This chapter is from the book
This chapter is from the book
This chapter is from the book
Saving Data Across Process Death
Configuration changes are not the only time the OS can destroy an activity even though the user does not intend it to. Each app gets its own process (more specifically, a Linux process) containing a single thread to execute UI-related work on and a piece of memory to store objects in.
An app’s process can be destroyed by the OS if the user navigates away for a while and Android needs to reclaim memory. When an app’s process is destroyed, all the objects stored in that process’s memory are destroyed. (You will learn more about Android application processes in Chapter 23.)
Processes containing resumed or paused activities get higher priority than other processes. When the OS needs to free up resources, it will select lower-priority processes first. Practically speaking, a process containing a visible activity will not be reclaimed by the OS. If a foreground process does get reclaimed, that means something is horribly wrong with the device (and your app being killed is probably the least of the user’s concerns).
But stopped activities are fair game to be killed. So, for example, if the user presses the Home button and then goes and watches a video or plays a game, your app’s process might be destroyed.
(As of this writing, activities themselves are not individually destroyed in low-memory situations, even though the documentation reads like that is the case. Instead, Android clears an entire app process from memory, taking any of the app’s in-memory activities with it.)
When the OS destroys the app’s process, any of the app’s activities and ViewModels stored in memory will be wiped away. And the OS will not be nice about the destruction. It will not call any of the activity or ViewModel lifecycle callback functions.
So how can you save UI state data and use it to reconstruct the activity so that the user never even knows the activity was destroyed? One way to do this is to store data in saved instance state. Saved instance state is data the OS temporarily stores outside of the activity. You can add values to saved instance state by overriding Activity.onSaveInstanceState(Bundle).
The OS calls Activity.onSaveInstanceState(Bundle) any time an activity that is not finished moves to the stopped state (such as when the user presses the Home button and then launches a different app). This timing is important, because stopped activities are marked as killable. If your app process is killed because it is a low-priority background app, then you can rest assured that Activity.onSaveInstanceState(Bundle) was already called.
The default implementation of onSaveInstanceState(Bundle) directs all of the activity’s views to save their state as data in the Bundle object. A Bundle is a structure that maps string keys to values of certain limited types.
You have seen this Bundle before. It is passed into onCreate(Bundle?):
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
...
}
When you override onCreate(Bundle?), you call onCreate(Bundle?) on the activity’s superclass and pass in the bundle you just received. In the superclass implementation, the saved state of the views is retrieved and used to re-create the activity’s view hierarchy.
Overriding onSaveInstanceState(Bundle)
You can override onSaveInstanceState(Bundle) to save additional data to the bundle, which can then be read back in onCreate(Bundle?). This is how you are going to save the value of currentIndex across process death.
First, in MainActivity.kt, add a constant that will be the key for the key-value pair that will be stored in the bundle.
Listing 4.10 Adding a key for the value (MainActivity.kt)
private const val TAG = "MainActivity"
private const val KEY_INDEX = "index"
class MainActivity : AppCompatActivity() {
...
}
Next, override onSaveInstanceState(Bundle) to write the value of currentIndex to the bundle with the constant as its key.
Listing 4.11 Overriding onSaveInstanceState(…) (MainActivity.kt)
override fun onPause() {
...
}
override fun onSaveInstanceState(savedInstanceState: Bundle) {
super.onSaveInstanceState(savedInstanceState)
Log.i(TAG, "onSaveInstanceState")
savedInstanceState.putInt(KEY_INDEX, quizViewModel.currentIndex)
}
override fun onStop() {
...
}
Finally, in onCreate(Bundle?), check for this value. If it exists, assign it to currentIndex. If a value with the key "index" does not exist in the bundle, or if the bundle object is null, set the value to 0.
Listing 4.12 Checking bundle in onCreate(Bundle?) (MainActivity.kt)
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
Log.d(TAG, "onCreate(Bundle?) called")
setContentView(R.layout.activity_main)
val currentIndex = savedInstanceState?.getInt(KEY_INDEX, 0) ?: 0
quizViewModel.currentIndex = currentIndex
...
}
onCreate accepts a nullable bundle as input. This is because there is no state when a new instance of the activity is launched by the user the first time, so in this case the bundle would be null. When the activity is re-created after rotation or process death, the bundle object will be non-null. The non-null bundle will contain any key-value pairs you add in onSaveInstanceState(Bundle). The bundle may also contain additional information added by the framework, such as the contents of an EditText or other basic UI widget state.
Rotation is easy to test. And, luckily, so is the low-memory situation. Try it out now to see for yourself.
On your device or emulator, find and click on the Settings icon within the list of applications. You need to access the developer options, which are hidden by default. If you are using a hardware device, you may have enabled the developer options in Chapter 2. If you are using an emulator (or you did not already enable developer options), go to System → About emulated device (or System → About Tablet/Phone). Scroll down and click (press) the Build number seven times in quick succession.
When you see You are now a developer!, use the Back button to return to the system settings. Scroll down to find Developer options (you may need to expand the Advanced section). On the Developer options screen you will see many possible settings. Scroll down to the Apps section and turn on the setting labeled Don’t keep activities, as shown in Figure 4.8.
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FIGURE 4.8 Don’t keep activities
Now, run GeoQuiz, press NEXT to move to another question, and press the Home button. Pressing the Home button causes the activity to be paused and stopped, as you know. The logs tell you that the stopped activity has also been destroyed, just as if the Android OS had reclaimed it for its memory. However, as the logs also show, onSaveInstanceState(Bundle) was also called – so there is hope.
Restore the app (using the list of apps on the device or emulator) to see whether your state was saved as you expected. Pat yourself on the back when GeoQuiz opens to the question you last saw.
Be sure to turn Don’t keep activities off when you are done testing, as it will cause a performance decrease. Remember that pressing the Back button instead of the Home button will always destroy the activity, regardless of whether you have this development setting on. Pressing the Back button tells the OS that the user is done with the activity.
Saved instance state and activity records
How does the data you stash in onSaveInstanceState(Bundle) survive the activity’s (and process’s) death? When onSaveInstanceState(Bundle) is called, the data is saved to the Bundle object. That Bundle object is then stuffed into your activity’s activity record by the OS.
To understand the activity record, let’s add a stashed state to the activity lifecycle (Figure 4.9).
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FIGURE 4.9 The complete activity lifecycle
When your activity is stashed, an Activity object does not exist, but the activity record object lives on in the OS. The OS can reanimate the activity using the activity record when it needs to.
Note that your activity can pass into the stashed state without onDestroy() being called. You can rely on onStop() and onSaveInstanceState(Bundle) being called (unless something has gone horribly wrong on the device). Typically, you override onSaveInstanceState(Bundle) to stash small, transient-state data that belongs to the current activity in your Bundle. Override onStop() to save any permanent data, such as things the user is editing, because your activity may be killed at any time after this function returns.
So when does the activity record get snuffed? When the activity finishes, it really gets destroyed, once and for all. At that point, your activity record is discarded. Activity records are also discarded on reboot. (For a reminder about what it means for an activity to finish, see the section called Finishing an activity in Chapter 3.)