1. CS499 – Mobile Application Development
Fall 2013
Threads & Android – Part 2

2. Concurrent Programming
Carnegie Mellon
Concurrent Programming
Multiple processes – each logical control flow is a process. Some form of IPC (InterProcess Communication) needed
Multiple threads – multiple logical control flows within a single process. Shared address space
I/O multiplexing – event driven concurrency. Shared address space

3. Android Threads Activities have a main thread – the UI thread
App components in the same process use the same main thread
User interaction, system callback & lifecycle methods are handled in the UI thread
It is essential that the UI remains responsive
UI toolkit not thread-safe.
A piece of code is thread-safe if it only manipulates shared data structures in a manner that guarantees safe execution by multiple threads at the same time.

4. Implications Two rules:
Do not block the UI thread
Blocking the UI thread hurts responsiveness
Long-running operations should be done in background thread
Don’t access/manipulate the UI toolkit from a non UI thread. Results are unpredictable
Q: How can the UI & background threads communicate in a thread-safe manner?

5. Consider the following Game App:
public class Demo extends Activity { int public void onCreate(Bundle state) { super.onCreate(state); setContentView(R.layout.demo); final View root – findViewById(R.id.root); final Drawable bg = root.getBackground(); final TextView msg = (TextView) findViewById(R.id.msg); final Game game = Game.newGame(); ((Button)findViewById(R.id.start)).setOnClickListener( new View.onClickListener() public void onClick(View v) { // initialize the game! } } });

6. Want to display animated background while waiting to initialize…
// Broken – do not use!!! void initGame(View root, Drawable bg, Game game, TextView resp, String level) { // if animation hasn’t started, make it start if (0 >= mInFlight++) { root.setBackgroundResource(R.anim.dots); ((AnimationDrawable)root.getBackground()).start(); } // initialize the game String msg = game.initialize(level); // if last initialization remove & clean up animation if (0 >= --mInFlight) { ((AnimationDrawable)root.getBackground()).stop(); root.setBackgroundDrawable(bg); resp.setText(msg);

7. Problem: UI thread is executing game
Problem: UI thread is executing game.initialize and hence can’t show the animation
background animation won’t work
if the initialize takes too long, the android framework will suspend it

8. Might be tempted to do the following:
public void onClick(View v) { new Thread(new Runnable() { public void run() { Bitmap b = loadImageFromNetwork(" mImageView.setImageBitmap(b); } }).start(); } Seems to meet Rule 1 but violates Rule 2

9. Solution: AsyncTask
Structured, safe, powerful, and easy-to-use way to manage UI and background threads correctly.
Generic class
class AsyncTask<Params,Progress,Result> { … }
Generic type parameters
Params – Types used in background work
Progress – Types used when indicating progress
Result – Types of result

10. AsyncTask methods Runs before doInBackground() Performs work
void onPreExecute()
Runs before doInBackground()
Result doInBackground(Params… params)
Performs work
can call void publishProgress(Progress…values)
void onProgressUpdate(Progress…values)
Invoked in response to publishProgress()
void onPostExecute(Result result)
Runs after doInBackground()

11. public void onClick(View v) { new DownloadImageTask()
public void onClick(View v) {     new DownloadImageTask().execute(" } private class DownloadImageTask extends AsyncTask<String, Void, Bitmap> {     /** The system calls this to perform work in a worker thread and       * delivers it the parameters given to AsyncTask.execute() */     protected Bitmap doInBackground(String... urls) {         return loadImageFromNetwork(urls[0]);     }         /** The system calls this to perform work in the UI thread and delivers       * the result from doInBackground() */     protected void onPostExecute(Bitmap result) {         mImageView.setImageBitmap(result);     } }

12. AsyncTask Created on UI thread. When ‘execute’d
UI thread invokes onPreExecute() to do any setup
UI thread creates new background thread that runs a doInBackground() method concurrently
Once background thread terminates, onPostExecute() is run in the UI thread.

13. public void onCreate(Bundle state) { super
public void onCreate(Bundle state) { super.onCreate(state); setContentView(R.layout.main); final View dots = findViewById(R.id.dots); final Drawable bg = dots.getBackground(); final TextView msg = ((TextView) findViewById(R.id.msg)); final Game game = Game.newGame(); ((Button) findViewById(R.id.start)).setOnClickListener( new View.OnClickListener() { public void onClick(View v) { new AsyncInitGame(dots, bg, game,msg).execute("basic"); } }); }

14. private final class AsyncInitGame extends AsyncTask<String, Void, String> { private final View root; private final Game game; private final TextView message; private final Drawable bg; public AsyncInitGame(View root, Drawable bg, Game game, TextView msg) { this.root = root; this.bg = bg; this.game = game; this. message = msg; } // runs on the UI protected void onPreExecute() { if (0 >= mInFlight++) { root.setBackgroundResource(R.anim.dots); ((AnimationDrawable)root.getBackground()).start();

15. // runs on the UI protected void onPostExecute(String msg) { if (0 >= --mInFlight) { ((AnimationDrawable)root.getBackground()).stop(); root.setBackgroundDrawable(bg); } message.setText(msg); // runs on a background protected String doInBackground(String … args) { return ((1 != args.length) || (null == args[0])) ? null : game.initialize(args[0]);

16. AsyncTask dataflow Creation Thread (UI) New Thread execute(args…)
doInBackground(args…) : result
doPostExecute(result)

17. private final class LoadIconTask extends Integer,Integer,Bitmap> { protected Bitmap doInBackground(Integer…resid) { Bitmap tmp = BitmapFactory.decodeResource( getResources(),resid[0]); for (int I = 1; i< 11;i++) { // some long running op… publishProgress(i*10); } return tmp; protected void onProgressUpdate(Integer... values) { progress.setProgress(values[0]); protected void onPostExecute(Bitmap result) { iview.setImageBitmap(result); }

18. Be careful!
UI thread should not modify data sent as parameters to AsyncTasks
doInBackground() must only make thread-safe references to variables inherited into its scope.
int mCount;
public void initButton(Button button) {
mCount = 0;
new View.OnClickListener() {
@Override public void onClick(View v) {
new AsyncTask<Void,Void,Void>() {
@Override
protected Void doInBackground(Void…args) {
mCount++; // NOT THREAD SAFE
return null; }
}.execute();
} });

19. However…
Beginning with Honeycomb (3.0), Android changed how it's running the AsyncTasks:
AsyncTask task = new AsyncTask1();
if (currentapiVersion >= android.os.Build.VERSION_CODES.HONEYCOMB) { task.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR); }
else { task.execute(); }

20. Handler
Threads can also communicate by posting Messages & Runnables to a Handler
Message
Can contain a code, data object & args
Recipient (Handler) implements response
Runnable
Contains an instance of the Runnable interface
Sender implements response

21. Handler sendMessage() post() Looper Puts Message on MessageQueue
Puts runnable on Message Queue
Looper
One per Thread
Dispatches MessageQueue entries
Calls handleMessage() for Messages
Calls run() for Runnables

22. Simple use of UI handler
public void onClick(View v) { new Thread(new Runnable() { public void run() { final Bitmap bitmap = loadImageFromNetwork(" mImageView.post(new Runnable() { public void run() { mImageView.setImageBitmap(bitmap); } }); } }).start(); }

23. Handler Two main uses for a Handler
Schedule Message/Runnable for future execution
Enqueue action to be performed on a different thread

24. Messages & Handlers Create Message and set Message content
Handler.obtainMessage()
Message.obtain()
Many variants. See documentation
Message parameters include
int arg1, arg2
int what
Object obj
Bundle data

25. Messages & Handlers sendMessage() sendMessageAtFrontOfQueue()
Queue message immediately
sendMessageAtFrontOfQueue()
Insert message immediately at front ofqueue
SendMessageAtTime()
Queue message at the stated time
sendMessageDelayed()
Queue message after the stated delay

26. public class SimpleThreadingExample extends Activity { private final static int SET_PROGRESS_BAR_VISIBILITY = 0; private final static int PROGRESS_UPDATE = 1; private final static int SET_BITMAP = 2; private ImageView iview; private ProgressBar progress; Handler handler = new Handler() public void handleMessage(Message msg) { switch (msg.what) { case SET_PROGRESS_BAR_VISIBILITY: { progress.setVisibility((Integer) msg.obj); break; } case PROGRESS_UPDATE: { progress.setProgress((Integer) msg.obj); case SET_BITMAP: { iview.setImageBitmap((Bitmap) msg.obj); }; …

27. @Override public void onCreate(Bundle savedInstanceState) { super
@Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); iview = (ImageView) findViewById(R.id.imageView1); progress = (ProgressBar) findViewById(R.id.progressBar1); final Button button = (Button) findViewById(R.id.loadButton); button.setOnClickListener(new OnClickListener() { public void onClick(View v) { new Thread(new LoadIconTask(R.drawable.icon, handler)).start(); } });

28. private class LoadIconTask implements Runnable { private final int resId; private final Handler handler; LoadIconTask(int resId, Handler handler) { this.resId = resId; this.handler = handler; } public void run() { final Bitmap tmp = BitmapFactory.decodeResource(getResources(), resId); Message msg = null; msg = handler.obtainMessage(SET_PROGRESS_BAR_VISIBILITY,ProgressBar.VISIBLE); handler.sendMessage(msg); for (int i = 1; i < 11; i++) { sleep(); msg = handler.obtainMessage(PROGRESS_UPDATE, i * 10); } msg = handler.obtainMessage(SET_BITMAP, tmp); msg = handler.obtainMessage(SET_PROGRESS_BAR_VISIBILITY, ProgressBar.INVISIBLE); private void sleep() { try { Thread.sleep(200); } catch (InterruptedException e) { e.printStackTrace();}

29. Runnables & Handlers boolean post(Runnable r)
Add Runnable to the MessageQueue
boolean postAtTime(Runnable r, long uptimeMillis)
Add Runnable to the MessageQueue. Run at a specific time (based on SystemClock.upTimeMillis())
boolean postDelayed(Runnable, long delayMillis)
Add Runnable to the message queue. Run after the specified amount of time elapses.

30. public class SimpleThreadingExample extends Activity { private ImageView iview; private final Handler handler = new public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); iview = (ImageView) findViewById(R.id.imageView1); final Button button = (Button) findViewById(R.id.loadButton); button.setOnClickListener(new OnClickListener() { public void onClick(View v) { new Thread(new LoadIconTask(R.drawable.icon)).start();} }); } private class LoadIconTask implements Runnable { int resId; LoadIconTask(int resId) { this.resId = resId; } public void run() { final Bitmap tmp = BitmapFactory.decodeResource(getResources(),resId); handler.post(new Runnable() { public void run() { iview.setImageBitmap(tmp);} }}

31. public class SimpleThreadingExample extends Activity { private Bitmap public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); final ImageView iview = (ImageView) findViewById(R.id.imageView1); final Button button = (Button) findViewById(R.id.loadButton); button.setOnClickListener(new OnClickListener() { public void onClick(View v) { new Thread(new Runnable() { public void run() { bitmap = BitmapFactory.decodeResource(getResources(),R.drawable.icon); iview.post(new Runnable() { public void run() { iview.setImageBitmap(bitmap); } });} }).start(); });

32. public class SimpleThreadingExample extends Activity { private Bitmap public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); final ImageView iview = (ImageView) findViewById(R.id.imageView1); final Button button = (Button) findViewById(R.id.loadButton); button.setOnClickListener(new OnClickListener() { public void onClick(View v) { new Thread(new Runnable() { public void run() { bitmap = BitmapFactory.decodeResource(getResources(), R.drawable.icon); SimpleThreadingExample.this.runOnUiThread(new Runnable() { iview.setImageBitmap(bitmap); } }); }).start();