Android Threading

1. Android'Threading'
Jussi'Pohjolainen'
Tampere'University'of'Applied'Sciencs'

2. UI'Thread'
• When'Android'app'is'launched'one'thread'is'
created.'This'thread'is'called'Main'Thread'or'
UI#Thread#
• UI'Thread'is'responsible'for'dispatching+events+
to+widgets+
• Avoid#doing#0me#consuming#tasks#in#UI#
Thread#since#it#leads#to#app#that#does#not#
respond#quickly#

3. TesAng'UI'Responsivess'
public class ThreadExample extends Activity implements OnClickListener {
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Button button = new Button(this);
button.setText("Do Time Consuming task!");
setContentView(button);
button.setOnClickListener(this);
}

@Override
public void onClick(View v) {
try {
for(int i=0; i<10; i++) {
System.out.println(i);
Thread.sleep(10000);
}

} catch (InterruptedException e) {
e.printStackTrace();
}
}
}

4. Result'

5. TesAng'Separate'Thread'
public class ThreadExample extends Activity implements OnClickListener, Runnable {

...
@Override
public void onClick(View v) {
Thread thread = new Thread(this);
thread.start();
}

@Override
public void run() {
try {
for(int i=0; i<10; i++) {
System.out.println(i);
Thread.sleep(10000);
}

} catch (InterruptedException e) {
e.printStackTrace();
}
}
}

7. How'about'influencing'the'UI?'
...

@Override
public void run() {
try {
for(int i=0; i<10; i++) {
button.setText("Iteration: " + i);
Thread.sleep(1000);
}

} catch (InterruptedException e) {
e.printStackTrace();
}
}

...

8. Problem'

9. Why?'
• Android'UI'Toolkit'is#not#thread#safe#
• If'you'want'to'manipulate'UI,'you#must#do#it#
inside#the#UI#thread#
• How'do'you'do'it'then?'You'can'use'
– Activity.runOnUiThread(Runnable)
– View.post(Runnable)
– View.postDelayed(Runnable, long)
– …

10. Activity.runOnUiThread(Runnable)
• The'given'acAon'(Runnable)'is'executed'
immediately'if'current'thread'is'UI'thread'
• If'current'thread'is'NOT'UI'thread,'the'acAon'
(Runnable)'is'posted'to'event'queue'of'the'UI'
Thread'

11. Example'of'RunOnUiThread'
public class ThreadExample extends Activity implements OnClickListener, Runnable {
...

@Override
public void onClick(View v) {
Thread t = new Thread(this);
t.start();
}

public void run() {
// lengthy operation
try {
Thread.sleep(2000);
} catch (InterruptedException e) { }

runOnUiThread(new Update());
}

class Update implements Runnable {
// This action is posted to event queue
public void run() {
button.setText("Finished!");
}
}
}

12. View.post(Runnable)
View.postDelayed(Runnable, Long)
• These'methods'are'of'view'and'are'use'for'
updaAng'the'view'
• AcAon'(Runnable)'is'placed'on'Message'
Queue'
• Runnable'acAon'runs'on'UI'Thread'
• postDelayed'method'for'delayed'acAon'

13. Using'post'
private int iteration;
...
@Override
public void run() {
try {
for(int i=0; i<10; i++) {
iteration = i;
button.post(new InfluenceUIThread());

Thread.sleep(1000);
}

} catch (InterruptedException e) {
e.printStackTrace();
}
}

class InfluenceUIThread implements Runnable {
public void run() {
button.setText("Iteration = " + iteration);
}
}

14. Using'Anonymous'Inner'Classes'
@Override
public void onClick(View v) {
new Thread(new Runnable() {
public void run() {
try {
for(int i=0; i<10; i++) {
iteration = i;
button.post(new Runnable() {
public void run() {
button.setText("Iteration = " + iteration);
}
});
Thread.sleep(1000);
}

} catch (InterruptedException e) {
e.printStackTrace();
}
}
}).start();
}
This'can'be'really'
confusing...'

15. AsyncTask
• Goal:'take'care'thread'management'for'you'
• Use'it'by'subclassing'it:'class'MyTask'extends'
AsyncTask'
• Override'onPreExecute(),'onPostExecute()'
and'onProgressUpdate()
– Invokes'in'UI'Thread'
• Override'doInBackground()
– Invokes'in'worker'thread'

16. Example'(Google'SDK)'
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));
         }
         return totalSize;
     }

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

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

 new DownloadFilesTask().execute(url1, url2, url3);

Params,'Progress,'Result'

17. public class ThreadExample extends Activity implements OnClickListener {
private Button button;

...

class MyBackgroundTask extends AsyncTask<Integer, Integer, Integer> {

protected Integer doInBackground(Integer... ints) {
int i = ints[0];
try {
for(i=0; i<10; i++) {
System.out.println("doInBackground!");
publishProgress(new Integer(i));
Thread.sleep(1000);
}
} catch(Exception e) {
e.printStackTrace();
}

return i;
}

protected void onProgressUpdate(Integer iteration) {
button.setText("Iteration = " + iteration);
}

protected void onPostExecute(Integer result) {
button.setText("Finished with result of: " + result);
}
}
}