Event Aggregator Pattern featuring Otto

May 2014 Android Listener presentation on the event aggregation pattern featuring Otto.

Transcript

1. Event Aggregator Pattern featuring Otto
3. Code
5. Problems 4 Tight coupling 4 Testability 4 Refactoring 4 Open/Closed Principle
6. Software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification. 1 Bertrand Meyer
7. What about listeners?
8. Listener Drawbacks 4 1:1 relationship between class and response 4 Conflicting interface methods 4 Naming conventions (e.g. handleChangeEvent vs. recordChangeInJournal). 4 Each event usually has its own interface.
9. Enter the Event Aggregator
10. 4 Brokers events between components via pub/sub communication 4 Components (activities, fragments, services) are blissfully ignorant of each other 4 Programming to a contract
12. Adding Otto dependencies { compile 'com.squareup:otto:1.3.+' }
13. Introducing the Bus public final class BusProvider { private static final Bus BUS = new Bus(); public static Bus getInstance() { return BUS; } private BusProvider() { // No instances. } }
14. Registration @Override protected void onResume() { super.onResume(); // Register ourselves so that we can provide the initial value. BusProvider.getInstance().register(this); } @Override protected void onPause() { super.onPause(); // Always unregister when an object no longer should be on the bus. BusProvider.getInstance().unregister(this); }
15. Publish Events Method public void post(Object event) { /*...*/ } Example BusProducer.getInstance().post(new LocationClearEvent());
16. Create Events Event objects are just POJOs that represent the types of events that occur in the app. public class LocationClearEvent { /*...*/ } public class LocationChangedEvent { /*...*/ }
17. Subscribe to Events 1. @Subscribe annotation 2. Method parameter of the event being subscribed to
18. @Subscribe public void onLocationChanged(LocationChangedEvent event) { // Action goes here } @Subscribe public void onLocationCleared(LocationClearEvent event) { // Action goes here }
19. public class AndroidBus extends Bus { private final Handler mainThread = new Handler(Looper.getMainLooper()); @Override public void post(final Object event) { if (Looper.myLooper() == Looper.getMainLooper()) { super.post(event); } else { mainThread.post(new Runnable() { @Override public void run() { AndroidBus.super.post(event); } }); } } }
20. Resources 4 http://square.github.io/otto/ 4 http://corner.squareup.com/2012/07/otto.html) 4 http://martinfowler.com/eaaDev/ EventCollaboration.html 4 https://code.google.com/p/guava-libraries/wiki/ EventBusExplained
21. Code http://j.mp/al-otto-code
22. Slides http://j.mp/al-otto-slides
23. @klmcmahon