MobAppDev

Shared Agendas, Loopers, Handlers, & Threads

Vladimir Kulyukin

Outline● Shared Agenda Thread Pattern● Loopers, Handlers, & Threads● Using Loopers & Handlers to Update GUIs on the Main

Thread ProgressBarSimulatedFileDownload ProgressBarNetworkFileDownload

Shared Agenda Thread Pattern

Shared Agenda● Sequential processing of messages and tasks is

accomplished on Android via the Shared Agenda Thread pattern

● Agenda is an AI term frequently used in AI robotics

● Pipeline is an OS term commonly used in the concurrency literature

Shared Agenda● Shared Agenda Thread pattern is used on many robotic

platforms (ActivMedia robots is one example)● Shared Agenda pattern or something very similar to it

are used in many UI frameworks such as Swing or Adobe Flex

● On Android, the general idea is to use the Shared Agenda thread to push sequentially executable tasks off the main UI thread

Shared Agenda● Shared Agenda is a thread that holds a queue of

messages and tasks● A message (android.os.Message) is a data

structure for someone else to process● A task (java.lang.Runnable) is a exectable unit

of CPU work● In Java, Runnables are used to run some code on

different Threads

Shared Agenda

● Threads can push new messages or tasks onto the queue at any time

● When there are messages on the queue, the Shared Agenda processes them one after another

● When there are no tasks on the Shared Agenda, it blocks

Loopers, Handlers, & Threads

Definitions● Looper (android.os.Looper) is a class that turns a thread

into a shared agenda thread with a message/task queue● Handler (android.os.Handler) is a class whose objects

other threads can use to push messages/tasks to the shared agenda

● The term Looper emphasizes the fact that a Looper object loops over the message queue executing messages/tasks one after another

Threads & android.os.Handler

● Thread in Android has a MessageQueue● MessageQueue is a queue of Message objects● Handler is associated with a specific Thread and its

MessageQueue● Handler, after it is associated with a Thread, can send

Messages and Runnables to the Thread's MessageQueue and for the Looper to execute them as they come off the MessageQueue

android.os.Handler● Handlers are used to post Messages and Runnables on a specific Shared

Agenda Thread● You can enqueue Runnables with

post(Runnable) postAtTime(Runnable, long) postDelayed(Runnable, long)

● The enqueued Runnables are called as they are received● You can send messages with

sendEmptyMessage(int)

sendMessage(Message)

sendMessageAtTime(Messasge, int)

sendDelayedMessage(Message, long)

android.os.Looper

● Looper runs a message loop for a Thread● Handler is used in conjunction with a Looper● Looper.prepare() creates a message queue that

a Handler can use to post messages● Looper.loop() processes messages as long as

the Thread runs

Using Loopers & Handlers to Simulate File Download

source code is here

App Specs

● Develop an application that consists of one activity that displays one button

● A click on a button displays in a progress bar that works on the scale from 0 to 100

● The application simulates a file download and posts update messages to the main UI thread from a separate thread via a Handler

App Screenshots

UI XML<LinearLayout android:layout_width="fill_parent" android:layout_height="fill_parent" android:orientation="vertical" > <Button android:id="@+id/btnStartProgress" android:layout_width="match_parent" android:layout_height="200px" android:text="Download File" /></LinearLayout>

Declaration of UI Componentspublic class ProgressBarSimulatedFileDownloadActivity extends Activity

implements OnClickListener {

Button mBtnStartProgress; ProgressDialog mProgressBar; private int mProgressBarStatus=0; // Handler to post messages on the main thread from a non-main threads. private Handler mProgressBarHandler=new Handler(); private long mFileSize=0; static final String PROGRESS_MESSAGE="File Downloading . . ."; static final String COMPLETION_MESSAGE="Download complete."; …}

UI XML Inflation in onCreate()

protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.progressbar_demo_layout); mBtnStartProgress =(Button) findViewById(R.id.btnStartProgress); mBtnStartProgress.setOnClickListener(this);}

Simulating File Download & Posting Messagespublic void simulateFileDownload() {

while( mProgressBarStatus < 100 ) { // do some task; downLoadFile method simulates a file download. mProgressBarStatus = downLoadFile(); // sleep for 500ms to simulate network traffic try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } // Use the post method of the handler object // to send messages to the progress bar object on the main UI thread. mProgressBarHandler.post(new Runnable() { @Override public void run() { // show the status mProgressBar.setProgress(mProgressBarStatus); If( mProgressBarStatus>=100 ) mProgressBar.setMessage(COMPLETION_MESSAGE); }}); }

//after Download is complete If( mProgressBarStatus>=100 ) { // sleep 2 seconds, so that you can see the 100% try { Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } // get rid of the progress bar object. mProgressBar.dismiss();

}

Computing Status of Progress Barprivate int downLoadFile() {

// pretend that the file size is 1,000,000 bytes = about 1MG

while (mFileSize <= 1000000) {

mFileSize++;

// scale the progress bar status, depending on the current size of the file.

if (mFileSize == 100000) { return 10; }

else if (mFileSize == 200000) { return 20; }

else if (mFileSize == 300000) { return 30; }

else if (mFileSize == 400000) { return 40; }

else if (mFileSize == 500000) { return 50; }

else if (mFileSize == 600000) { return 60; }

else if (mFileSize == 700000) { return 70; }

else if (mFileSize == 800000) { return 80; }

else if (mFileSize == 900000) { return 90; }

}

return 100; // the file has been downloaded

}

Mounting File Download on a Thread in onCreate()

// 1. Create a thread Thread thread = new Thread(){

public void run(){

simulateFileDownload();

}

};

// 2. Start the thread

thread.start();

Using Loopers & Handlers to Do Real File Download

source code is here

App Specs

● Develop an application that consists of one activity that displays one button

● A click on a button displays in a progress bar that works on the scale from 0 to 100

● The application downloads a real image file and posts update messages to the main UI thread from a separate thread via a Handler

App Screenshots

UI XML

<LinearLayout android:layout_width="fill_parent"

android:layout_height="fill_parent"

android:orientation="vertical" > <Button

android:id="@+id/btnStartProgress"

android:layout_width="match_parent"

android:layout_height="200px"

android:text="Download File" /> <ImageView

android:id="@+id/FileDownloadimageView"

android:layout_width="match_parent"

android:layout_height="wrap_content" /></LinearLayout>