Activities in the system are managed as an activity stack.
When a new activity is started, it is placed on the top of the stack and becomes the running activity -- the previous activity always remains below it in the stack, and will not come to the foreground again until the new activity exits.
An activity has essentially four states:
If an activity in the foreground of the screen (at the top of the stack), it is
active or
running.
If an activity has lost focus but is still visible (that is, a new non-full-sized or transparent activity has focus on top of your activity), it is paused.
A paused activity is completely alive (it maintains all state and member information and remains attached to the window manager), but can be killed by the system in extreme low memory situations.
If an activity is completely obscured by another activity, it is stopped.
It still retains all state and member information, however, it is no longer visible to the user so its window is hidden and it will often be killed by the system when memory is needed elsewhere.
If an activity is paused or stopped, the system can drop the activity from memory by either asking it to finish, or simply killing its process. When it is displayed again to the user, it must be completely restarted and restored to its previous state.
There are
three key loops you may be interested in monitoring within your activity:
The entire lifetime of an activity happens between the first call to onCreate(Bundle) through to a single final call to onDestroy().
An activity will do all setup of "global" state in onCreate(), and release all remaining resources in onDestroy(). For example, if it has a thread running in the background to download data from the network, it may create that thread in onCreate() and then stop the thread in onDestroy().
The visible lifetime of an activity happens between a call to
onStart() until a corresponding call to
onStop(). During this time the user can see the activity on-screen, though it may not be in the foreground and interacting with the user. Between these two methods you can maintain resources that are needed to show the activity to the user.
For example, you can register an IntentReceiver in
onStart() to monitor for changes that impact your UI, and unregister it in
onStop() when the user an no longer see what you are displaying. The
onStart() and
onStop() methods can be called multiple times, as the activity becomes visible and hidden to the user.
The foreground lifetime of an activity happens between a call to
onResume() until a corresponding call to
onPause(). During this time the activity is in front of all other activities and interacting with the user.
An activity can frequently go between the resumed and paused states -- for example when the device goes to sleep, when an activity result is delivered, when a new intent is delivered -- so the code in these methods should be fairly lightweight.
The entire lifecycle of an activity is defined by the following Activity methods. All of these are hooks that you can override to do appropriate work when the activity changes state. All activities will implement onCreate(Bundle) to do their initial setup; many will also implement onPause() to commit changes to data and otherwise prepare to stop interacting with the user.
You should always call up to your superclass when implementing these methods.
protected void onCreate(Bundle icicle);
Called when the activity is first created. This is where you should do all of your normal static set up: create views, bind data to lists, etc. This method also provides you with a Bundle containing the activity's previously frozen state, if there was one.
Always followed by ..
protected void onStart();
Called when the activity is becoming visible to the user.Followed by onResume() if the activity comes to the foreground, or onStop() if it becomes hidden.
protected void onRestart();
Called after your activity has been stopped, prior to it being started again. Always followed by onStart()
protected void onResume();
Called when the activity will start interacting with the user. At this point your activity is at the top of the activity stack, with user input going to it.
Always followed by onPause().
protected void onPause();
Called when the system is about to start resuming a previous activity. This is typically used to commit unsaved changes to persistent data, stop animations and other things that may be consuming CPU, etc. Implementations of this method must be very quick because the next activity will not be resumed until this method returns.
Followed by either onResume() if the activity returns back to the front, or onStop() if it becomes invisible to the user.
protected void onStop();
Called when the activity is no longer visible to the user, because another activity has been resumed and is covering this one. This may happen either because a new activity is being started, an existing one is being brought in front of this one, or this one is being destroyed.
Followed by either onRestart() if this activity is coming back to interact with the user, or onDestroy() if this activity is going away.
protected void onDestroy();
The final call you receive before your activity is destroyed. This can happen either because the activity is finishing (someone called finish() on it, or because the system is temporarily destroying this instance of the activity to save space. You can distinguish between these two scenarios with the isFinishing() method.
