Design from app

Look at an app, measure it, and rebuild its design system in SwiftUI. The result is a design kit (design-kit skill) with the reference app's numbers, and one reference screen that matches the screenshot.

Guessing gets close. Measuring gets it right. Two scripts do the measuring, next to this skill:

• scripts/measure.swift: sizes, positions, text heights, and colors from a screenshot.
• scripts/compare.swift: a side-by-side, a diff, and the share of pixels that differ.

Run them with swift "${CLAUDE_SKILL_DIR}/scripts/measure.swift" .... All coordinates are points (pixels / 2 for a Retina capture).

1. Get the screenshots

Ask for screenshots, or take them with the clone skill. You need:

• The window only, at 2x, no desktop. Save them in design/reference/.
• One screen with the most parts on it (a sidebar, a list, a toolbar), plus close-ups of menus, dialogs, and dark mode if they matter.
• The window size: measure.swift info shot.png.

Check: info prints the window size in points, and every screenshot uses the same scale.

2. List the parts

Look at each screenshot and name every distinct part: window chrome, sidebar row, section header, primary action, list row, checkbox, date chip, icon button, divider. Start design/KIT.md from the design-kit skill's kit-spec.md with one section per part.

Check: every visible element on the main screenshot belongs to a part.

3. Measure

Fill design/KIT.md with numbers, not estimates. The recipes and the traps are in measuring.md. In short:

Check: every token and every part size in design/KIT.md has a number from a command.

4. Find the font and the icons

• Most apps on the Mac use the system font, even web apps. Start there. When the letters differ (a, g, t, digits), name the font and pick it or an open alternative with a license that allows bundling.
• Text sizes come from step 6: build, measure the same word in both images, scale.
• Icons: note the size, stroke weight, corners, and whether selected rows switch to a filled form. Match with SF Symbols at a light weight first; draw the ones that do not match as shapes (design-kit parts.md, Icons).

Check: design/KIT.md names the font, the icon source, and the icon size and weight.

5. Build the kit and one screen

Call the Skill tool with design-kit and build the tokens, the parts, the window chrome, and the gallery from design/KIT.md. Then build the main screenshot's screen from kit parts only, with the same data and the same window size.

Check: a snapshot of that screen has the same size in pixels as the reference.

6. Compare until it matches

swift "${CLAUDE_SKILL_DIR}/scripts/compare.swift" design/reference/main.png /tmp/<App>-shots/<page>-light.png /tmp/<App>-compare
swift "${CLAUDE_SKILL_DIR}/scripts/compare.swift" ... /tmp/<App>-compare-sidebar 0 0 220 300

Open side-by-side.png and diff.png with the Read tool. Then measure the same rows, edges, and words in both images, and fix the tokens, not the views. Order: window size, text sizes, columns and left edges, row heights, colors, icons.

Check: under 2% of pixels differ on the whole window, and every row and left edge is within 1.5 pt of the reference.

7. Make it yours

Before the app ships to anyone else, replace what belongs to the other company: its name, logo, illustrations, and any font or icon it owns. Keep the structure, the spacing, and the feel. Write the replacements in design/KIT.md.

Done when design/KIT.md has a measured number for every token and part, the kit's gallery shows every part, the reference screen passes step 6, and step 7 lists what was replaced.