Programming Sideways: Asynchronous Techniques for Android
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The document discusses asynchronous programming techniques for Android, emphasizing the challenges of multithreading and the importance of non-blocking UI operations. It explores various frameworks like Reactive Extensions (Rx) and event buses to simplify asynchronous operations and improve code readability. Additionally, it addresses the use of fragments in Android and the potential for creating reactive applications by connecting UI events to data streams.
![Ok, when it is my turn?
HOLLYWOOD
PRINCIPLE
"don't
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call
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managed
execuKon
LISTENERS
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Programming Sideways: Asynchronous Techniques for Android
- 1. ASYNCHRONOUS TECHNIQUES FOR ANDROID: PROGRAMMING SIDEWAYS Emanuele Di Saverio AND02 Senior Design Technologist emanuele.disaverio@gmail.com hazam emanueledisaverio
- 2. Who? "Agilist in a Mobile world" • Mobile Java Developer (2007) • Android (2009) • h@p://www.androidavanzato.it (2012) Emanuele Di Saverio Senior Design Technologist @
- 3. TWO RECURRING PROBLEMS, WITH SIMILAR SYMPTOMS
- 4. 1ST PROBLEM
- 5. A (biased) TALE ABOUT SERVERS
- 6. The S in C/S • a server program in (connected) network compuKng while (true) { connect(); read(); parseRequest(); process(); sendResponse(); disconnect(); }
- 7. MPM Prefork (Apache) • let's fork() and spawn more processes! full address space copied around
- 8. MPM Worker (Apache) • handful processes spawn a set of threads context switch + shared memory
- 9. Multi Threaded Programming • Memory is shared, threads spawn faster • Problem of synchronizaKon – solvable only with low-‐level OS primiKves
- 10. Multi Threaded Programming • Memory is shared, threads spawn faster • Problem of synchronizaKon – solvable only with low-‐level OS primiKves "… non-‐trivial mul/-‐threaded programs are incomprehensible to humans. It is true that the programming model can be improved: design pa;erns, atomicity, improved languages, and formal methods. However, these techniques merely chip away at the unnecessarily enormous non-‐determinism of the threading model. The model remains intrinsically intractable." Edward A. Lee – The Problem with Threads
- 11. CONTEXT SWITCH has fixed cost.
- 12. Context Switch
- 13. Context Switch hint: human mind work same way
- 14. Single Thread Event Old technique – New Trend Nginx / NodeJS / Jetty • User Code is executed by single thread • I/O is on separate thread
- 15. MOBILE UI ARCHITECTURE a.k.a. looked from a high enough level, they're all the same.
- 16. The Loop while (true) { processInputEvents(); processSysEvents(); measure(); layout(); draw(); } input sys measure layout draw thread
- 18. Ok, when it is my turn? HOLLYWOOD PRINCIPLE "don't call me, I'll call you" managed execuKon LISTENERS Button a; [...] a.setOnClickListener( new View.OnClickListener() { public void onClick(View v) { //do stuff } }); object that receives callback through define interface class MyActivity extends Activity { protected void onCreate(Bundle b) { //create my UI } onStop() {...} onStart(){...} }
- 19. The Android Loop All callbacks are executed in Main Thread! • AcKvity • Service • Fragment • animaKons • OnClicks • OnTouch • OnWhatever...
- 20. android.os.Looper Make every thread an event-‐based message queue public void run() { Looper.prepare(); this.handler = new Handler() handleMessage(int what) if (what == 0x666) //do stuff }; Looper.loop();} handler.sendMessage(Message.obtain(0x666));
- 21. 60 FPS -> 16 ms per frame ALL YOUR CODE NEEDS TO FIT IN 16ms
- 22. The Loop while (true) { processInputEvents(); processSysEvents(); measure(); layout(); draw(); } input sys measure layout draw thread 16ms
- 23. DON'T BLOCK UI THREAD no network calls -‐ no file I/O -‐ no intensive computaKon
- 24. DON'T BLOCK UI THREAD no network calls -‐ no file I/O -‐ no intensive computaKon USE OTHER THREADS!
- 25. Asynchronous Java Java was designed with multithreaded programming in mind (thread ,synchronized, volatile, memory model). Don't rely on that – probabilities you really understand what they do are small. java.util.concurrent.* full of goodies to handle concurrency
- 26. Asynchronous Java You don't really create Threads that much Executors – Thread pool implemented for you ExecutorService executor = Executors.newFixedThreadPool(10); executor.execute(new Runnable() { run() { //do stuff } });
- 27. Future – a computation Stock Option Want to have result of an async execuKon? Future<String> f1 = executor.submit( new Callable<String> { String call() { return fetchNetworkContent(); } }); f1.get(); //locks current thread
- 28. Future – combining? Future<String> f1 = executor.submit(...); Future<Integer> f2 = executor.submit(...); Future<Integer> f3 = executor.submit( new [CALLABLE](f1.get())); Future<Integer> f4 = executor.submit(new [CALLABLE]f2.get()));
- 29. Future – combining? Future<String> f1 = executor.submit(...); Future<Integer> f2 = executor.submit(...); Future<Integer> f3 = executor.submit( new [CALLABLE](f1.get())); Future<Integer> f4 = executor.submit(new [CALLABLE]f2.get())); Future<String> f3 = executor.submit(new Callable<String>() { call() { return new CallToRemoteServiceC(f1.get()).call(); }});
- 30. Future – Other Approaches List<Future> futures = new ArrayList<Future>(); Iterator<Future<?>> i = futures.iterator(); while (i.hasNext()) { Future<?> f = i.next(); if (f.isDone()) { doMoreWork(f.get()); i.remove(); } }
- 31. GET() BLOCKS COMBINATION IS A PAIN
- 32. Callback Gets around the early blocking problem by pre-‐ declaring the funcKon that will be executed public interface Callback<T> { public void done(T result, Exception error); }
- 33. Callback fetchNetworkContentAsync(Callback<Response> cb) { mExecutor.execute(new Runnable(){ ... try { Response r = fetchNetworkContent(); cb.done(r, null); } catch(Exception exc) { cb.done(null, exc); }); }
- 34. WAIT, WE NEED TO BE IN MAIN THREAD
- 35. Callback fetchNetworkContentAsync(Callback<Response> cb) { mExecutor.execute(new Runnable(){ ... try { Response r = fetchNetworkContent(); mHandler.post(new Runnable() { cb.done(r, null); }); } catch(Exception exc) { cb.done(null, exc); }); }
- 36. Callback fetchNetworkContentAsync(Callback<Response> cb) { mExecutor.execute(new Runnable(){ ... try { Response r = fetchNetworkContent(); mExecutor.execute(new Runnable() { try { Response r2 = fetchNetworkContent2(r); mHandler.post(new Runnable() { cb.done(r2, null); }); } catch(Exception exc) { cb.done(null, exc); }); }); } catch(Exception exc) { cb.done(null, exc);
- 37. Callback fetchNetworkContentAsync(Callback<Response> cb) { CountDownLatch cd = new CountDownLatch(2); Response r1, r2; mExecutor.execute(new Runnable(){ Response r1 = fetchNetworkContent(); cd.countDown(); }); mExecutor.execute(new Runnable(){ Response r2 = fetchNetworkContent(); cd.countDown(); }); mExecutor.execute(new Runnable(){ cd.await(); Response r = new Response(r1,r2); mHandler.post(new Runnable() { cb.done(r, null); });
- 38. ANDROID SDK HELP
- 39. Loader & AsyncTask They help in the field of gedng the asynchronous task happen in a way that is compaKble with Android threading model, but fall short in: • Combining • ManipulaKng • Error Handling • Streaming Data Handling
- 40. AsyncTask
- 41. NEED MORE ASYNC EXPRESSIVENESS a.k.a. my head is hurKng
- 44. Reactive Extensions (Rx) Developed by Microsof for .NET (C#) 2009 – Erik Meijer -‐ to support distributed cloud applicaEons Open Source port of the semanKcs to • Rx.ObjC, RxJava, Rx.JS, Scala • JavaVM based version is developed by Nejlix – Ben Christensen
- 45. Rx (Java): Concepts Observable EnKty that can fetch a stream of events asyncronously, if you subscribe an observer to it Observer You can imperaKvely noKfy an observer about an event
- 46. Rx (Java): Sample Observable<Employee> loadEmployeeWithName(String name) { Observable<Employee> observable = Observable.create( new Func1<Observer<Employee>, Subscription>() { public Subscription call(final Observer<Employee> observer) { List<Employee> employee = loadEmployeeFromDB(name); for (Employee emp : employees) { try { observer.onNext(emp); } catch (Exception e) { observer.onException(e); }} observer.onCompleted(); return Subscriptions.empty(); } }); return observable;} loadEmployeeWithName("Aldo").subscribe(new Observer<Employee>() { public void onNext(Employee emp) { showEmployeeDataOnScreen(emp); } public void onError(Exception e) { handleException(e); } public void onCompleted() { doneLoadingEmployees(); }});
- 47. "Async" expressed for collections IMPERATIVE • T next() • throws ExcepKon • return REACTIVE • onNext(T) • onError(ExcepKon) • onCompleted()
- 48. Rx (Java): Sample Observable<Employee> loadEmployeeWithName(String name) { Observable<Employee> observable = Observable.create( new Func1<Observer<Employee>, Subscription>() { public Subscription call(final Observer<Employee> observer) { List<Employee> employee = loadEmployeeFromDB(name); for (Employee emp : employees) { try { observer.onNext(emp); } catch (Exception e) { observer.onException(e); }} observer.onCompleted(); return Subscriptions.empty(); } }); return observable;} loadEmployeeWithName("Aldo").subscribe(new Observer<Employee>() { public void onNext(Employee emp) { showEmployeeDataOnScreen(emp); } public void onError(Exception e) { handleException(e); } public void onCompleted() { doneLoadingEmployees(); }}); STILL SYNC
- 49. Rx (Java): Sample Observable<Employee> loadEmployeeWithName(String name) { Observable<Employee> observable = Observable.create( new Func1<Observer<Employee>, Subscription>() { public Subscription call(final Observer<Employee> observer) { executor.postRunnable(new Runnable() {... List<Employee> employee = loadEmployeeFromDB(name); for (Employee emp : employees) { try { mHandler.post(new Runnable() { ... observer.onNext(emp); ... } } catch (Exception e) { mHandler.post(new Runnable() { ... observer.onException(e); }) }} mHandler.post(new Runnable() { observer.onCompleted(); }); return Subscriptions.empty(); } }); }); return observable;}
- 50. So what is Rx really doing? Rx is man-‐in-‐the middle between method calling and callback execuKon onNext() != onNext() We have control over scheduling of methods
- 51. Rx (Java): Sample Observable<Employee> loadEmployeeWithName(String name) { Observable<Employee> observable = Observable.create( new Func1<Observer<Employee>, Subscription>() { public Subscription call(final Observer<Employee> observer) { List<Employee> employee = loadEmployeeFromDB(name); for (Employee emp : employees) { try { observer.onNext(emp); } catch (Exception e) { observer.onException(e); }} observer.onCompleted(); return Subscriptions.empty(); } }); observable = observable.subscribeOn(Schedulers.executor(mExecutor)); observable = observable.observeOn(AndroidScheduler.getInstance()); return observable; }
- 52. Rx (Java): Sample Observable<Employee> loadEmployeeWithName(String name) { Observable<Employee> observable = Observable.create( new Func1<Observer<Employee>, Subscription>() { public Subscription call(final Observer<Employee> observer) { List<Employee> employee = loadEmployeeFromDB(name); for (Employee emp : employees) { try { observer.onNext(emp); } catch (Exception e) { observer.onException(e); }} observer.onCompleted(); return Subscriptions.empty(); } }); observable = observable.subscribeOn(Schedulers.executor(mExecutor)); observable = observable.observeOn(AndroidScheduler.getInstance()); return observable; } loadEmployeeWithName("Aldo").subscribe(new Observer<Employee>() { public void onNext(Employee emp) { showEmployeeDataOnScreen(emp); }
- 53. Rx (Java): Features Observer is executed afer someone subscribed Since we are decoupling method call and callback execuKon, we can easily manipulate and combine.
- 55. Filter Observables Observable.filter(obs1, new Func1<>() {});
- 56. Zip Observables Observable.zip(obs1, obs2, new Func1<a,b>() {});
- 57. Rx (Java): Features Each observer can be scheduled to have his subscribe happen on any thread, and all observaKon happen on any other thread! Observable<Image> getImage(URL url) { AtomicBoolean found = new AtomicBoolean(false); obs1 = //get image from in memory cache obs2 = //get image from disc cache obs2 = obs2 .subscribeOn(Schedulers.executor(highprioexecutor)) .observeOn(AndroidScheduler.getInstance()); obs3 = //get image from network obs3 = obs3 .subscribeOn(Schedulers.executor(lowprioexecutor)) .observeOn(AndroidScheduler.getInstance()); return Observable.concat(obs1, obs2, obs3); }
- 58. Rx (Java): Features Connectable version of Observable can be subscribed mulKple Kmes, doesn't restart but goes on, can even replay past events! Observable<Tweet> obs = createObservableThatStreamTweet(); void initObs() { obs.replay(); obs = obs.connect(); } Observable<Tweet> getPopularTweets() { return obs; }
- 59. Reactive the world • We could wrap all applicaKon in reacKves: click events could generate events that can route to DB calls. View could subscribe to DB observable and... Yes. Maybe.
- 61. Reactive Over Services <<sync>> Employee DAO <<sync>> Paypal API <<async>> Android NFC Intents RX Layer UI / Controllers FragmentA FragmentA AcKvity Views
- 62. 2ND PROBLEM
- 63. Fragments
- 64. Fragments NOT A UI but a LIFECYCLE ABSTRACTION!
- 65. Fragments • OS can create new instances you don't know about • Retrieve with findByTag() • Can be a@ached to one of many AcKviKes subclass
- 66. Fragments / Communication public class MasterFragment extends Fragment { private Listener listener; public interface Listener { void onItemSelected(int position){} onAttach(Activity a) { super.onAttach(a); listener = (Listener) a; }} public class HomeActivity extends Activity implements MasterFragment.Listener { onItemSelected(int position) { }}
- 67. Fragments / Communication AcKvity1 FragmentA FragmentB AcKvity2 FragmentC AcKvity3 FragmentA FragmentD FragmentC
- 68. Fragments / Communication public class HomeActivity extends Activity implements MasterFragment.Listener, DetailFragment.Listener, PopupFragment.Listener { ... }} ListFragment lf = getFragmentManager().findFragmentByTag("LIST"); if (lf == null) { getActivity().onItemSelected(); }
- 69. Need a Bus
- 70. Event Bus AcKvity1 FragmentA FragmentB BUS AcKvity2 FragmentQ AcKvity1 Service
- 71. Event Bus AcKvity1 FragmentA FragmentB BUS AcKvity2 FragmentQ AcKvity1 Service
- 72. Excellent library by : OMo Event Bus • Also called Publish / Subscribe in distributed systems • An agent posts a message on an event (topic) • All objects registered on that topic, receive event • Decouple caller by receiver + synchronous delivery
- 73. Otto bus.post(new AnswerAvailableEvent(42)); bus.register(this); ... @Subscribe public void answerAvailable(AnswerAvailableEvent event) { // TODO: React to the event somehow! } ... bus.unregister(this); static Bus bus = new Bus();
- 74. Otto • Event delivered! • O@o is not rocket science: • ReflecKon to spot methods that declare to handle some object as event • search for method that accept some event and deliver the event • More features...
- 75. Otto • Event delivered! • O@o is not rocket science: • ReflecKon to spot methods that declare to handle some object as event • search for method that accept some event and deliver the event • More features... Remember to call unregister()! Memory (acKvity) leak danger
- 76. DIY BUS
- 77. DIY Bus events = integers parameters = object or integers MyObject obj = createMyObject(); Bus.i().publish(0x2324, obj); Bus.i().subscribe(new Invokable() { invoke(int event, Object param) { MyObject obj = (MyObject) param; } }, 0x2324);
- 78. DIY Bus events = integers parameters = object or integers MyObject obj = createMyObject(); Bus.i().publish(0x2324, obj); Bus.i().subscribe(new Invokable() { invoke(int event, Object param) { MyObject obj = (MyObject) param; } }, 0x2324); Bus.java is 90 lines of code!
- 79. WRAPPING UP!
- 80. BOOST JAVA OBJ-C C# JS you need to work your way towards asynchronous expressiveness
- 81. SIMPLE ABSTRACTIONS CAN TURN YOUR WORLD Rx is a simple abstracKon that enables a truckload of features Event Bus is so old, is so simple, and works so well.
- 82. SERVER & CLIENT scale is orders of magnitude smaller, different problems, same techniques.
- 83. :)
- 85. Links • RxJava by Nejlix hMps://github.com/NePlix/RxJava/wiki • O@o by Square hMp://square.github.io/oMo • DIY Bus hMps://gist.github.com/hazam/ 8ac4abd927588d0bd04b • EPFL ReacKve on Coursera hMps://www.coursera.org/course/reacEve
- 86. Grazie.Non dimenKcare di riempire il modulo di feedback AND02 emanuele.disaverio@gmail.com hazam emanueledisaverio