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Android development Training programme
Day 3
17/11/2012
Dhiraj Karalkar
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@COPYRIGHTS BY Dhiraj P. karalkar [email protected]
Day 3 Session 1
Background Processing in an Android
Threads
Android supports the usage of the Threads class to perform asynchronous processing. Android also supplies the java.util.concurrent package to perform something in the background, e.g.
using the ThreadPools and Executor classes.
If you need to update the user interface from a new Thread, you need to synchronize with the user interface thread.
You can use the android.os.Handler class or the AsyncTasks class for this.
Handler
The Handler class can update the user interface. A Handler provides methods for receiving instances of the Message or Runnable class.
To use a handler you have to subclass it and override the handleMessage() to process messages. To
process a Runnable you can use the post() method. You only need one instance of a Handler in your Activity.
You thread can post messages via the sendMessage(Message msg) method or via the
sendEmptyMessage() method.
AsyncTask
The AsyncTask class encapsulates the creation of Threads and Handlers. An AsyncTask is started via the execute() method.
The execute() method calls the doInBackground() and the onPostExecute() method.
The doInBackground() method contains the coding instruction which should be performed in a background thread.
This method runs automatically in a separate Thread.
The onPostExecute() method synchronize itself again with the user interface thread and allows to update it.
This method is called by the framework once the doInBackground() method finishes.
To use AsyncTask you must subclass it. AsyncTask uses generics and varargs. The parameters are the
following AsyncTask <TypeOfVarArgParams , ProgressValue , ResultValue> .
TypeOfVarArgParams is passed into the doInBackground() method as input, ProgressValue is used for
progress information and ResultValue must be returned from doInBackground() method and is passed to
onPostExecute() as parameter.
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@COPYRIGHTS BY Dhiraj P. karalkar [email protected]
Handler with Example
A Handler allows you to send and process Message and Runnable objects associated with a thread's
MessageQueue. Each Handler instance is associated with a single thread and that thread's message queue. When you create a new Handler, it is bound to the thread / message queue of the thread that is creating it -
- from that point on, it will deliver messages and runnables to that message queue and execute them as
they come out of the message queue.
How do the 2 threads (parent/UI and the worker threads) communicate? Here comes the Handler. By definition – “A Handler allows you to send and process Message and Runnable objects associated with a thread's MessageQueue. Each Handler instance is associated with a single thread and that thread's message queue.” So, let us take the handler from the main thread and see how we can use it to communicate with a child thread. When a handler is created, it is associated by default with the current thread. So, we have this piece of code in the main activity class: Now, I spawn a new thread through a method that is called when I click a button. So, let us see the button piece of code first: Here I am providing the xml layout and the HandlerActivity source code for the button click event
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@COPYRIGHTS BY Dhiraj P. karalkar [email protected]
Here is the piece of code where the Handler is initialized
Now, on click of the button, the fetchData() method is invoked.
Since it is time consuming, I am starting a ProgressDialog just to inform the end user that some activity is
happening. Then, I start the thread, make it sleep for 800 milliseconds and send back an empty message through the message handler. The messageHandler.sendEmptyMessage(0) is the callback on the parent
thread’s messageHandler to inform that the child thread has finished its work. In this example I am
sending an empty message. But this can be the means of communication and exchange of data from the child thread to the parent Thread.
Now the control returns to handleMessage() call back method. That method shown in the beginning just
dismisses the progressDialog.
In real life cases, within this thread we can do things like calling web-services, calling web-sites to fetch
specific data or doing network IO operations and returning actual data that needs to be displayed on the front-end.0
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@COPYRIGHTS BY Dhiraj P. karalkar [email protected]
AsyncTask with Example
AsyncTask enables proper and easy use of the UI thread. This class allows to perform background
operations and publish results on the UI thread without having to manipulate threads and/or handlers.
The AsyncTask class encapsulates the creation of Threads and Handlers. An AsyncTask is started via the
execute() method.
The execute() method calls the doInBackground() and the onPostExecute() method. The doInBackground() method contains the coding instruction which should be performed in a
background thread. This method runs automatically in a separate Thread.
The onPostExecute() method synchronize itself again with the user interface thread and allows to update it. This method is called by the framework once the doInBackground() method finishes.
To use AsyncTask you must subclass it. AsyncTask uses generics and varargs. The parameters are the
following AsyncTask <TypeOfVarArgParams , ProgressValue , ResultValue> .
TypeOfVarArgParams is passed into the doInBackground() method as input, ProgressValue is used for progress information and ResultValue must be returned from doInBackground() method and is passed to
onPostExecute() as parameter.
In this example we will use an instance of the AsyncTask class to download the content of a webpage. We use Android HttpClient for this. Create a new Android project called ThreadAsynkTaskExample
with an Activity called ThreadAsynkTaskActivity. Add the android.permission.INTERNET permission to your <filenname>AndroidManifest.xml</filenname> file. . Create the following layout.
On the buttonClick we are executing the AsynkTask WebPageDownloadTask as task.execute()
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Following is the AsynkTask in our scenario the WebPageDownLoadTask is extending AsyncTask<String, Void, String> of android.os.AsyncTask package
This WebPageDownLoadTask contains the overridden method of the AsynksTask
i) onPreExecute() : in this method we are started the the Progress bar
ii) doInBackground(String... urls): in the doInBackground we have a logic of downloading the text from the specific url
iii) onPostExecute(String result): In this method we are dismissing the ProgressBar and adding
text to the TextView
Here is the source code for the Same
If you run your application and press your button then the content of the defined webpage should be read in the background. Once this process is done your TextView will be updated.
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@COPYRIGHTS BY Dhiraj P. karalkar [email protected]
Session2
Broadcast Receivers
The concept of Broadcast Receivers, one of the fundamental blocks of Android, is very simple. These are applications that will respond to any intent that is broadcast by other applications. Provided the broadcast intent matches the intent specified against the receiver in the AndroidManifest.xml This goes to automatically imply that many activities, events, services or the like can broadcast intents expecting the appropriate action to be automatically taken. So, to begin with, let us see the various Broadcast events that are given by the platform itself. Here is a standard list obtained from the android documentation:
ACTION_TIME_TICK
ACTION_TIME_CHANGED
ACTION_TIMEZONE_CHANGED
ACTION_BOOT_COMPLETED
ACTION_PACKAGE_ADDED
ACTION_PACKAGE_CHANGED
ACTION_PACKAGE_REMOVED
ACTION_PACKAGE_RESTARTED
ACTION_PACKAGE_DATA_CLEARED
ACTION_UID_REMOVED
ACTION_BATTERY_CHANGED
ACTION_POWER_CONNECTED
ACTION_POWER_DISCONNECTED
ACTION_SHUTDOWN We will define a broadcast receiver which listens to telephone state changes. If the phone receives a phone call then our receiver will be notified and log a message. Here is the manifest file for the BroadCastReceiver Example , You must register the receiver as shown in the following code
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@COPYRIGHTS BY Dhiraj P. karalkar [email protected]
Here is the MyBroadcastReceiver extending BroadcastReceiver class , when any Telephony call arrived and disconnected then the onReceive(Context context, Intent intent) get called .
You can even register BroadcastReceiver progrmatically by using
Context.registerReceiver(BroadCastReceiver bcr, IntentFilter ifilter );
To view the output of this application , install this application in your mobile Phone and make a call the
BroadcastReceiver gets called
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@COPYRIGHTS BY Dhiraj P. karalkar [email protected]
Session 3
Understanding Services in android
Role of services
Service life cycle methods
What is service ?
A Service is an application component representing either an application's desire to perform a longer-running operation while not interacting with the user or to supply functionality for other applications to
use. Each service class must have a corresponding <service> declaration in its package's
AndroidManifest.xml. Services can be started with Context.startService() and Context.bindService().
If user wants to create a longer background action like Playing the songs even if your activity is closed ,
or to download some files from the server in the background then Service used to do the Background processes without having UI ..
To create an application to run in the background of other current activities, one needs to create a Service.
The Service can run indefinitely (unbounded) or can run at the lifespan of the calling activity(bounded).
Please note that a Service has a different lifecycle than activities therefore have different methods. But to
begin a service in the application a call to startService() which envokes the service onCreate() method and
onStart() beginning running the service.
context.startService() | ->onCreate() - >onStartCommand() [service running]
Calling the applications stopService() method to stop the service.
context.stopService() | ->onDestroy() [service stops]
Something that we didn't use in this example is bindService() which just calls the services onCreate()
method but does not call the onStartCommand(). onBindService() is used to create persistance
connection to the service.
context.onBindService() | ->onCreate() [service created]
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@COPYRIGHTS BY Dhiraj P. karalkar [email protected]
Service Example
This Services Demo is simple as it plays an audio file and by listening to click events of the buttons invokes the MyService service. Add any of the audio file to your res >> raw folder of an application Note : you must add the following tag in the manifest file to register your service <service android:enabled="true" android:name="MyService" /> Layout file : activity_service_demo.xml
Here is your ServicesDemo which extends Activity and implements OnClickListener
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@COPYRIGHTS BY Dhiraj P. karalkar [email protected]
When the user clicks on the start Button then startService() method gets called which invokes your MyService’s onCreate() method , in this method we have initialize the MediaPlayer right now for this
ignore the Functionality of the Media player just concentrate on the Service .
After that the OnStart() method get called of the MyService where we have started the Media Player
Your service is started now even if you close your activity the service will run in the Background , that is the basic use of using the Service in Android .
Now to stop the media file you need to stop the service , refer the ServicesDemo above ,
Onclicking the stopbutton the stopService() method gets called which stops your service running in the background , this stopService() method calls the onDestroy() method of the MyService
Which stops the service running in the Background
Here is the code to start your service (here MyService)which plays an audio file in a background
