by Gerard Sans | @gerardsans

Bending time

with Schedulers and RxJS 6

Bending time

with Schedulers and RxJS 6

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Woot!? Event Loop?

Macro & microtasks Quiz

const log = console.log;
const macro = v => setTimeout(() => log(v));
const micro = v => Promise.resolve().then(() => log(v));

log(1);
macro(2);
micro(3);
log(4);

a) 1 2 3 4
b) 1 4 2 3
c) 1 4 3 2

Event Loop

waits microtasks

demo

JavaScript: call me maybe?

setTimeout Quiz

setTimeout(() => log(1));
setTimeout(() => log(2), 0);
log(3)

// a) 1 2 3
// b) 3 2 1
// c) 3 1 2

YAY!

setTimeout Quiz

setTimeout(() => log(1), 1000);
setTimeout(() => log(2), 2000);
setTimeout(() => log(3), 3000);

// a) 1 2 3 (wait aprox. 1s)
// b) 1 2 3 (wait aprox. 3s)
// c) 1 2 3 (wait aprox. 6s)

setInterval(task, 50)

Time

50ms

task 1

task 2

task 3

task < delay

setInterval(task, 50)

Time

50ms

task 1

task 2

task 3

task > delay

task 1

task 2

task 3

Reality

Ideal

Delay is only

time to enter

Event Loop

RxJS help me!

audit auditTime buffer bufferCount bufferTime bufferToggle bufferWhen catchError combineAll combineLatest concat concatAll concatMap concatMapTo count debounce debounceTime defaultIfEmpty delay delayWhen dematerialize distinct distinctUntilChanged distinctUntilKeyChanged elementAt endWith every exhaust exhaustMap expand filter finalize find findIndex first groupBy ignoreElements isEmpty last map mapTo materialize max merge mergeAll mergeMap mergeMap as flatMap mergeMapTo mergeScan min multicast observeOn onErrorResumeNext pairwise partition pluck publish publishBehavior publishLast publishReplay race reduce repeat repeatWhen retry retryWhen refCount sample sampleTime scan sequenceEqual share shareReplay single skip skipLast skipUntil skipWhile startWith subscribeOn switchAll switchMap switchMapTo take takeLast takeUntil takeWhile tap throttle throttleTime throwIfEmpty timeInterval timeout timeoutWith timestamp toArray window windowCount windowTime windowToggle windowWhen withLatestFrom zip zipAll

103 operators

Synchronous Operators

Synchronous by default

Examples: of, from, range
Default Scheduler: queue

Asynchronous Operators

Asynchronous by default

Examples: timer, interval
Scheduler: async
Primitive: setInterval

Changing default Scheduler

of(1).subscribe(v => l(v));
l(2);

// 1 2


import { asyncScheduler } from 'rxjs';
of(1, asyncScheduler).subscribe(v => l(v));
l(2);

// 2 1

Least Concurrency Principle

What's new in RxJS 6!

RxJS v6 Goodies

Friendly imports
- rxjs, rxjs/operators
EcmaScript spec compliant
- Pipeable operators
- Avoid reserved keywords
Simplified API
- Result Selectors removed

Operator Renames

do throw switch finally

tap throwError switchAll finalize

Listing odd numbers

import { interval } from 'rxjs';
import { filter } from 'rxjs/operators'; 

const isOddNumber = (x: number) => x%2!==0;
interval(1000).pipe(
  filter(isOddNumber)
);

// --0--1--2--3--4--5-- interval
// -----1-----3-----5-- filter

Error Handling

import { throwError } from 'rxjs';
import { catchError, finalize } from 'rxjs/operators';

throwError("💩").subscribe({
  error: e => l(`Error: ${e}`)
})

throwError("💩").pipe(
  catchError(e => of("No worries. Sorted!😃")),
  finalize(() => { /* code */ }),
).subscribe(v => l(v));

Scheduler operators

1

Timeline

Emitted values

complete

next

Observable

Sync Implementation

// (111|)

let a$ = Observable.create(observer => {
  observer.next(1);
  observer.next(1);
  observer.next(1);
  observer.complete();
});

let subscription = a$.subscribe({
  next: v => log(v),
  complete: () => log('|')
});

Async Implementation

// --1--1--1|

let a$ = Observable.create(observer => {
  setTimeout(() => observer.next(1));
  setTimeout(() => observer.next(1));
  setTimeout(() => observer.next(1));
  setTimeout(() => observer.complete());
});

let subscription = a$.subscribe({
  next: v => log(v),
  complete: () => log('|')
});

subscribeOn

1

complete

next

1

map(v => v)

subscribeOn(asyncScheduler)

.subscribe()

subscribeOn

Changes Source Execution
Only used once

observeOn

1

complete

next

1

map(v => v)

observeOn(asyncScheduler)

.subscribe()

observeOn

Changes Notifications Execution
- Next, Error, Complete
Can be used before each operator

subscribeOn Quiz

of(1).pipe(subscribeOn(asyncScheduler))
  .subscribe({
    next: x => l(x),
    complete: () => l('3') 
  });
l('2');

// a) 1 2 3
// b) 2 1 3
// c) 1 3 2

Queue Scheduler

Overview

Executes Synchronously
Tasks execute in order
Waits until current task ends before executing next one
Performant (precedes Event Loop)

QueueScheduler Example

import { queueScheduler } from 'rxjs';

queueScheduler.schedule(() => log(1)); 
log(2);
queueScheduler.schedule(() => log(3)); 

// 1 2 3

queueScheduler.schedule(() => {
  queueScheduler.schedule(() => log(1)); 
  log(2);
  queueScheduler.schedule(() => log(3)); 
});

// 2 1 3

Asap Scheduler

Overview

Executes Asynchronously (micro)
Tasks execute before next tick
Relays on Promises
Performant (precedes Event Loop)

AsapScheduler Example

import { asapScheduler } from 'rxjs';
import { queueScheduler } from 'rxjs';

setTimeout(() => log(1));
asapScheduler.schedule(() => log(2)); 
queueScheduler.schedule(() => log(3));

// 3 2 1

Async Scheduler

Overview

Executes Asynchronously (macro)
Relays on setInterval
Less performant (uses Event Loop)

AsyncScheduler Example

import { asyncScheduler } from 'rxjs';
import { queueScheduler } from 'rxjs';

asapScheduler.schedule(() => log(2)); 
asyncScheduler.schedule(() => log(1));
queueScheduler.schedule(() => log(3));

// 3 2 1

Cancelling tasks

import { asyncScheduler } from 'rxjs';

const s = asyncScheduler;
const DELAY = 0;
let subscription;

subscription = s.schedule(v => log(v), DELAY, 1);
s.schedule(v => log(v), DELAY, 2);
log(3);
subscription.unsubscribe();

// 3 
// 2

Internal Time

import { asyncScheduler } from 'rxjs';

const s = asyncScheduler;
const DELAY = 2000;
const start = Date.now();

s.schedule(v => log(v), DELAY, 1);
s.schedule(v => log(v), DELAY, 2);
s.schedule(() => log(`${s.now()-start}ms`), DELAY);
log(3);

// 3
// 1
// 2 
// 2008ms

Animations Scheduler

Overview

Executes Asynchronously
Relays on requestAnimationFrame
Adapts to Device Frame Rate
Slows down when is not active
Balances CPU/GPU load

Animations

FRAMES

Frame Rates

60 FPS

Time

16.66ms

1000/60ms

paint

frame

paint

frame

paint

frame

paint

frame

paint

frame

Stutter or Losing Frames!

60 FPS

Time

16.66ms

using setInterval

const token;
const paintFrame = () => {
  // animation code
  token = setInterval(paintFrame, 1000/60);
}
paintFrame();
 
setTimeout(() => clearInterval(token), 2000);

Issues

Ignores Device Frame Rate
Runs all the time (batteries enemy)
Ignores current CPU/GPU load

requestAnimationFrame

60 FPS

Time

16.66ms

1000/60ms

paint

frame

paint

frame

paint

frame

requestAnimationFrame

const token;
const paintFrame = (timestamp) => {
  // animation code
  token = requestAnimationFrame(paintFrame)
}
requestAnimationFrame(paintFrame);
 
setTimeout(() => cancelAnimationFrame(token), 2000);

AnimationFrameScheduler

import { animationFrameScheduler } from 'rxjs';
const s = animationFrameScheduler;
const DELAY = 0;
const state = { angle: 0 }
const div = document.querySelector('.circle');

const work = state => {
  let {angle} = state;
  div.style.transform = `rotate(${angle}deg)`;
  s.schedule(work, DELAY, { angle: ++angle%360 });
} 
s.schedule(work, DELAY, state);

demo

Wrapping up

What is a Scheduler?

Data Structure
Execution context
Virtual clock

Schedulers Overview

Type	Execution	Primitives
queueScheduler	Sync	scheduler.schedule(task, delay) scheduler.flush()
asapScheduler	Async (micro)	Promise.resolve().then(() => task)
asyncScheduler	Async (macro)	id = setInterval(task, delay) clearInterval(id)
animationFrameScheduler	Async	id = requestAnimationFrame(task) cancelAnimationFrame(id)

Thanks

Bending time with Schedulers and RxJS 6

By Gerard Sans

Bending time with Schedulers and RxJS 6

Observables have been very popular because of their many qualities: asynchronous processing, composition, performance, powerful operators. But usually there's a less covered feature that lies beneath. That is: Schedulers. In this talk we are going to cover Schedulers in depth, going from the basic APIs to more obscure features to bend time to our will!

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