简化版 useState 数据流笔记

let componentHooks = [];
let currentHookIndex = 0;

// useState 在 React 中是如何工作的（简化版）
function useState(initialState) {
  let pair = componentHooks[currentHookIndex];
  if (pair) {
    // 这不是第一次渲染
    // 所以 state pair 已经存在
    // 将其返回并为下一次 hook 的调用做准备
    currentHookIndex++;
    return pair;
  }

  // 这是我们第一次进行渲染
  // 所以新建一个 state pair 然后存储它
  pair = [initialState, setState];

  function setState(nextState) {
    // 当用户发起 state 的变更，
    // 把新的值放入 pair 中
    pair[0] = nextState;
    updateDOM();
  }

  // 存储这个 pair 用于将来的渲染
  // 并且为下一次 hook 的调用做准备
  componentHooks[currentHookIndex] = pair;
  currentHookIndex++;
  return pair;
}

function Gallery() {
  // 每次调用 useState() 都会得到新的 pair
  const [index, setIndex] = useState(0);
  const [showMore, setShowMore] = useState(false);

  function handleNextClick() {
    setIndex(index + 1);
  }

  function handleMoreClick() {
    setShowMore(!showMore);
  }

  let sculpture = sculptureList[index];
  // 这个例子没有使用 React，所以
  // 返回一个对象而不是 JSX
  return {
    onNextClick: handleNextClick,
    onMoreClick: handleMoreClick,
    header: `${sculpture.name} by ${sculpture.artist}`,
    counter: `${index + 1} of ${sculptureList.length}`,
    more: `${showMore ? "Hide" : "Show"} details`,
    description: showMore ? sculpture.description : null,
    imageSrc: sculpture.url,
    imageAlt: sculpture.alt,
  };
}

function updateDOM() {
  // 在渲染组件之前
  // 重置当前 Hook 的下标
  currentHookIndex = 0;
  let output = Gallery();

  // 更新 DOM 以匹配输出结果
  // 这部分工作由 React 为你完成
  nextButton.onclick = output.onNextClick;
  header.textContent = output.header;
  moreButton.onclick = output.onMoreClick;
  moreButton.textContent = output.more;
  image.src = output.imageSrc;
  image.alt = output.imageAlt;
  if (output.description !== null) {
    description.textContent = output.description;
    description.style.display = "";
  } else {
    description.style.display = "none";
  }
}

let nextButton = document.getElementById("nextButton");
let header = document.getElementById("header");
let moreButton = document.getElementById("moreButton");
let description = document.getElementById("description");
let image = document.getElementById("image");
let sculptureList = [
  {
    name: "Homenaje a la Neurocirugía",
    artist: "Marta Colvin Andrade",
    description:
      "Although Colvin is predominantly known for abstract themes that allude to pre-Hispanic symbols, this gigantic sculpture, an homage to neurosurgery, is one of her most recognizable public art pieces.",
    url: "https://react.dev/images/docs/scientists/Mx7dA2Y.jpg",
    alt: "A bronze statue of two crossed hands delicately holding a human brain in their fingertips.",
  },
  {
    name: "Floralis Genérica",
    artist: "Eduardo Catalano",
    description:
      "This enormous (75 ft. or 23m) silver flower is located in Buenos Aires. It is designed to move, closing its petals in the evening or when strong winds blow and opening them in the morning.",
    url: "https://react.dev/images/docs/scientists/ZF6s192m.jpg",
    alt: "A gigantic metallic flower sculpture with reflective mirror-like petals and strong stamens.",
  },
  {
    name: "Eternal Presence",
    artist: "John Woodrow Wilson",
    description:
      'Wilson was known for his preoccupation with equality, social justice, as well as the essential and spiritual qualities of humankind. This massive (7ft. or 2,13m) bronze represents what he described as "a symbolic Black presence infused with a sense of universal humanity."',
    url: "https://react.dev/images/docs/scientists/aTtVpES.jpg",
    alt: "The sculpture depicting a human head seems ever-present and solemn. It radiates calm and serenity.",
  },
  {
    name: "Moai",
    artist: "Unknown Artist",
    description:
      "Located on the Easter Island, there are 1,000 moai, or extant monumental statues, created by the early Rapa Nui people, which some believe represented deified ancestors.",
    url: "https://react.dev/images/docs/scientists/RCwLEoQm.jpg",
    alt: "Three monumental stone busts with the heads that are disproportionately large with somber faces.",
  },
  {
    name: "Blue Nana",
    artist: "Niki de Saint Phalle",
    description:
      "The Nanas are triumphant creatures, symbols of femininity and maternity. Initially, Saint Phalle used fabric and found objects for the Nanas, and later on introduced polyester to achieve a more vibrant effect.",
    url: "https://react.dev/images/docs/scientists/Sd1AgUOm.jpg",
    alt: "A large mosaic sculpture of a whimsical dancing female figure in a colorful costume emanating joy.",
  },
  {
    name: "Ultimate Form",
    artist: "Barbara Hepworth",
    description:
      "This abstract bronze sculpture is a part of The Family of Man series located at Yorkshire Sculpture Park. Hepworth chose not to create literal representations of the world but developed abstract forms inspired by people and landscapes.",
    url: "https://react.dev/images/docs/scientists/2heNQDcm.jpg",
    alt: "A tall sculpture made of three elements stacked on each other reminding of a human figure.",
  },
  {
    name: "Cavaliere",
    artist: "Lamidi Olonade Fakeye",
    description:
      "Descended from four generations of woodcarvers, Fakeye's work blended traditional and contemporary Yoruba themes.",
    url: "https://react.dev/images/docs/scientists/wIdGuZwm.png",
    alt: "An intricate wood sculpture of a warrior with a focused face on a horse adorned with patterns.",
  },
  {
    name: "Big Bellies",
    artist: "Alina Szapocznikow",
    description:
      "Szapocznikow is known for her sculptures of the fragmented body as a metaphor for the fragility and impermanence of youth and beauty. This sculpture depicts two very realistic large bellies stacked on top of each other, each around five feet (1,5m) tall.",
    url: "https://react.dev/images/docs/scientists/AlHTAdDm.jpg",
    alt: "The sculpture reminds a cascade of folds, quite different from bellies in classical sculptures.",
  },
  {
    name: "Terracotta Army",
    artist: "Unknown Artist",
    description:
      "The Terracotta Army is a collection of terracotta sculptures depicting the armies of Qin Shi Huang, the first Emperor of China. The army consisted of more than 8,000 soldiers, 130 chariots with 520 horses, and 150 cavalry horses.",
    url: "https://react.dev/images/docs/scientists/HMFmH6m.jpg",
    alt: "12 terracotta sculptures of solemn warriors, each with a unique facial expression and armor.",
  },
  {
    name: "Lunar Landscape",
    artist: "Louise Nevelson",
    description:
      "Nevelson was known for scavenging objects from New York City debris, which she would later assemble into monumental constructions. In this one, she used disparate parts like a bedpost, juggling pin, and seat fragment, nailing and gluing them into boxes that reflect the influence of Cubism’s geometric abstraction of space and form.",
    url: "https://react.dev/images/docs/scientists/rN7hY6om.jpg",
    alt: "A black matte sculpture where the individual elements are initially indistinguishable.",
  },
  {
    name: "Aureole",
    artist: "Ranjani Shettar",
    description:
      'Shettar merges the traditional and the modern, the natural and the industrial. Her art focuses on the relationship between man and nature. Her work was described as compelling both abstractly and figuratively, gravity defying, and a "fine synthesis of unlikely materials."',
    url: "https://react.dev/images/docs/scientists/okTpbHhm.jpg",
    alt: "A pale wire-like sculpture mounted on concrete wall and descending on the floor. It appears light.",
  },
  {
    name: "Hippos",
    artist: "Taipei Zoo",
    description:
      "The Taipei Zoo commissioned a Hippo Square featuring submerged hippos at play.",
    url: "https://react.dev/images/docs/scientists/6o5Vuyu.jpg",
    alt: "A group of bronze hippo sculptures emerging from the sett sidewalk as if they were swimming.",
  },
];

// 使 UI 匹配当前 state
updateDOM();

let componentHooks = [];
let currentHookIndex = 0;

updateDOM();

function updateDOM() {
  currentHookIndex = 0;
  let output = Gallery();

  // 把 output 写入 DOM，并绑定事件处理函数
}

const [index, setIndex] = useState(0);

componentHooks[0] = [0, setIndex 对应的 setState]
currentHookIndex = 1

const [showMore, setShowMore] = useState(false);

componentHooks[1] = [false, setShowMore 对应的 setState]
currentHookIndex = 2

componentHooks
├── 槽位 0: [0,     setIndex]
└── 槽位 1: [false, setShowMore]

本次 Gallery 快照
├── index = 0
└── showMore = false

let sculpture = sculptureList[index];

return {
  onNextClick: handleNextClick,
  onMoreClick: handleMoreClick,
  header: `${sculpture.name} by ${sculpture.artist}`,
  more: `${showMore ? "Hide" : "Show"} details`,
  description: showMore ? sculpture.description : null,
  imageSrc: sculpture.url,
  imageAlt: sculpture.alt,
};

index = 0
    ↓
sculpture = sculptureList[0]
    ↓
header、imageSrc、imageAlt 使用第一件雕塑

showMore = false
    ↓
more = "Show details"
description = null

function handleNextClick() {
  setIndex(index + 1);
}

nextButton.onclick = output.onNextClick;

第 1 次 Gallery()
├── index₁ = 0
└── handleNextClick₁ → 读取 index₁

第 2 次 Gallery()
├── index₂ = 1
└── handleNextClick₂ → 读取 index₂

function setState(nextState) {
  pair[0] = nextState;
  updateDOM();
}

handleNextClick
├── 记住本次渲染的 index
└── 记住 setIndex
          └── setIndex 记住 componentHooks[0] 中的 pair

handleMoreClick
├── 记住本次渲染的 showMore
└── setShowMore 记住 componentHooks[1] 中的 pair

function handleMoreClick() {
  setShowMore(!showMore);
}

index = 1
showMore = false

setShowMore(!false);
// setShowMore(true)

componentHooks
├── 槽位 0: [1,    setIndex]
└── 槽位 1: [true, setShowMore]

index = 1
showMore = true
more = "Hide details"
description = sculpture.description

currentHookIndex = 2

currentHookIndex = 0;

componentHooks     → 保留，里面是跨渲染的状态
currentHookIndex   → 归零，新一轮从第一个 Hook 开始匹配

第一个 useState  → 创建 componentHooks[2]
第二个 useState  → 创建 componentHooks[3]

componentHooks   = 保存数据的磁带
currentHookIndex = 磁带读取头

新一轮渲染开始时，磁带仍保留，但读取头必须回到起点。

function Gallery({ canShowDetails }) {
  const [index, setIndex] = useState(0);

  if (canShowDetails) {
    const [showMore, setShowMore] = useState(false);
  }

  const [theme, setTheme] = useState("light");
}

槽位 0 → index
槽位 1 → showMore
槽位 2 → theme

槽位 0 → index
槽位 1 → theme 错误读到原来的 showMore

theme === false;

setTheme("dark");

function Gallery({ canShowDetails }) {
  const [index, setIndex] = useState(0);
  const [showMore, setShowMore] = useState(false);
  const [theme, setTheme] = useState("light");

  const shouldShowDetails = canShowDetails && showMore;
}

// 在真实 React 中正确
setIndex((i) => i + 1);

// 在当前教学实现中会把函数本身存进 pair[0]，导致后续代码出错

事件函数读取 index/showMore 快照
        ↓
setter 修改自己记住的 pair
        ↓
updateDOM 重置 Hook 游标
        ↓
Gallery 重新执行并读取新状态
        ↓
output 更新 DOM 和事件函数

// A
if (canShowDetails) {
  const [showMore, setShowMore] = useState(false);
}

// B
const [showMore, setShowMore] = useState(false);
const shouldShowDetails = canShowDetails && showMore;