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.
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:
Use a combination of ViewModel instances, the onSaveInstanceState() method, 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 more information about saving your activity UI state, see
Save UI states.
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 the Multi-window support page.
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 Multi-window support.
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().
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.
By default, the
onSaveInstanceState()
callback does not fire in this case. This behavior assumes the user taps Back with
no expectation of returning to the same instance of the activity.
However, you can override the
onBackPressed() method to
implement custom behavior, such as displaying a dialog that asks the user to
confirm that they want to exit your app.
If you override the onBackPressed() method, we
highly recommend that you still invoke super.onBackPressed() from
your overridden method. Otherwise the system Back behavior might be jarring to the
user.
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.