Scroll State Machine UIの作り方｜スクロール演出を状態管理で整理する方法

if (scrollY > 200) {
  title.textContent = "STEP 2";
}
if (scrollY > 500) {
  title.textContent = "STEP 3";
}
if (scrollY > 800) {
  card.classList.add("active");
}

const progress = 0.0〜1.0 の値に正規化する
const state = progress に応じて 0, 1, 2 を返す
render(state)

<section class="scroll-state-demo" id="demo">
  <div class="demo-sticky">
    <div class="panel">
      <p class="eyebrow">Scroll State Machine</p>
      <h2 id="stateTitle">Intro</h2>
      <p id="stateText">
        スクロール位置を直接使うのではなく、まず state に変換してから UI を更新します。
      </p>

      <div class="state-dots" aria-label="State indicator">
        <span class="dot active"></span>
        <span class="dot"></span>
        <span class="dot"></span>
      </div>
    </div>
  </div>

  <div class="demo-content">
    <div class="step-block">
      <span class="step-label">STEP 1</span>
      <h3>Intro</h3>
      <p>まずは導入。スクロール演出全体の入り口です。</p>
    </div>

    <div class="step-block">
      <span class="step-label">STEP 2</span>
      <h3>Focus</h3>
      <p>次に注目ポイントへ。主役のメッセージや要素を見せます。</p>
    </div>

    <div class="step-block">
      <span class="step-label">STEP 3</span>
      <h3>Finish</h3>
      <p>最後はまとめ。流れの着地地点をはっきり見せます。</p>
    </div>
  </div>
</section>

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  font-family: sans-serif;
  background: #0f1115;
  color: #f5f7fb;
}

.scroll-state-demo {
  display: grid;
  grid-template-columns: minmax(260px, 420px) 1fr;
  gap: 40px;
  max-width: 1100px;
  margin: 0 auto;
  padding: 80px 24px;
}

.demo-sticky {
  position: relative;
}

.panel {
  position: sticky;
  top: 80px;
  padding: 28px;
  border-radius: 20px;
  background: linear-gradient(180deg, #171a21 0%, #101319 100%);
  border: 1px solid rgba(255,255,255,0.08);
}

.eyebrow {
  margin: 0 0 12px;
  font-size: 12px;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: #9aa4b2;
}

.panel h2 {
  margin: 0 0 16px;
  font-size: clamp(28px, 4vw, 44px);
}

.panel p {
  margin: 0;
  line-height: 1.8;
  color: #d6dbe4;
}

.state-dots {
  display: flex;
  gap: 10px;
  margin-top: 24px;
}

.dot {
  width: 10px;
  height: 10px;
  border-radius: 999px;
  background: rgba(255,255,255,0.18);
  transition: transform 0.25s ease, background 0.25s ease;
}

.dot.active {
  background: #ffffff;
  transform: scale(1.4);
}

.demo-content {
  display: grid;
  gap: 28px;
}

.step-block {
  min-height: 72vh;
  padding: 32px;
  border-radius: 20px;
  background: rgba(255,255,255,0.04);
  border: 1px solid rgba(255,255,255,0.06);
  display: flex;
  flex-direction: column;
  justify-content: center;
}

.step-label {
  display: inline-block;
  margin-bottom: 14px;
  font-size: 12px;
  letter-spacing: 0.12em;
  color: #9aa4b2;
}

.step-block h3 {
  margin: 0 0 12px;
  font-size: clamp(24px, 3vw, 36px);
}

.step-block p {
  margin: 0;
  line-height: 1.8;
  color: #d6dbe4;
}

@media (max-width: 820px) {
  .scroll-state-demo {
    grid-template-columns: 1fr;
    gap: 24px;
  }

  .panel {
    top: 16px;
  }

  .step-block {
    min-height: 52vh;
  }
}

const section = document.querySelector(".scroll-state-demo");
const title = document.getElementById("stateTitle");
const text = document.getElementById("stateText");
const dots = document.querySelectorAll(".dot");

const stateContent = [
  {
    title: "Intro",
    text: "スクロール位置を直接使うのではなく、まず state に変換してから UI を更新します。"
  },
  {
    title: "Focus",
    text: "state を1つ決めておけば、テキスト・背景・インジケーターなどをまとめて切り替えられます。"
  },
  {
    title: "Finish",
    text: "Scroll Story UI を場当たり的な実装から、整理された設計へ変えやすくなります。"
  }
];

let currentState = -1;

function clamp(value, min, max) {
  return Math.min(Math.max(value, min), max);
}

function getProgress() {
  const rect = section.getBoundingClientRect();
  const windowHeight = window.innerHeight;

  const total = rect.height - windowHeight;
  const current = -rect.top;

  if (total <= 0) return 0;

  return clamp(current / total, 0, 1);
}

function getStateFromProgress(progress) {
  if (progress < 0.33) return 0;
  if (progress < 0.66) return 1;
  return 2;
}

function renderState(state) {
  if (state === currentState) return;
  currentState = state;

  title.textContent = stateContent[state].title;
  text.textContent = stateContent[state].text;

  dots.forEach((dot, index) => {
    dot.classList.toggle("active", index === state);
  });
}

function update() {
  const progress = getProgress();
  const state = getStateFromProgress(progress);
  renderState(state);
}

window.addEventListener("scroll", update, { passive: true });
window.addEventListener("resize", update);
window.addEventListener("load", update);

const progress = clamp(current / total, 0, 1);

function getStateFromProgress(progress) {
  if (progress < 0.33) return 0;
  if (progress < 0.66) return 1;
  return 2;
}

title.textContent = stateContent[state].title;
text.textContent = stateContent[state].text;

if (state === currentState) return;