Clyune Component GalleryCover Reveal · How it works & code
The page starts with a large cover.
Runtime logic
Visiting `/scroll-fx/cover`, DemoHost lazy-loads `demos/kits/kit-scroll.js` (registry `mod: 'kit-scroll'`) and calls `mount(host, { variant: 'cover' })`. mount resolves the variant to family `cover`; `withControls` builds the control panel (6 presets + sliders). `createCanvasStage` creates the `<canvas>` (HiDPI ≤ 2×, ResizeObserver) and the info panel. The family’s `build(S)` initialises state and returns the per-frame callback. Every `requestAnimationFrame`: read controls, advance a speed-scaled clock (dt ≤ 50 ms), run the frame callback; returned readouts go to the info panel. Changing parameters: colour/speed/size apply live; structural ones remount; leaving the page cancels rAF, removes listeners and frees resources.
Environment
Canvas 2D: 2D canvas drawing; requestAnimationFrame: Display-synced animation loop; ResizeObserver: Canvas follows container size; Pointer Events: Unified mouse/touch/pen events; devicePixelRatio: Crisp HiDPI rendering (capped at 2×) · react 18.3.1, react-router-dom 6.30.6, vite 5.4.21
Dependencies
react 18.3.1
Imported APIs
withControls — Wraps the demo in the shared control panel (presets, sliders, reset); hot-updates or remounts on change; paramsFromSpec — Turns the spec (colour/speed/size/count/extras) into control-panel parameter definitions; createCanvasStage — Shared Canvas 2D stage: canvas + info panel, HiDPI, one rAF loop, pointer state, idle auto-pointer; hueOf — Hue of a colour (0–360°); clamp — Clamps a value to a range; TAU — Constant 2π; fmt — Number formatting (fixed decimals); F — Formula block HTML; S.clear — Clears with the background colour (translucent = trails); S.isAutoDemo — Whether auto demo is on; S.onFrame — Registers a per-frame callback; its return value feeds the info panel; ctx.createLinearGradient() — Creates a linear gradient; ctx.fillRect() — Fills a rectangle; ctx.beginPath() — Starts a new path; ctx.arc() — Adds a circular arc (center, radius, start/end angle); ctx.fill() — Fills the current path; ctx.fillText() — Draws filled text; ctx.moveTo() — Moves the pen without drawing; ctx.arcTo() — Adds an arc using tangent lines; ctx.closePath() — Closes the current sub-path; ctx.stroke() — Strokes the current path; ctx.save() — Saves the drawing state (transform, styles, clip); ctx.rect() — Adds a rectangle to the path; ctx.clip() — Turns the current path into the clipping region
Source code
frontend/src/demos/kits/kit-scroll.js: FAM.cover (L423-L437)
Code walkthrough
L423-437: Calls the factory `scFam()` to produce the full definition (spec + doc + build) from the options below. L423-423: Title shown in the info and control panels: “Cover Slides Out”. L423-423: Range/default of the count slider. L423-423: Extra demo-specific slider/toggle: Cover Height. L423-423: Range/default of the size slider. L423-423: Default main colour. L424-424: Help text of the count parameter. L424-424: Help text of the extra parameter. L424-424: Help text of the size parameter. L425-425: Preset buttons: name → parameter overrides (5). L426-426: Body of the “Notes” section. L427-427: Main formula of the “Formula” section. L427-427: Second formula line. L428-437: Per-frame draw function: draws with Canvas 2D `createLinearGradient`, `fillRect`, `beginPath`, `arc`, `fill`; math via `Math.max`, `Math.min`, `Math.round`; returns readouts `mode`, `cell`, `pageH` to the info panel.
Principles
Cover offset = 0.5 · scrollY, title transparency = 1 - scrollY / Cover High Contents y = cover high - scrollY Cover slides out: The page starts with a big cover. When you scroll down, the cover background moves up at half speed (vision), the title During the shift, the title fades and shrinks, and the contents card below slides up to cover the cover at normal speed. Back at the top, the cover appeared. Image simulates a scrollable web page: scrollY 表示页面已经向上滚过了多少像素，这里它自动来回滚动（右侧细条就是滚动条）。元素在屏幕上的位置 = Page Location − scrollY；The degree to which the elements enter the vision is usually used scroll Event or event IntersectionObserver Let's figure this out 0~1 The progress is driven by animations such as transparency, offset, scaling, etc.