Different events, some user-triggered and some system-triggered, can cause an
Activity to transition from one state to another. This document describes
some common cases in which such transitions happen and how to handle those
transitions.
For more information about activity states, see The activity lifecycle. To
learn about how the ViewModel class can help you manage the activity
lifecycle, see the ViewModel overview.
For most activity changes, you don't need to respond directly to callbacks in
the activity lifecycle. Since Compose rebuilds UIs from state, you can take
advantage of automatic recomposition by ensuring that state is stored in an
appropriate place, such as rememberSaveable or ViewModel.
Configuration change occurs
There are a number of events that can trigger a configuration change. Perhaps the most prominent example is a change between portrait and landscape orientations. Other cases that can cause configuration changes include changes to language settings or input device.
When a configuration change occurs, the activity is destroyed and recreated. This triggers the following callbacks in the original activity instance:
A new instance of the activity is created, and the following callbacks are triggered:
In Compose, it's not typical to interact with these callbacks directly. Instead,
use the Lifecycle API to observe state changes. In Compose, you can use
LocalLifecycleOwner.current to obtain the current lifecycle and add an
observer, letting you respond to events.
Use a combination of ViewModel instances, rememberSaveable, or persistent
local storage to preserve an activity's UI state across configuration changes.
Deciding how to combine these options depends on the complexity of your UI data,
use cases for your app, and consideration of the speed of retrieval versus
memory usage. For most use cases, you should hoist state into a ViewModel and
use rememberSaveable to ensure state persists through both configuration
changes and system-initiated process death. For more information about saving
your activity UI state, see Save UI states.
When an activity is recreated due to a configuration change, the initial
composition is disposed of. Using ViewModel or rememberSaveable ensures your
UI state is restored in the new composition.
For more information, see Lifecycle in Jetpack Compose and State and Jetpack Compose.
Handle multi-window cases
When an app enters multi-window mode, available in Android 7.0 (API level 24) and higher, the system notifies the running activity of a configuration change, thus going through the lifecycle transitions described previously.
This behavior also occurs if an app already in multi-window mode gets resized. Your activity can handle the configuration change itself, or it can allow the system to destroy the activity and recreate it with the new dimensions.
For more information about the multi-window lifecycle, see the explanation of the multi-window lifecycle in Support multi-window mode.
In multi-window mode, although there are two apps that are visible to the user, only the one the user is interacting with is in the foreground and has focus. That activity is in the Resumed state, while the app in the other window is in the Paused state.
When the user switches from app A to app B, the system calls onPause on
app A and onResume on app B. It switches between these two methods
each time the user toggles between apps.
For more details about multi-window mode, refer to Support multi-window mode.
Activity or dialog appears in foreground
If a new activity or dialog appears in the foreground, taking focus and
partially covering the activity in progress, the covered activity loses focus
and enters the Paused state. Then, the system calls onPause on it.
When the covered activity returns to the foreground and regains focus, the
system calls onResume.
If a new activity or dialog appears in the foreground, taking focus and
completely covering the activity in progress, the covered activity loses focus
and enters the Stopped state. The system then, in rapid succession, calls
onPause and onStop.
When the same instance of the covered activity returns to the foreground, the
system calls onRestart, onStart, and onResume on the
activity. If it is a new instance of the covered activity that comes to the
background, the system does not call onRestart, only onStart and
onResume.
Recomposition is not affected by dialogs appearing in the foreground. However,
side effects tied to lifecycle, such as flows and animations, should use
lifecycle-aware APIs (such as collectAsStateWithLifecycle) to
automatically pause and resume work as needed. For more information, see State
and Jetpack Compose.
User taps or gestures Back
If an activity is in the foreground and the user taps or gestures Back, the
activity transitions through the onPause, onStop, and
onDestroy callbacks. The activity is destroyed and removed from the
back stack.
In a single-activity app, like most Compose apps, rememberSaveable won't
maintain state if the composable is removed from the navigation backstack. This
is because, when the user taps Back, there is no expectation that they will
return to the same instance, so all state is cleared.
To implement custom Back behavior, such as displaying a dialog that asks the
user to confirm that they want to exit your app, use the
NavigationEventHandler API.
System kills app process
If an app is in the background and the system needs to free up memory for a
foreground app, the system can kill the background app. When the system kills an
app, there is no guarantee that onDestroy is called in the app.
To learn more about how the system decides which processes to destroy, read Activity state and ejection from memory and Processes and app lifecycle.
To learn how to save your activity UI state when the system kills your app process, see Saving and restoring transient UI state.