Android life cycle

1. Android Life Cycle
Wei-Tsung Lin

2. Introduction
●Android is a mobile operating system.
●There are many interrupts during normal
using, such as a call.
●Also, most desktop users have been familiar
with multi-task operating system, expecting
that they can browsing the web while
listening to the musics.
●However, the memory is not unlimited in
mobile devices.
●So, Android introduced "Life Cycle".

3. Activity
●In Android, each application is a process,
called activity.
●Android (aka Dalvik VM) maintains an
unique activity stack to observer the status
of process, and manager the memory.
●Life cycle of an activity is managed by
Android OS, not by application itself.

4. Status of an activity
●Active
●Paused
●Stopped
●Dead

5. Active
●"Active" is the status that an activity is
running.
●There will be only one "Active" activity. The
others are int the status of "Pause",
"Stopped", and "Dead".

6. Paused
●"Paused" means an activity is in the
background.
●Creating toast, dialog, or receiving a phone
call will make an running activity paused.
●User cannot interact with an activity which is
paused.

7. Stopped
●The activity has already exit the screen, no
other action is running.
●We can wake an stopped activity up by
using "Notification" or multi-task button.

8. Dead
●The activity has been finished manually, or
garbage collected by system.

9. Low memory
●When memory is running out, Dalvik will
follow the recycle rule to release memory.

1.Independent Activity/Service/Intent Receiver
2.Stopped Activity (by LRU)
3.Service
4.Visiable Activity
5.Running Activity

10. Activity API
●OnCreate()
●OnStart()
●OnResume()
●OnPause()
●OnStop()
●OnRestart()
●OnDestroy()

11. ANR
●Android will prompt you a notification that
"Application Not Responding" when an
activity executing without response for a long
time.
●Default timeout is 5 seconds.
●To avoid that, developers shouldn't execute
a long time task in the main thread, such as
accessing Internet, database, or playing
music.

12. AsyncTask
private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> {
protected Long doInBackground(URL... urls) {
int count = urls.length;
long totalSize = 0;
for (int i = 0; i < count; i++) {
totalSize += Downloader.downloadFile(urls[i]);
publishProgress ((int) ((i / (float) count) * 100));
// Escape early if cancel() is called
if (isCancelled()) break;
}
return totalSize;
}

protected void onProgressUpdate(Integer... progress) {
setProgressPercent (progress[0]);
}

protected void onPostExecute(Long result) {
showDialog ("Downloaded " + result + " bytes");
}
}

13. Service
●Four components in Android: Activity,
Service, BroadcastReceiver,
ContentProvider
●Except ContentProvider, the others main
objects are running in the main thread.
●The life cycle of Service is most persistent
(longest).
●System will kill a paused activity in very high
possibility.

14. Service
●Putting a long time task into a thread is not
enough.
●Consider rotating your device. Actually the
activity is killed, and then system recreate a
new activity rotated. The thread will be killed
too!
●You should not only put the long time task
into a new thread, but also a Service.
●Although the original activity is killed, the
service will execute as usual. You can make
inter-process communication later.

15. MessageQueue
●Android implement a message queue to
handle messages, which called Looper.
●It's very helpful when you want to update UI
outside the main thread(UI thread).
●A handler can push a message into a looper,
dispatch, and handler the message in the
queue.
●When creating a handler, it will bind to the
looper of current activity automatically. Also,
you can assign a specific looper to bind.

16. IntentService
●IntentService is actually a Service.
●Using IntentService, you don't need to
implement Looper, Message, and Handler
yourself.

Intent intent = new Intent(MainActivity.this, MyIntentService.class);
intent.putExtra("tag", "message");
startService(intent);

in MyIntentService:

@Override
protected void onHandleIntent(Intent intent) {
String message = intent.getStringExtra("tag");
}

17. Reference
http://developer.android.
com/training/basics/activity-lifecycle/index.html
http://developer.android.
com/guide/components/services.html
http://developer.android.
com/reference/android/os/Handler.html
http://developer.android.
com/reference/android/os/Looper.html
http://developer.android.
com/reference/android/app/IntentService.html