Intro RxJs - Stream all the things

Introduction to Reactive Extensions with code examples using RxJs

1. Intro RX Stream all the things
2. Mark van straten @markvanstraten Q42
3. “RX is a library for composing asynchronous and event-based programs by using observable sequences”
4. Value Promise<Value> Enumerable<Value> ????
5. Value Promise<Value> Enumerable<Value> Observable<Value> (Subject/Observer is Dual to Iterator - Erik Meijer - http://csl.stanford.edu/~christos/pldi2010.fit/meijer.duality.pdf)
6. A stream of future events is an Observable Listening to an observable is a Subscription State changes on an observable are done using Operators Concurrency is managed using Schedulers
7. Promise { onResolved, onRejected } Observable { onNext, onError, onCompleted }
8. SHOW ME CODE
9. UI Interaction function onDoubleClick(target, callback) { let clicks = 0; let self = this; return target.on('click', () => { clicks++; if(clicks == 1) { setTimeout(() => { if(clicks === 2) { callback.call(self); } clicks = 0; }, 300); } }); const target = document.querySelector('body'); onDoubleClick(target, () => console.log(‘got doubleClick’) const target = document.querySelector('body'); const clickStream = Rx.Observable.fromEvent(target, 'click'); clickStream .buffer(clickStream.debounce(300)) .map(list => list.length) .filter(x => x == 2) .subscribe(doubleClick => console.log('got doubleClick'));
10. API call retry logic // using observables function getUser(userId) { const getUserDetailsFallbacks = [ getUserFromServer(userId), getUserFromCache(userId), getEmptyUserObject(userId), ]; return Rx.Observable .catch(getUserDetailsFallbacks) .toPromise(); // optional } // using promises function getUser(userId) { return getUserFromServer(userId) .catch(err => getUserFromCache(userId) .catch(err => getEmptyUserObject(userId)) ); }
11. REAL LIFE EXAMPLE
12. I want to publish question(s) and retrieve my answer. If the answer is not in time throw an error.
13. Match response to request const allAnswersStream = redis.subscribe(‘answers:*’) .map(parseAnswer); function getAnswerForQuestion(q) { const answerStream = allAnswersStream .filter(a => a.id === q.id); return redis.publish(‘question:${q.id}’,q) .zip(answerStream) .take(1) /* only need one answer */ .timeout(2 * 1000) /* ms */ .map(qa => qa[1]);//only need the answer }
15. OMG THE DOCUMENTATION
16. “transform the items emitted by an Observable into Observables, then flatten the emissions from those into a single Observable” Also known as flatMap() (http://reactivex.io/documentation/operators/flatmap.html)
18. A LOT of operators (http://reactivex.io/documentation/operators.html)
19. Start with these • map • filter • scan • mergeMap • switchMap • combineLatest • concat • do
20. Find your RX Operator http://reactivex.io/rxjs/
22. TESTING
23. Seen this? describe('MyService', function() { // ... setup stuff it('Retrieves user details', function(done) { // increased timeout because getUser sometimes takes a long time to complete this.timeout(5000); const promise = service.getUser(1) .then(user => { expect(user).to.not.be.null; }); expect(promise).to. eventually.be.fulfilled.and.notify(done); }); });
24. RX lets you do async in virtual time for testing
25. const scheduler = new Rx.TestScheduler(); const getUserFromServerStream = scheduler.createColdObservable( onNext(100, ({ id: 1, name: 'myUser' })), onCompleted(110) ); mocha.stub('getUserFromServer').returns(getUserFromServerStream); const results = scheduler.startScheduler( () => getUser(1) ); console.log(results.messages) // [0]=onNext(100, ({ id: 1, name: 'myUser' }))), [1]=onComplete(110)
26. PITFALLS
27. Most observables are lazy (cold) and only start when subscribed() to
28. Observables might never complete. Use take(x) for clarity
29. It’s easy to interleave Promises in Rx Use defer(() => myPromise()) to make them lazy
30. Multiple subscribe() calls will (default) create their own observable. Use share() to make these multicast compatible.
31. TAKEAWAYS
32. Rx is a complex tool for reducing cognitive load when talking about the complex problem called asynchronicity
33. Reference material RxJS 5 Thinking Reactively - Ben Lesh https://www.youtube.com/watch?v=3LKMwkuK0ZE http://reactivex.io http://rxmarbles.com/