Clyune Component GalleryBack-to-top Bubble · How it works & code
After scrolling past a certain distance, a circular button appears in the bottom-right corner. The progress ring around the button shows how much of the page has been scrolled.
Runtime logic
Visiting `/scroll-fx/totop`, DemoHost lazy-loads `demos/kits/kit-scroll.js` (registry `mod: 'kit-scroll'`) and calls `mount(host, { variant: 'totop' })`. mount resolves the variant to family `totop`; `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.save() — Saves the drawing state (transform, styles, clip); ctx.beginPath() — Starts a new path; ctx.arc() — Adds a circular arc (center, radius, start/end angle); ctx.fill() — Fills the current path; ctx.stroke() — Strokes the current path; ctx.moveTo() — Moves the pen without drawing; ctx.lineTo() — Adds a straight line to a point; ctx.restore() — Restores the saved drawing state; ctx.fillText() — Draws filled text; ctx.arcTo() — Adds an arc using tangent lines; ctx.closePath() — Closes the current sub-path; ctx.fillRect() — Fills a rectangle; ctx.rect() — Adds a rectangle to the path
Source code
frontend/src/demos/kits/kit-scroll.js: FAM.totop (L493-L505)
Code walkthrough
L493-505: Calls the factory `scFam()` to produce the full definition (spec + doc + build) from the options below. L493-493: Title shown in the info and control panels: “Back to top”. L493-493: Range/default of the count slider. L493-493: Extra demo-specific slider/toggle: Appearance threshold. L493-493: Range/default of the size slider. L493-493: Default main colour. L494-494: Help text of the count parameter. L494-494: Help text of the extra parameter. L494-494: Help text of the size parameter. L495-495: Preset buttons: name → parameter overrides (5). L496-496: Body of the “Notes” section. L497-497: Main formula of the “Formula” section. L497-497: Second formula line. L498-505: Per-frame draw function: draws with Canvas 2D `save`, `beginPath`, `arc`, `fill`, `stroke`, `moveTo` …; math via `Math.min`, `Math.floor`, `Math.max`, `Math.hypot`, `Math.round`; returns readouts `mode`, `cell`, `pageH` to the info panel.
Principles
Show = Progress > Threshold (Left Bandwidth 0.05) Progress Ring Angle = 360 12:00 · Progress Back to top: When the page scrolls more than a certain distance, a circle button emerges at the lower right corner, and the progress loop of the outer circle of the button shows how much. Clicking on the button will smooth-roll back to the top ({top: 0, behavior: "smooth"}). The button hides when the scroll distance is less than the threshold and avoids disturbing reading. 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.