Advanced Observable Patterns in Angular apps with RxJS

Contents

Part 1 - Reactive programming and RxJS

  • Creating observables
  • Operators
  • Higher order observables
  • Error-handling

 

Part 2 - Observables in Angular

  • EventEmitters
  • Http
  • Forms
  • Router

INTRODUCTION

Reactive Programming

// concert spectators
let inMainStand = 300;
let inVipStand = 50;

// code...

let total = inMainStand + inVipStand; // 350

// code...

inMainStand += 60;

console.log(total); // total = ?
total = inMainStand + inVipStand; // 410
  • imperatively recalculate the value of total

Reactive Programming

Andre Staltz - Reactive Programming: Why It Matters

Reactive Programming

Product

Vendor

Cart

Cupon

Sale

Payment

User profile

Invoice

Reactive Programming

Cart

Invoice

Add product...

...update the total

Reactive Programming

Cart

Invoice

import {Invoice} from './invoice';

class Cart {
    
    constructor(invoice: Invoice){ ... }
    
    addProduct(product) {
        ...
        this.invoice.update(product); // needs to be public
        ...
    }
}

Passive

Proactive

Reactive Programming

Cart

Invoice

import {Cart} from './cart';

class Invoice {
    
    constructor(cart: Cart){ ... }
    
    init() {
        ...
        this.cart.onCartChange( callback )

    }
}

Reactive

Listenable

Reactive Programming

Cart

Cupon

Sale

Payment

Invoice

  • What does it affect? - look inside
  • How does it work? - find usages

Passive programming

  • remote setters and updates

Reactive Programming

Cart

Cupon

Sale

Payment

Invoice

  • What does it affect? - find usages of events
  • How does it work? - look inside

Reactive programming

  • events, observation and self updates

Reactive Programming

Interruptions in web applications

  • API events
  • WebSockets
  • User-triggered events
  • Timers (setTimeout, setInterval)

Reactive Programming

(Global) Event Bus

Image credits:

https://hackernoon.com/communicating-between-components-in-java-using-guava-event-bus-2f8a1e4e18dd

Reactive Programming

Observer pattern

Image credits:

https://www.dofactory.com/javascript/observer-design-pattern

Reactive Programming

Observer vs PubSub

Reactive Programming

Managing side-effects

SYNC

ASYNC

SINGLE

MULTIPLE

function

Reactive Programming

Managing side-effects

SYNC

ASYNC

SINGLE

MULTIPLE

function

generators

Reactive Programming

Managing side-effects

SYNC

ASYNC

SINGLE

MULTIPLE

function

promises

generators

Reactive Programming

Managing side-effects

SYNC

ASYNC

SINGLE

MULTIPLE

function

promises

generators

observables

OBSERVABLES

Special credits

Michael Hladky
@Michael_Hladky

 

- marble diagrams

- inspirational talks

Ben Lesh
@BenLesh

 

inspirational talks -

 work on RxJS -

OBSERVABLES

  • A stream of data ( 0 or more values of any type)

 

  • Pushed over any amount of time (can end, but not necessarily)

 

  • Cancelable ( can be stopped from emitting a value - unsubscribed)

 

  • Lazy - won't emit values until we subscribe to them

 

  • Just a function that takes an observer and returns a function

OBSERVABLES

Dealing with async in our apps:

  • Dom events
  • Animations
  • AJAX
  • WebSockets
  • Server Events

 

AJAX with callbacks

getData(function(successResult) {
    // do something with the data
});
getData(
  function(successResult) {
    // do something with the data
  },
  function(faliureError) {
    // handle the error
  },
);

AJAX with callbacks

let result = fetch('api/users.json');

result
  .then(success => {
    // handle success
  })
  .catch(error => {
    // handle error
  })

AJAX with promises

result
  .then(...)
  .then(...)
  .then(...)

Observables

Unifying callbacks, promises and event handlers

Shape of an observable:

  • A function
  • accepts an observer ( an object with `next`, `error` and `complete` methods on it)
  • returns a cancellation function

 

Creating an Observable

let observable$ = new Observable(() => {

})

Observable

  • read-only - consumer
  • plain function
  • exposes 3 channels: next, error and complete

excersise file: https://stackblitz.com/edit/adv-rxjs-1

Creating an Observable

let observable$ = new Observable(observer => {
  observer.next(1);
  observer.next(2);
})

Observer

  • write-only - producer
  • instance passed to observable
  • provide next, error and complete methods
  • just an interface

Creating an Observable

let observable$ = new Observable(observer => {
  observer.next(1);
  observer.next(2);
})

observable$.subscribe(value => {
  console.log(value)
}

Subscription

  • triggers the observable execution
  • returns an unsubscribe() method that stops the observable

Creating an Observable

let observable$ = new Observable(observer => {
  observer.next(1);
  observer.next(2);

  return () => {
    // cleanup resources when done
  };
})

const subscription = observable$.subscribe(value => {
  console.log(value)
})

subscription.unsubscribe(); // stop emitting values

Creating an Observable

let observable$ = new Observable(observer => {
  observer.next(1);
  observer.next(2);

  observer.error(new Error('Bad'));
})

const subscription = observable$.subscribe(
  value => { console.log(value)},
  error => { console.log(error.message)}
)

Creating an Observable

Creation functions

  • of(value1, value2, value3)

  • from(promise/itterable/observable)

  • fromEvent(target, eventName)

  • interval(time)

  • timer(time)

Creating a promise

let promise = new Promise((resolve, reject) => {

  doAsyncThing((err, value) => {
    if (err) {
      reject(err);
    } else {
      resolve(value)
    }
  })
})

promise.then(successFn, errorFn);

Creating an observable

let promise = new Promise((resolve, reject) => {

  doAsyncThing((err, value) => {
    if (err) {
      reject(err);
    } else {
      resolve(value)
    }
  })
})

promise.then(successFn, errorFn);

Hot vs Cold observables

COLD is when your observable creates the producer

// COLD
var cold = new Observable((observer) => {
  var producer = new Producer();
  // have observer listen to producer here
});
  • producer is created and activated during subscription
  • unicast => everyone gets their own instance
  • observables are "cold" by default

Hot vs Cold observables

  • HOT is when your observable closes over the produce

// HOT
var producer = new Producer();
var cold = new Observable((observer) => {
  // have observer listen to producer here
});
  • producer is created and activated outside and independent of subscription
  • multicast => shared reference to the producer

Subjects

Observables are unicast - each subscriber manages its own execution context

Subjects are multicast observables - values are multicasted to many Observers

Types of subjects

  • BehaviorSubject - has the notion of "current value" 
  • ReplaySubject - can record part of it's execution

Operators

Observables are collections of pushed values or events that we can:

  • query (filter)
  • transform (map)
  • accumulate (reduce)
  • join
  • flatten
  • more...

Array functions

let array = [1,2,3,4,5];

array
  .map(v => v * v)
  .filter(v => v > 5 && v < 20)
  .reduce((acc, curr) => acc + curr, 0)

// 25

Array functions

Operators

let observable$ = from([1,2,3,4,5]);

observable$
  .pipe(
    map(v => v * v),
    filter(v => v > 5 && v < 20),
    reduce((acc, curr) => acc + curr, 0)
  )
  .subscribe(val => console.log(val))

// 25

excersise file: https://stackblitz.com/edit/adv-rxjs-2

map

filter

scan

reduce

tap

Custom operators

const toUpperCase = 
  (source: Observable<any>) => {
    return new Observable((observer) => {
      return source.subscribe({
        next(x) {
          observer.next( x.toUpperCase() )
        }
      })
    }
  )}

from('hello')
  .pipe(toUpperCase)
  .subscribe(val => l(val))

excersise file: https://stackblitz.com/edit/adv-rxjs-3

Custom operators

const pow = (p = 1) =>
  (source: Observable<any>) => {
    return new Observable((observer) => {
      return source.subscribe({
        next(val) {
          observer.next( Math.pow(val, p) )
        }
      })
    }
  )}

from([1, 2, 3])
  .pipe(pow(3))
  .subscribe(val => l(val))

Operators can take parameters as well

Stopping streams

Higher order observbles

const buttonObs$ = fromEvent(querySelector('button'), 'click');

buttonObs$.subscribe(() => {
  http$.get('/api/users').subscribe( data => {
    // handle loaded data
  })
})

excersise file: https://stackblitz.com/edit/rxjs-adv-4

Flatning operators

mergeMap

Flatning operators

concatMap

Flatning operators

switchMap

Flatning operators

exhaustMap

Combination operators

zip

Combination operators

combineLatest

Combination operators

forkJoin

Combination operators

withLatestFrom

https://rxjs.dev/operator-decision-tree

Error Handling

const obs$ = new Observable(observer => {
  observer.closed;    // false 
  observer.next(1);   // emit 1
  observer.complete();
  observer.closed;    // true
  observer.next(2);   // won't emit
});
  • Completing a stream

excersise file: https://stackblitz.com/edit/rxjs-adv-5

Error Handling

  • A stream can only error once
const obs$ = new Observable(observer => {
  observer.closed;    // false 
  observer.next(1);   // emit 1
  observer.error(new Error('Bad!'));
  observer.closed;    // true
  observer.next(2);   // won't emit
});

obs$
  .subscribe({
    next: v => console.log(v),
    error: e => console.log(e.message)
  });

Error Handling

  • A stream can only error once
const obs$ = new Observable(observer => {
  observer.next(1);
  observer.error(new Error('BANG!'));
}).pipe(
  catchError(err => {
    console.log('Intercepted error:' + err);
    return of('I got this');
  })
)

obs$.subscribe(v => console.log(v));

// 1   'I got this'

Error Handling

  • Recovering after an error - the retry operator
const getData$ = http.get('/api/users')
    .pipe(
        retry(3),
        catchError(() => of('Something went wrong');
    )

getData$.subscribe(value => console.log(value));

Schedulers

Schedulers control timing around when an event occurs

 

RxJS Schedulers

  • queue - During same "job", but on a queue
  • asap - Next "job", aka, "microtask"
  • async - Next "timeout"
  • animationFrame - Next requestAnimationFrame

excersise file: https://stackblitz.com/edit/rxjs-adv-6

RxJS in Angular

Observables in Angular

Angular power-up -> async pipe

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