Functioning in React: A Deep-Dive into useState

The State Hook is now a core part of React. But what does it mean for a function to have state? This talk explores this question, touching on the history of state, the source code behind the hook, and the impact stateful functions have on the code we write.

1. Functioning in React: A Deep-Dive into useState Jake Worth, Hashrocket
2. Ponzi Scheme @jwworth 2
3. "[I]f he doesn't understand something, he just asks you. He doesn't care if he sounds foolish... he will ask the most obvious question without any sort of concern about it... so he asks lots and lots of 'dumb,' in the best sense of that word, questions. He'll say to someone, 'I don't understand. Explain that to me.' He'll just keep asking questions until he gets it right." — Malcolm Gladwell1 1 Ferriss 2016, 573. @jwworth 3
4. ! ------|------ @jwworth 4
5. ! ------|---- - @jwworth 5
6. » ! Jake Worth / @jwworth » " Hashrocket » # Today I Learned » $ https://www.jakeworth.com/ @jwworth 6
7. The Tower of Abstractions @jwworth 7
8. useState @jwworth 8
9. Who is this talk for? @jwworth 9
10. Disclaimer @jwworth 10
11. State @jwworth 11
12. Pitch: Understanding useState will make us better developers! @jwworth 12
13. Agenda 1. History of State 2. State Hooks Deep-Dive 3. Hooks In My Code? @jwworth 13
14. 1. History of State @jwworth 14
15. History of State: CS "Stateful means the computer or program keeps track of the state of interaction, usually by setting values in a storage field designated for that purpose. Stateless means there is no record of previous interactions and each interaction request has to be handled based entirely on information that comes with it."2 2 Whatis.com 2019. @jwworth 15
16. History of State: CS "Stateful means the computer or program keeps track of the state of interaction, usually by setting values in a storage field designated for that purpose. Stateless means there is no record of previous interactions and each interaction request has to be handled based entirely on information that comes with it."2 2 Whatis.com 2019. @jwworth 16
17. History of State: CS » Stateless interaction: HTTP » Stateful interaction: Browser & localStorage » Imperative programming languages: JavaScript @jwworth 17
18. History of State: CS "In computer science, imperative programming is a programming paradigm that uses statements that change a program's state."3 3 Wikipedia 2019. @jwworth 18
19. History of State: React » ES3 module pattern » ES6 component classes » Stateless functional components » Class-fields syntax » Stateful functional components (Hooks) @jwworth 19
20. History of State: React Sandbox: https://codesandbox.io/s/react-state-history-bwh2x @jwworth 20
21. History of State: React 0.13 // ES3 module pattern function PirateShip(initialProps) { return { state: { cannonsCount: 0 }, render: function() { return <div>{this.state.cannonsCount} cannons</div>; }, }; } @jwworth 21
22. History of State: React 0.13 // ES6 component classes class PirateShip extends React.Component { constructor(props) { super(props); this.state = { cannonsCount: 0 }; } render() { return <div>{this.state.cannonsCount} cannons</div>; } } @jwworth 22
23. History of State: React 0.13 // Updater function this.setstate((state, props) => { return { cannonsCount: state.cannonsCount + 1 }; }); // First-argument object this.setstate({ cannonsCount: 100 }); @jwworth 23
24. History of State: React 0.14 "In idiomatic react code, most of the components you write will be stateless."4 4 Alpert "React v0.14" 2015. @jwworth 24
25. History of State: React 0.14 // ES6 arrow function var PirateShip = props => { var cannonsCount = props.cannonsCount; return <div>{cannonsCount} cannons</div>; }; @jwworth 25
26. History of State: React 0.14 & ES6 // Class-fields syntax class PirateShip extends React.Component { state = { cannonsCount: 0 }; render() { return <div>{this.state.cannonsCount} cannons</div>; } } @jwworth 26
27. History of State: React 16.8 // useState function PirateShip() { const [cannonsCount, setCannonsCount] = useState(0); return <div>{cannonsCount} cannons</div> } @jwworth 27
28. History of State: React 16.8 // useState w/ multiple slices function PirateShip() { const [cannonsCount, setCannonsCount] = useState(0); const [anchored, setAnchored] = useState(false); // return JSX } @jwworth 28
29. History of State: React 16.8 // useState function PirateShip() { const [cannonsCount, setCannonsCount] = useState(0); // return JSX } @jwworth 29
30. 2. State Hooks Deep-Dive @jwworth 30
31. State Hooks Deep-Dive: Below » ! Multiple packages » " Type-safety » # Dependency injection » $ Interesting comments » % Leaps of faith @jwworth 31
32. State Hooks Deep-Dive: SHA React SHA: 34aaec6f908e85d56cbc510677bc1c85d87aa57b @jwworth 32
33. // Our code function PirateShip() { const [cannonscount, setCannonsCount] = useState(0); // return JSX } @jwworth 33
34. // Our code import React, { useState } from 'react'; // function PirateShip() { const [cannonscount, setCannonsCount] = useState(0); // return JSX } @jwworth 34
35. State Hooks Deep-Dive: Split » React 0.14 Package split: react & react-dom » Renderers: react-dom, react-dom/server, react-native, react- test-renderer, react-art @jwworth 35
36. // packages/react/index.js const React = require('./src/React'); // TODO: decide on the top-level export form. // This is hacky but makes it work with both // Rollup and Jest. ! module.exports = React.default || React; @jwworth 36
37. // packages/react/src/React.js import { useState } from './ReactHooks'; const React = { useState, }; export default React; @jwworth 37
38. // packages/react/src/React.js import { useState } from './ReactHooks'; const React = { useState, __SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED: ReactSharedInternals, // }; export default React; @jwworth 38
39. // packages/react/src/React.js import { useState } from './ReactHooks'; const React = { useState, }; export default React; @jwworth 39
40. // packages/react/src/ReactHooks.js export function useState<S>(initialState: (() => S) | S) { const dispatcher = resolveDispatcher(); return dispatcher.useState(initialState); } @jwworth 40
42. // packages/react/src/ReactHooks.js export function useState<S>(initialState: (() => S) | S) { const dispatcher = resolveDispatcher(); return dispatcher.useState(initialState); // } @jwworth 42
43. "[T]he base class setState() implementation has been an illusion all along. It doesn't do anything except forwarding the call to the current renderer. And [the] useState Hook does exactly the same thing." — Dan Abramov5 5 Abramov "How Does setState Know What to Do?" 2018. @jwworth 43
44. // packages/react/src/ReactHooks.js import ReactCurrentDispatcher from './ReactCurrentDispatcher'; function resolveDispatcher() { const dispatcher = ReactCurrentDispatcher.current; return dispatcher; } @jwworth 44
45. // packages/react/src/ReactCurrentDispatcher.js import type {Dispatcher} from 'react-reconciler/src/ReactFiberHooks'; const ReactCurrentDispatcher = { current: (null: null | Dispatcher), }; export default ReactCurrentDispatcher; @jwworth 45
46. > const immutable = { permanent: "don't update me" } undefined > immutable["permanent"] = 'uh oh' 'uh oh' > immutable { permanent: 'uh oh' } @jwworth 46
47. // packages/react/src/ReactCurrentDispatcher.js import type {Dispatcher} from 'react-reconciler/src/ReactFiberHooks'; const ReactCurrentDispatcher = { current: (null: null | Dispatcher), // }; export default ReactCurrentDispatcher; @jwworth 47
48. "[T]he base class setState() implementation has been an illusion all along. It doesn't do anything except forwarding the call to the current renderer." — Dan Abramov5 5 Abramov "How Does setState Know What to Do?" 2018. @jwworth 48
49. // packages/react-dom/src/server/ReactPartialRenderer.js import { Dispatcher, } from './ReactPartialRendererHooks'; ReactCurrentDispatcher = ReactSharedInternals.ReactCurrentDispatcher; ReactCurrentDispatcher.current = Dispatcher; @jwworth 49
50. // packages/react-dom/src/server/ReactPartialRendererHooks.js export const Dispatcher: DispatcherType = { useState, }; @jwworth 50
51. // packages/react-dom/src/server/ReactPartialRendererHooks.js export function useState<S>( initialState: (() => S) | S, ): [S, Dispatch<BasicStateAction<S>>] { return useReducer(basicStateReducer, (initialState: any)); } export const Dispatcher: DispatcherType = { useState, }; @jwworth 51
52. // packages/react-dom/src/server/ReactPartialRendererHooks.js // useReducer (abridged) workInProgressHook = createWorkInProgressHook(); workInProgressHook.memoizedState = state; const dispatch: Dispatch<A> = (queue.dispatch: any); return [workInProgressHook.memoizedState, dispatch]; @jwworth 52
53. // packages/react-dom/src/server/ReactPartialRendererHooks.js function createHook(): Hook { return { memoizedState: null, queue: null, next: null, }; } @jwworth 53
54. // Our code const [cannonscount, setCannonsCount] = useState(0); @jwworth 54
55. // Our code const [cannonscount, setCannonsCount] = useState(0); ‑ const [WIPHookMemoizedState, dispatch] = useState(0); @jwworth 55
56. State Hooks Deep-Dive: Resurfacing » React checks types and creates an interface for a dispatcher » The renderer nullifies and sets a real dispatcher » The dispatcher returns an array of memoized state and a setter function » Rendering and reconciliation occurs @jwworth 56
57. 3. Hooks In My Code? @jwworth 57
58. Hooks are great and you should use them! @jwworth 58
59. Hooks are everywhere! @jwworth 59
60. Hooks solve problems! @jwworth 60
61. State management is important! @jwworth 61
62. Should I refactor my existing components to use hooks? @jwworth 62
63. Hooks are great and you should use them! @jwworth 63
64. Conclusion @jwworth 64
65. Thank you! Jake Worth / @jwworth @jwworth 65
66. Further Reading » "RFC: React Hooks", Reactjs RFCs, accessed September, 2019, https://github.com/reactjs/rfcs/pull/68. » Abramov, Dan. 2018. "How Does setState Know What to Do?". Overreacted.io, December 9, 2018. https://overreacted.io/how-does-setstate-know-what-to-do/. » Abramov, Dan. 2018. "Why Do We Write super(props)?". Overreacted.io, November 30, 2018. https://overreacted.io/why-do-we-write-super-props/. » Albert, Sophie. 2015. "React v0.14". React.js Blog, October 7, 2015. https://reactjs.org/blog/2015/10/07/react-v0.14.html. » Alpert, Sophie. 2015. "React's Package Split". React.js Blog. July 3, 2015. https://reactjs.org/blog/2015/07/03/react-v0.14-beta-1.html#two-packages. » Clark, Andrew. 2016. "React Fiber Architecture". GitHub Gist. July 18, 2016. https://github.com/acdlite/react-fiber-architecture/blob/master/README.md. » Clark, Lin. 2017. "A Cartoon Intro to Fiber". Filmed at React Conf 2017, San Francisco, CA. Video. https://youtu.be/ZCuYPiUIONs. » Cowan, Paul. 2019. "Frustrations With Hooks". LogRocket Blog, September 9, 2019. https://blog.logrocket.com/frustrations-with-react-hooks/. » Dhuyvetters, Geoffrey. 2016. "What is React Fiber? And how can I try it out today?". GitHub Gist, December 20, 2016. https://gist.github.com/duivvv/2ba00d413b8ff7bc1fa5a2e51c61ba43. » Ehrenberg, Daniel and Morrison, Jeff. "Class field declarations for JavaScript". ECMT TC39, accessed September, 2019, https://github.com/tc39/proposal-class-fields. » Ferriss, Tim. 2016. Tools of Titans: The Tactics, Routines, and Habits of Billionaires, Icons, and World-Class Performers . New York: Houghton Mifflin Harcourt. » Giamir Buoncristiani, Giamir. 2017. "What is React Fiber?". Giamir.com. April 23, 2017. https://giamir.com/what-is-react-fiber. » Iffland, David. 2017. "React Fiber: A Closer Look at the New Engine of React". May 1, 2017. https://www.infoq.com/news/2017/05/react-fiber-closer-look/. » Markbåge, Sebastian. 2015. "React v0.13.0 Beta 1". React.js Blog. January, 2015. https://reactjs.org/blog/2015/01/27/react-v0.13.0-beta-1.html. » McCracken, Harry. 2014. "This 1981 Computer Magazine Cover Explains Why We're So Bad at Tech Predictions". Time Magazine, April 14, 2014, https://time.com/60505/this-1981-computer-magazine- cover-explains-why-were-so-bad-at-tech-predictions/. » Rouse, Margaret. s.v. "stateless". Whatis.com. Accessed September, 2019, https://whatis.techtarget.com/definition/stateless. » Flow Documentation, s.v. "Generics", accessed September 2019, https://flow.org/en/docs/types/generics/. » Flow Documentation, s.v. "Importing and Exporting Types", accessed September 2019, https://flow.org/en/docs/types/modules/#importing-and-exporting-types-. » Flow Documentation, s.v. "Type Aliases", accessed September 2019, https://flow.org/en/docs/types/aliases/. » Wikipedia, s.v. "Imperative Programming", accessed September 2019, https://en.wikipedia.org/wiki/Imperative_programming. @jwworth 66