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73 lines
3.3 KiB
HTML
73 lines
3.3 KiB
HTML
<!doctype html>
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<meta charset="utf-8">
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<title>28 IntersectionObserver visibility</title>
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<link rel="stylesheet" href="../_style.css">
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<script src="../_lib.js"></script>
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<body>
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<main>
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<p>Goal: scroll until the hidden <b>TARGET</b> button comes into view, then click it.
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Page uses an <code>IntersectionObserver</code> to track when TARGET crosses the
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viewport. Real scrolling produces a sequence of intersection updates with
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gradually increasing <code>intersectionRatio</code> (0 → 1) across animation
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frames. Programmatic teleports (<code>scrollIntoView({behavior:"instant"})</code>,
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direct <code>scrollTop=</code>) produce exactly one observer callback with
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<code>ratio≈1</code> and no rAF samples in between — fail.</p>
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<div id="scroller" style="height:240px;overflow:auto;border:1px solid #555;background:#fff;color:#111">
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<div style="height:1600px;padding:8px">keep scrolling …</div>
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<button id="target" style="margin:0 auto 1600px;display:block;padding:14px 22px;background:#1f7a1f;color:#fff;border:0;border-radius:6px">TARGET</button>
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</div>
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</main>
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<script>
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Challenge.init({ id: "intersection-visibility", instructions: "scroll until TARGET visible, then click" });
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const scroller = document.getElementById("scroller");
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const target = document.getElementById("target");
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const ratioSamples = [];
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const rafSamples = [];
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const io = new IntersectionObserver((entries) => {
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for (const e of entries) ratioSamples.push({ r: e.intersectionRatio, t: performance.now() });
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}, { root: scroller, threshold: Array.from({length: 21}, (_,i) => i/20) });
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io.observe(target);
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// Track rAF ticks during which scroll was happening.
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let lastScrollTop = scroller.scrollTop;
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let scrollingFrames = 0;
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function tick() {
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if (scroller.scrollTop !== lastScrollTop) {
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scrollingFrames++;
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rafSamples.push({ st: scroller.scrollTop, t: performance.now() });
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lastScrollTop = scroller.scrollTop;
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}
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requestAnimationFrame(tick);
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}
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requestAnimationFrame(tick);
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target.addEventListener("click", (e) => {
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if (!e.isTrusted) return Challenge.fail("TARGET click isTrusted=false");
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// Need the target to actually be in the viewport at click time.
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const rect = target.getBoundingClientRect();
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const sRect = scroller.getBoundingClientRect();
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const inViewport = rect.top < sRect.bottom && rect.bottom > sRect.top;
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if (!inViewport) return Challenge.fail("TARGET clicked while still off-screen");
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// Require at least a gradient of intersection ratios from low to high.
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const lows = ratioSamples.filter(s => s.r > 0 && s.r < 0.3).length;
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const mids = ratioSamples.filter(s => s.r >= 0.3 && s.r < 0.7).length;
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const highs = ratioSamples.filter(s => s.r >= 0.7).length;
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Challenge.log("io", { samples: ratioSamples.length, lows, mids, highs, scrollingFrames });
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const bad = [];
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if (ratioSamples.length < 4) bad.push(`only ${ratioSamples.length} IntersectionObserver samples (teleport)`);
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if (lows === 0) bad.push("no low-ratio IO samples — target appeared instantly");
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if (mids === 0) bad.push("no mid-ratio IO samples — no smooth approach");
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if (scrollingFrames < 5) bad.push(`only ${scrollingFrames} rAF frames saw scroll motion`);
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if (bad.length) Challenge.fail(...bad);
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else Challenge.pass(`IO gradient seen: lows=${lows} mids=${mids} highs=${highs}, rafFrames=${scrollingFrames}`);
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});
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</script>
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</body>
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