What Tamagotchis can teach you about ES6 generators

Hi, I’m Jenn. Senior Frontend Engineer @ ClassPass @gurlcode

@gurlcode

SVG @gurlcode

SVG Controls @gurlcode

SVG Canvas Controls @gurlcode

Canvas var canvas = document.querySelector(‘canvas’); var context = canvas.context(‘2d’); context.drawImage(‘tamagotchi.png’, …); @gurlcode

Animations context.drawImage(image, 0, …coordinates); context.clear(); context.drawImage(image, 200, …coordinates); context.clear(); context.drawImage(image, 400, …coordinates);

function bounce(x) { return () => { context.drawImage(…, x, …); context.clear(); } } setTimeout(bounce(0), 300); setTimeout(bounce(200), 600); setTimeout(bounce(400), 900); setTimeout(bounce(200), 1200); setTimeout(bounce(0), 1500);

I want to resolve an animation, then handle another animation. var promise = new Promise((resolve, reject) => { …things resolve(value); }; promise().then((value) => …more_things); @gurlcode

Animation with Promises function animate(draw, ms) { return new Promise((resolve, reject) => { var animation = () => { context.clear(); var isComplete = draw(resolve); } } if (!isComplete) { setTimeout(() => (animation), ms); } animation(); }); @gurlcode

Animation with Promises function bounceUp() { var frameWidth = 200; var frameCount = 2; var currentFrame = 0; } return animate((resolve) => { context.drawImage(frameWidth * currentFrame, …); context.clear(); currentFrame++; if (currentFrame > frameCount) { resolve(); return true; } }, ms); @gurlcode

function bounce() { return bounceUp .then(bounceDown); } bounce() .then(bounce); .then(bounce); @gurlcode

function loop() { return idle() .then(loop); } @gurlcode

Except… we got problems @gurlcode

Thenable Hell bounce(() => { moveRight(() => { moveLeft(() => { // Am I in hell? }); }); })

Thenable Hell bounce() bounce(() => { moveRight(() => { moveLeft(() => { // Am I in hell? }); }); }) .then(() => moveRight(20)) .then(() => moveLeft(40)) .then(() => { var shouldFeed = … if () {… else { … }) .then(bounce) .then( // Still in hell. )

Promises are Unbreakable Vows @gurlcode

I want to pause an animation, & yield to an event. function* generator() { // pause } @gurlcode

Generators 101 @gurlcode

function* count() { yield 1; yield; yield 3; } @gurlcode

function* count() { yield 1; yield; yield 3; } var generator = count(); Generator {}; @gurlcode

function* count() { yield 1; yield; yield 3; } var generator = count(); Generator {}; generator.next(); { value: 1, done: false } @gurlcode

function* count() { yield 1; yield; yield 3; } var generator = count(); Generator {}; generator.next(); { value: 1, done: false } generator.next(); { value: undefined, done: false } generator.next(); { value: 3, done: false } generator.next(); { value: undefined, done: true } @gurlcode

function* add() { const num = yield; yield 2 + num; yield 4 + num; } var generator = add(); Generator {}; generator.next(); { value: undefined, done: false } generator.next(2); { value: 4, done: false } generator.next(); { value: 6, done: false } generator.next(); { value: undefined, done: true } @gurlcode

Infinite Generators @gurlcode

function* forever(num) { while (true) { yield 2 + num; } } var generator = forever(2); Generator {}; @gurlcode

function* forever(num) { while (true) { num = yield 2 + num; } } var generator = forever(2); Generator {}; generator.next(); { value: 4, done: false } generator.next(2); { value: 6, done: false } generator.next(2); { value: 8, done: false } generator.next(2); { value: 10, done: false } { value: …, done: false } @gurlcode

yield* @gurlcode

function* outer() { yield 1; yield* inner(); yield 2; } function* inner() { yield “a”; yield “b”; } var generator = outer(); Generator {}; @gurlcode

function* outer() { yield 1; yield* inner(); yield 2; } function* inner() { yield “a”; yield “b”; } generator.next(); { value: 1, done: false } generator.next(); { value: “a”, done: false } generator.next(); { value: “b”, done: false } generator.next(); { value: 2, done: false } generator.next(); { value: undefined, done: true }

Canceling a Generator @gurlcode

function* forever(num) { while (true) { yield 2 + num; return; } } var generator = forever(2); Generator {}; generator.next(); { value: 4, done: false } generator.next(); { value: undefined, done: true } generator.next(); { value: undefined, done: true } @gurlcode

function* forever(num) { while (true) { yield 2 + num; } } var generator = forever(2); Generator {}; generator.next(); { value: 4, done: false } generator.return(); { value: undefined, done: true } @gurlcode

Resuming the generator occurs outside of the generator. function* generatorFunc() { yield 1; yield 2; } var generator = generatorFunc(); generator.next(); @gurlcode

s n u r s r o t a r e n e g @gurlcode

Coroutines! @gurlcode

Coroutines are a general control structure whereby control flow is cooperatively passed between two different routines. @gurlcode

function* bounceUp() { yield context.drawImage(…); yield context.drawImage(…); yield context.drawImage(…); // …etc } coroutine(bounceUp); @gurlcode

function* bounceUp() { yield new Promise(…); yield new Promise(…); yield new Promise(…); // …etc } coroutine(bounceUp); @gurlcode

function nextResponse(value) { const response = generator.next(value); if (response.done) { return; } handleAsync(response.value); } @gurlcode

function handleAsync(value) { value.then(nextResponse); } @gurlcode

function bounce() { return bounceUp .then(bounceDown); }

coroutine(function* bounce() { yield bounceUp; yield bounceDown; }); @gurlcode

co(function* () { yield promise(); yield (callback) => { // do some things callback() }; yield generatorFunction; }); https://github.com/tj/co @gurlcode

You can think sequentially about async code. function* idle() { yield* bounce; yield* moveRight; yield* moveLeft; yield* bounce; } @gurlcode

@gurlcode

coroutine(function* loop() { let done; let animation = idle(); while (true) { if (pending) { yield event; } while (!done && !pending) { const next = animation.next(); done = next.done; yield next.value; } animation.return(); animation = idle(); done = false; } }); @gurlcode

coroutine(function* loop() { let done; let animation = idle(); while (true) { if (pending) { yield* event(); } while (!done && !pending) { const next = animation.next(); done = next.done; yield next.value; } animation.return(); animation = idle(); done = false; } }); @gurlcode

function* feed() { if (hunger === 0) { yield* dislike(); } } function* dislike() { yield drawFrame(‘dislike’, 0); yield drawFrame(‘dislike’, 1); } hunger—-; yield* eat(); @gurlcode

function* feed() { if (hunger === 0) { yield* dislike(); } } hunger—-; yield* eat(); function* eat() { yield drawFrame(‘eat’, 0); yield drawFrame(‘eat’, 1); } @gurlcode

@gurlcode

Thank you! @gurlcode

What Tamagotchis can teach you about ES6 generators

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function bounce() { return bounceUp .then(bounceDown); }

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