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Properly stopping and restarting your activity is an important process in the activity lifecycle that ensures your users perceive that your app is always alive and doesn't lose their progress. There are a few of key scenarios in which your activity is stopped and restarted:
The class provides two lifecycle methods,
and
, which allow you to specifically handle how your activity handles being stopped and restarted. Unlike the paused state, which identifies a partial UI obstruction, the stopped state guarantees that the UI is no longer visible and the user's focus is in a separate activity (or an entirely separate app).
Note: Because the system retains your instance in system memory when it is stopped, it's possible that you don't need to implement the
and
(or even
methods at all. For most activities that are relatively simple, the activity will stop and restart just fine and you might only need to use
to pause ongoing actions and disconnect from system resources.
When your activity receives a call to the method, it's no longer visible and should release almost all resources that aren't needed while the user is not using it. Once your activity is stopped, the system might destroy the instance if it needs to recover system memory. In extreme cases, the system might simply kill your app process without calling the activity's final
callback, so it's important you use
to release resources that might leak memory.
Although the method is called before
, you should use to perform larger, more CPU intensive shut-down operations, such as writing information to a database.
For example, here's an implementation of that saves the contents of a draft note to persistent storage:
@Overrideprotected void onStop() { super.onStop(); // Always call the superclass method first // Save the note's current draft, because the activity is stopping // and we want to be sure the current note progress isn't lost. ContentValues values = new ContentValues(); values.put(NotePad.Notes.COLUMN_NAME_NOTE, getCurrentNoteText()); values.put(NotePad.Notes.COLUMN_NAME_TITLE, getCurrentNoteTitle()); getContentResolver().update( mUri, // The URI for the note to update. values, // The map of column names and new values to apply to them. null, // No SELECT criteria are used. null // No WHERE columns are used. );}
When your activity is stopped, the object is kept resident in memory and is recalled when the activity resumes. You don’t need to re-initialize components that were created during any of the callback methods leading up to the Resumed state. The system also keeps track of the current state for each
in the layout, so if the user entered text into an
widget, that content is retained so you don't need to save and restore it.
Note: Even if the system destroys your activity while it's stopped, it still retains the state of the objects (such as text in an
) in a
(a blob of key-value pairs) and restores them if the user navigates back to the same instance of the activity (the talks more about using a
to save other state data in case your activity is destroyed and recreated).
When your activity comes back to the foreground from the stopped state, it receives a call to . The system also calls the
method, which happens every time your activity becomes visible (whether being restarted or created for the first time). The
method, however, is called only when the activity resumes from the stopped state, so you can use it to perform special restoration work that might be necessary only if the activity was previously stopped, but not destroyed.
It's uncommon that an app needs to use to restore the activity's state, so there aren't any guidelines for this method that apply to the general population of apps. However, because your
method should essentially clean up all your activity's resources, you'll need to re-instantiate them when the activity restarts. Yet, you also need to instantiate them when your activity is created for the first time (when there's no existing instance of the activity). For this reason, you should usually use the
callback method as the counterpart to the
method, because the system calls both when it creates your activity and when it restarts the activity from the stopped state.
For example, because the user might have been away from your app for a long time before coming back it, the method is a good place to verify that required system features are enabled:
@Overrideprotected void onStart() { super.onStart(); // Always call the superclass method first // The activity is either being restarted or started for the first time // so this is where we should make sure that GPS is enabled LocationManager locationManager = (LocationManager) getSystemService(Context.LOCATION_SERVICE); boolean gpsEnabled = locationManager.isProviderEnabled(LocationManager.GPS_PROVIDER); if (!gpsEnabled) { // Create a dialog here that requests the user to enable GPS, and use an intent // with the android.provider.Settings.ACTION_LOCATION_SOURCE_SETTINGS action // to take the user to the Settings screen to enable GPS when they click "OK" }}@Overrideprotected void onRestart() { super.onRestart(); // Always call the superclass method first // Activity being restarted from stopped state }
When the system destroys your activity, it calls the method for your
. Because you should generally have released most of your resources with
, by the time you receive a call to
, there's not much that most apps need to do. This method is your last chance to clean out resources that could lead to a memory leak, so you should be sure that additional threads are destroyed and other long-running actions like method tracing are also stopped.
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