This document discusses messaging in Android applications. It covers Android application lifecycles and activities, message handlers, and multithreading concepts. The key points are: 1) Android uses message handlers to allow asynchronous messaging between application components. A message contains a description and data, while a handler processes messages and runnables on a thread. 2) The Android UI toolkit and activities must be accessed from the main thread to avoid errors. Additional threads should be used to avoid blocking the main thread. 3) Listeners provide interfaces for processing specific events in a multithreaded manner according to Android's rules.

1. Android –
Message
Yong Heui Cho @ Mokwon University
Some of slides are referred to:
[1] Nitin Ramchandani, Android OS, slideshare.

2. 2
Smart Device Structure
3. Basic Android OS
4. Android – Application
Framework
5. Android – Message
6. Android – BroadcastReceiver

3. 3
Application
Application Structure
Activity
Context
OS
Resources
Service
lifecycle

4. 4
Application Lifecycle
I. In Android, every application runs in their
own process.
II. Processes are started or stopped as needed
to run application components.
III. A process may be killed to reclaim resources.
□ Courtesy to Nitin Ramchandani, Android OS, slideshare.

5. 5
Activity Lifecycle (I)

6. 6
• onCreate(Bundle): This is called when the activity first
starts up.
•onStart( ): This indicates the activity is about to be
displayed to the user.
• onResume( ): This is called when your activity can start
interacting with the user. This is a good place to start
animations and music.
• onPause( ): This runs when the activity is about to go
into the background, usually because another activity has
been launched in front of it. This is where you should save
your program’s persistent state, such as a database record
being edited.
Activity Lifecycle (II)
□ Courtesy to Nitin Ramchandani, Android OS, slideshare.

7. 7
• onStop( ): This is called when your activity is no longer
visible to the user and it won’t be needed for a while. If
memory is tight, onStop( ) may never be called (the system
may simply terminate your process).
• onRestart( ): If this method is called, it indicates your
activity is being redisplayed to the user from a stopped state.
• onDestroy( ): This is called right before your activity is
destroyed. If memory is tight, onDestroy( ) may never be
called (the system may simply terminate your process).
Activity Lifecycle (III)
□ Courtesy to Nitin Ramchandani, Android OS, slideshare.

8. 8
•onSaveInstanceState(Bundle): Android will call this
method to allow the activity to save per-instance state,
such as a cursor position within a text field. Usually you
won’t need to override it because the default
implementation saves the state for all your user interface
controls automatically.
• onRestoreInstanceState(Bundle): This is called when
the activity is being reinitialized from a state previously
saved by the onSaveInstanceState( ) method. The default
implementation restores the state of your user interface.
Save & Restore
□ Courtesy to Nitin Ramchandani, Android OS, slideshare.

9. 9
Lifecycle Comparison
Run Terminate
onCreate() onDestroy()
onStart() onStop()
onResume() onPause()
onRestart() -
onRestoreInstanceState() onSaveInstanceState()

10. 10
Summary of Lifecycle

11. 11
Android vs. Windows
type Android Windows
CPU optimal fast
battery small power supply
memory
mobile DRAM
small
DRAM
large
message handler
message (Message)
or action (String)
message (int)
termination app lifecycle permanent
function call call & proceed
call & wait
or call & proceed

12. 12
MVC Components

13. 13
Android Msg Handler
• Message: containing a description and arbitrary data
object
• Runnable: a command that can be executed
• Handler: allows you to send and process Message and
Runnable
• Looper: used to run a message loop for a thread

14. 14
Concept of Multithread

15. 15
Android Rules
• Do not block the UI thread (or main
thread).
• Do not access the Android UI toolkit
from outside the UI thread.

16. 16
Event Listener
• Interface to process specific events