Simplest Android Activity Lifecycle

Question

I noticed that the Android Developers Activity section has been updated since I started my app, but I am still unclear what the simplest Activity Lifecycle is.

As far as I can make out:

onCreate, onResume and onPause are the essential ones.

The activity may be deleted any time after onPause, so I should save my whole app state to a file onPause and not rely on onStop or onDestroy. Also, onSaveInstanceState is not called before every onPause so is not really worth using.

Rather than trying to write loads of code to handle all the scenarios, why not destroy the Activity at the end of its onPause?

The Lifecycle would then be onCreate and onResume before it is active, then onPause when it becomes inactive. Other methods would not be needed.

I'd use onCreate to call setContentView and set up view listeners, but everything else would be put in onResume, including loading the restored state from a file? As stated earlier, onPause would save the state to a file and destroy the activity.

As far as I can see, the only disadvantage of this might be that when a popup is on screen, the activity is deleted and has to be recreated when the popup is closed, meaning the activity won't be visible behind the popup (although I have not tested this)

It may take a bit longer to restart the activity, but since the system could have deleted the activity anyway without any notice, you have to save the whole state anyway.

Any thoughts?

Update: I suppose what I was thinking of was where a 'front page' activity calls a game activity. The frontpage activity would call the game activity when the player clicks 'Play'

The game activity would set up its views and listeners etc. in onCreate, and in onResume it would load a file containing the game state, or start a new game if no file existed.

onPause of the game, it writes the game state to the file, then whatever happens to the game activity (nothing, or gets stopped/destroyed, or whatever) the onResume method would always load all the data back in again from the file.

That's sort of what I was thinking, if that makes any sense?

Update2: I've devised a simple solution which I've documented in an answer below, if anyone's interested!

It doesn't support the Android Activity Lifecycle 'Paused' and 'Stopped' states. Once it is no longer displayed it kills itself and has to be restarted manually, but it does carry on from where you left off!

Answer 1

Are you looking for this?

To further answer your question, yes, as you can plainly see from the above diagram the "simplest" (i.e. smallest number of method calls) lifecycle is indeed onCreate(); onStart(); onResume(); onPause();.

You should also know about onSaveInstanceState() and onRetainNonConfigurationInstance(). These are NOT lifecycle methods.

All these methods are very well documented. Please read this documentation thoroughly.

To clarify things further, here are a couple of real-life scenarios:

1. Activity is running, other activities come on top of it, onPause is called. System runs out of memory, calls onSaveInstanceState, kills activity. User pressed back a few times, activity has to be re-instantiated (preferably using the data saved in onSaveInstanceState).
2. Activity is running, user presses back. At this point onPause->onDestroy are called, without calling onSaveInstanceState.

You should understand the essential difference between onPause and onSaveInstanceState. The former is always called, while the latter is only called when the activity instance might be re-instantiated in the future. Following this train of thought, your users will expect two things:

1. When they navigate away from your Activity and later come back to it, they want it in the exact same instance that they left it (this would be achieved using onSaveInstanceState). They don't expect that if they exit your activity. However:
2. They will expect that data they have entered will be persisted (which will be done in onPause). For example, if they started composing a message, they'll expect to see it as a draft the next time they come back, even if they exited the activity.

You should understand how these methods are supposed to be used in order to get what your users expect. How you actually use them is up to you, your needs, and your app's nature.

Answer 2

Android system is handling the lifecycle: i.e. instantiating activities and calling lifecycle methods. So I don't know what you mean by "destroy the activity". You, as a developer, have no such ability.

Second, activity lifecycle flow can be sometimes confusing (I know I struggled with it at the beginning). So, just implement all lifecycle methods and put logging statements in them. Then try all real-life use cases (including receiving a call during app use) to see how lifecycle methods are called.