (waiting for resume). To resume activity B, manifest configuration android:launchMode="singleInstance" is required so android will use existing activity (paused B) instead of creating new one (B2) when navigating back to activity B (startIntent(B.class)).

Details: launchMode singleInstance creates singleton activity in new task (task = set of activities with the same group id i. e. affinity, normally app activities have the same affinity i. e. app = single task).

Drawback: transition animations does not work with singleInstance mode. It seems that singleInstance task may not be fully initialized at animation time. For more details: Custom animation doesnt work on SingleInstance Activity.

Saving activity state to SharedPreferences

Saving activity state to SharedPreferences on "back-press", restoring activity state from preferences in onCreate(Bundle) like in following link Save activity state to SharedPreferences.

Drawback: unable to save Bundle state (only primitives: putInt, putString, ...).

Others

Some of the methods listed in Share data between activities can be used. Answers from this link refers to developer.android.com/guide/appendix/faq/framework.html which is unfortunately broken. Here is an alternative source http://wing-linux.sourceforge.net/guide/appendix/faq/framework.html.

About sharing data via Application object:

  • Don't Store Data in the Application Object,
  • Using the Android Application class to temporary persist data.

  • Based on Application class documentation, using static singletons over Application is more preferable.

    Base class for maintaining global application state. ...

    Note: There is normally no need to subclass Application. In most situations, static singletons can provide the same functionality in a more modular way. If your singleton needs a global context (for example to register broadcast receivers), include Context.getApplicationContext() as a Context argument when invoking your singleton's getInstance() method.

It seems that putting data in Application object or static singleton is so far the best solution for this problem.

QUESTION

Is there any build-in solution for this (without need of overriding onBackPressed())? For example saving activity on back-stack with it's state. If not, what is the common pattern to save activity state in such situation?

RELATED POSTS (just to link them with this post)

How do I save temporary data on android?

matoni
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2 Answers2

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For an alternative way to handle this, you can map your view's states to a data structure and save it as an object notation (such as json) to your local and when your activity is created / re-created you can read your state from local and bind them with your views.

For more information and example, you can check Spotify's presentations and open source project which contains information about how they manage application's ui from apis. (I know it's not exactly what you're trying to do but you may find some tricks.)

If you need to save your data in your activities, you can use Local Db or in-memory cache.

savepopulation
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When you back out of an activity, the activity is not just destroyed programmatically, but also conceptually. In other words, the user expects it to be gone. We talk about saving activity state in situations where the activity object is destroyed but the user perceives that it still exists, such as during configuration changes or when it's in the background or backstack.

What you're trying to save is properly thought of as application state, not activity state. As such, SharedPreferences is an appropriate place for it. Whether it's the best solution is a matter of opinion and depends on your use case.

Kevin Krumwiede
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  • Hi, thanks for thoughts. I saw similar suggestion somewhere else, I think this solution is almost perfect, but with SharedPreferences I'm not able to save Bundle object (Fragment's state). Can you think of any other solution that is commonly used? – matoni Jul 27 '17 at 20:17
  • @matoni I don't know of a simple way to save an entire `Bundle`. But I would suggest that each `Fragment` should be responsible for saving its own state, and question why it's in a `Bundle` in the first place. – Kevin Krumwiede Jul 27 '17 at 20:22
  • I have activity where I can put content fragment. Fragment contains `ViewPager` which consists of pages (fragments). I need to save state of these pages. First though was to export state of the `ViewPager's` adapter (`adapter.saveState()` which returns `Bundle`). Unfortunately, bundle contains only identifiers of the pages (fragments) so this bundle can restore state of the `ViewPager` only if fragments are alive (e. g. on configuration change). It seems that I need to save pages on my own (maybe I will save them in string format so I would be able to `SharedPreferences`). – matoni Jul 27 '17 at 21:04
  • Do you think it is a good idea to store state (in any form) in static field of the activity? Maybe I'm asking stupid questions, but I can't find any relevant resource on this topic. The only thing I found was: [How do I pass data between Activities/Services within a single application?](http://wing-linux.sourceforge.net/guide/appendix/faq/framework.html). As I can't find similar article on [developer.android.com](https://developer.android.com) I don't know whether It is out-dated or not. – matoni Jul 27 '17 at 21:14