Promises - The Unsung Heroes ofJavaScript


The Presentation inside:

Slide 0

PROMISES THE UNSUNG HEROES OF ASYNC IN JAVASCRIPT DEAN RADCLIFFE • OK GROW! • @DEANIUSDEV


Slide 1

A SPORTING BET "I BET YOU $20 THE REDS WON'T BEAT THE CUBS THIS SEASON..."


Slide 2

TWO PROMISES ➢ Begin Season then Cubs Lose then Season Ends ↩ I Pay You ➢ Begin Season then Season Ends ↩ You Pay Me


Slide 3

TWO PROMISES ➢ Begin Season then Cubs Lose // ^ What if we made the bet after a loss ? then Season Ends ↩ I Pay You ➢ Begin Season then Season Ends ↩ You Pay Me


Slide 4

PROMISES: EVENTS, AT ANY TIME A Promise represents a point in time which is: ▸ ASAP, if something has already happened ▸ ASAP the moment it happens


Slide 5

PROMISES: VALUES, AT ANY TIME A Promise provides access to a value whether: ▸ It is already known ▸ It is being computed in this turn of the Event Loop ▸ It may be un-knowable (maybe use a TimeOut)


Slide 6

PROMISES: ONE OR MORE CONDITIONS & ACTIONS A Promise can represent an entire chain of events, with branching possibilities. var firstStep = new Promise() var allSteps = firstStep .then(isReady => isReady || makeReady) .then(lastStep) Promise.await(allSteps)


Slide 7

OUR BET - MAKE IT SO ➢ let ourBet = BeginSeason() .then(Promise.race(CubsLoseToReds(), SeasonEnds())) .then(cubsLose => cubsLose ? iPayYou : youPayMe) //.then(iPayYou, youPayMe) ▸ CubsLoseToReds and SeasonEnds return Promises ▸ CubsLose should return truthy (such as the score), ▸ SeasonEnds should return falsy, or throw or reject


Slide 8

UNSUNG HEROES OF ASYNC ▸ Standard, explicit contract of behavior ▸ A promise can resolve or reject ▸ Its state may change 1x from pending to fulfilled ▸ p.then(youPayMe, iPayYou) ▸ Or: p.then((value)=>{}) .catch((err)=>{})


Slide 9

BACK ON THE CHAIN GANG Each step in a .then chain: RETURNS A CUMULATIVE PROMISE FOR ALL BEFORE ➢ let chain = promise1 .then(promise2) .then(action1)


Slide 10

BACK ON THE CHAIN GANG Each step in a .then chain may: RETURN A VALUE / RETURN A PROMISE FOR A NEW VALUE ➢ BeginSeasonLossless .then(CubsLoseToReds) // <-- a Promise .then(iPayYou)


Slide 11

BACK ON THE CHAIN GANG Each step in a .then chain may: RAISE AN EXCEPTION / RETURN REJECTED PROMISE ➢ BeginSeasonLossless .then(Promise.race(CubsLoseToReds,TimeOutAtSeasonEnd)) .then(iPayYou) .catch(youPayMe)


Slide 12

EXCEPTIONS ▸ An uncaught exception in a .then is OK! ▸ A catch at the end can handle errors for the whole chain


Slide 13

COMIC RELIEF CAT PICTURE


Slide 14

QUESTION: HOW DO THESE EVENTS BEHAVE ? If you add an event listener after an event has happened, does it make a sound ? document.addEventListener("DOMContentLoaded") // vs. $(document).ready // vs. $(document).on("ready")


Slide 15

QUESTION: HOW DO THESE EVENTS BEHAVE ? If you add an event listener after an event has happened, does it make a sound ? document.addEventListener("DOMContentLoaded") // vs. $(document).ready // <<-- only this one - like a Promise // vs. $(document).on("ready")


Slide 16

PROVIDED ITS LOADED, 1 ➢ DomContentLoadedPromise.then(function () { console.log('Im ready') }); ↩ Promise {[[PromiseStatus]]: "resolved"} ----Im ready


Slide 17

PROVIDED ITS LOADED, 2 ➢ DomContentLoadedPromise.then(function () { console.log('Im ready') }); ↩ Promise {[[PromiseStatus]]: "pending"} -----


Slide 18

PROVIDED ITS LOADED, 2.. ➢ DomContentLoadedPromise.then(function () { console.log('Im ready') }); ↩ Promise {[[PromiseStatus]]: "pending"} ----...


Slide 19

PROVIDED ITS LOADED, 3 ➢ DomContentLoadedPromise.then(function () { console.log('Im ready') }); ↩ Promise {[[PromiseStatus]]: "pending"} ----... _____ Im ready


Slide 20

WHAT'S WRONG WITH CALLBACKS ? Implicit, not formalized standard of behavior: 1. Function that takes 2 arguments ▸ first argument is an error ▸ second argument is the result ▸ Never pass both ▸ error should be instanceof Error


Slide 21

WHAT'S WRONG WITH CALLBACKS ? Implicit, not formalized standard of behavior (cont.) 1. Must never execute on the same tick of the event loop 2. Must be passed as last argument to function 3. Return value is ignored 4. Must not throw / must pass resulting errors 5. Must never be called more than once


Slide 22

WHAT'S WRONG WITH CALLBACKS ? ▸ Burdensome to check & propogate errors correctly function doAsyncStuff (cb) { try { doStuff1(function(err, value) { if (err) return cb(err); try { doStuff2(value, function(err, value2) { if (err) return cb(err); cb(null, value2); }); } catch (e) { cb(e); } }) } catch (e) { cb(e); } }


Slide 23

WHAT'S RIGHT WITH PROMISES ? BEAUTIFUL TO CHECK ERRORS, EXCEPTION OR ASYNC! // Promises FTW !! function doAsyncStuff () { return doStuff1() .then(doStuff2) .catch(function(err){ /*...*/}) }


Slide 24

THE EVENT LOOP (AKA TASK QUEUE)


Slide 25

THE EVENT LOOP (AKA TASK QUEUE)


Slide 26

THE 2 WAYS OF CODING JS


Slide 27

PROMISES AND REACTIVITY Keep an upperCase var in sync with a variable called lowerCase let lowerCase = new ReactiveVar let upperCase = new ReactiveVar


Slide 28

PROMISES AND REACTIVITY Keep an upperCase var in sync with a variable called lowerCase let lowerCase = new ReactiveVar let upperCase = new ReactiveVar Tracker.autorun(() => { let x = lowerCase.get() upperCase.set(x.toUpperCase()) })


Slide 29

PROMISES AND REACTIVITY Keep an upperCase var in sync with a variable called lowerCase lowerCase.set(x).then((x) => { upperCase.set(x.toUpperCase()) })


Slide 30

PATTERN RETURNING A PROMISE FROM A METEOR METHOD


Slide 31

RETURNING A PROMISE FROM A METEOR METHOD Return a Promise from a Method Return an Eventual Value from a Method


Slide 32

RETURNING A PROMISE FROM A METEOR METHOD Return a Promise from a Method Return an Eventual Value from a Method Eventually return a value from a Method


Slide 33

RETURNING A PROMISE FROM A METHOD, 1 Meteor.methods({ futureTime() { return Promise.await( ); } })


Slide 34

RETURNING A PROMISE FROM A METHOD, 2 Meteor.methods({ futureTime() { return Promise.await(new Promise((resolve) => { //some callback code which should call 'resolve' })); } })


Slide 35

RETURNING A PROMISE FROM A METHOD, 3 Meteor.methods({ futureTime() { return Promise.await(new Promise((resolve) => { setTimeout(() => { console.log("its the future now") resolve(new Date()) }, 5000) })); } })


Slide 36

RETURNING A PROMISE FROM A METHOD, 4 Meteor.methods({ futureTime() { let actions = new Promise((resolve) => setTimeout(()=> resolve(new Date), 5000)) .then(logAndReturn) .catch(logAndReturn); return Promise.await(actions); } })


Slide 37

ASYNC SHEEP


Slide 38

CALLING A METHOD Meteor.promise("futureTime") .then(log)


Slide 39

CALLING A METHOD, NOW AND LATER ➢ Promise.resolve(new Date) .then(log) .then(() => Meteor.promise("futureTime")) .then(log) --Fri Sep 18 2015 22:11:26 GMT-0500 (CDT) ↩ Promise {[[PromiseStatus]]: "pending"} --Fri Sep 18 2015 22:11:31 GMT-0500 (CDT)


Slide 40

CALLING A METHOD, FUNNILY Promise.resolve(new Date) .then(log) .then(() => "futureTime") .then(Meteor.promise) .then(log) --Fri Sep 18 2015 22:11:46 GMT-0500 (CDT) ↩ Promise {[[PromiseStatus]]: "pending", --Fri Sep 18 2015 22:11:51 GMT-0500 (CDT)


Slide 41

var ComicReliefCatPicture; // TODO fill in


Slide 42

PROMISES AND REACTIVITY okgrow:promise A promise's resolution can invalidate a computation ? Awwwwwww yeah !


Slide 43

DEFINE A HELPER AROUND A PROMISE <template name="example1Demo"> <input id="arg1" value="foo"/> <input id="arg2" value="bar"/> <textarea> {{meteorPromise "addWithSleep" "arg1" "arg2"}} </textarea> </template>


Slide 44

RETURN A PROMISE FROM A HELPER // {{meteorPromise "addWithSleep" "arg1" "arg2"}} Template.example1Demo.helpers({ meteorPromise: function (methodName, part1, part2) { var template = Template.instance(), value1 = template[part1].get(), value2 = template[part2].get(); // want to - but cant do this ! return Meteor.callPromise(methodName, value1, value2); } });


Slide 45

RETURN A PROMISE FROM A HELPER // {{meteorPromise "addWithSleep" "arg1" "arg2"}} Template.example1Demo.helpers({ meteorPromise: ReactivePromise(theHelperFunction, {pending: "loading..."}) }); var theHelperFunction = function (methodName, part1, part2) { var template = Template.instance(), value1 = template[part1].get(), value2 = template[part2].get(); return Meteor.callPromise(methodName, value1, value2); }


Slide 46

TMTOWTDI CALLBACKS, PROMISES, FIBERS, REACTIVITY, AND ES6 GENERATORS ARE ALL WAYS TO BE ASYNC! (YOU CAN ALWAYS CONVERT)


Slide 47

QUESTIONS ??


Slide 48

PROMISES Я AWESOME


Slide 49

PROMISES Я AWESOME AND SO ARE YOU!


Slide 50

DEAN RADCLIFFE, OK GROW! @DEANIUSDEV


Slide 51


×

HTML:





Ссылка: