UI Prototyping Skill

Task lists and specs commit you to one layout before you've seen the alternatives. The cost is quiet but large: you anchor on the first arbitrary structure an agent guessed, then spend the rest of the phase fighting feature creep around it. This skill front-loads divergent exploration — many real, named SwiftUI variations you flip between in Xcode's canvas — so the layout you carry into /apple:plan is one you chose, not one you defaulted into.

> Agents are collaborators, not designers. You always have final say. Go wide, remix, repeat.

Why this exists (the trap it removes)

A vague prompt — "make a UI for a book club" — produces one arbitrary layout, silently guesses at features you never asked for (polling? a photo gallery?), and anchors you on a flawed start. By the time you've bent it to the features you do want, it looks clunky and inelegant. Three disciplines remove the trap, and this skill enforces all three:

| Discipline | What it means | What it prevents | |---|---|---| | Specificity | Bake the exact features in; nothing extra | Feature creep, arbitrary navigation elements | | Stylistic intent | Name the mood — warm coffee-shop palette? paper & typography? clean editorial? | Point-of-view-less, generic layouts | | Multiplicity | Ask for many divergent options at once | Anchoring on the first guess |

Early is your only cheap chance to explore wide. Spend it.

Input

• .planning/APP.md — features, audience, platform. The feature list is exactly what you bake into every variation.
• Optional target screen (arg). If none, prototype the primary screen implied by <mvp-features>.
• If APP.md is missing, that's fine — elicit the essentials first via AskUserQuestion (this is the novice front door): the one screen, its 3–5 must-have features, the mood, and 0–2 reference apps. You do not need to know how to write the "good prompt" — the skill assembles it from your answers.

The method — go wide → remix → make lived-in → tune

Stage 1 — Go wide (divergent variations)

1. Assemble the brief (via AskUserQuestion if it isn't already pinned down in APP.md): the screen, the 3–5 must-have features (baked in — this is what stops feature creep), the mood cue, reference apps.
2. Generate 6–10 genuinely divergent variations of that ONE screen. Divergence is measured by organizing metaphor, not paint: vary tab vs. grid→detail vs. single-scroll vs. dashboard/standings, the navigation shape, typography (New York / serif vs. SF), density, and color/mood. Two variations that differ only in tint = one wasted slot.
3. Each variation gets its own #Preview with a descriptive, memorable name — "Cozy", "Editorial", "Club Hub", "Blueprint Atelier". The name is the handle you'll use when you remix. Put each in its own file (or a clearly separated struct) so the named previews render side by side in Xcode's canvas.

swift #Preview("Cozy") { CozyHomeView(club: .sample) } #Preview("Club Hub") { ClubHubHomeView(club: .sample) }

1. Only the requested features appear. If the app needs no polling or gallery, no variation invents one.
2. It compiles. xcodebuild build (or the canvas) — you're carrying this forward, not screenshotting a dead mock.

Present the named set; the user flips between them and reacts. Expect duds ("…well, it was worth a shot") — range is the goal, not a batting average.

Stage 2 — Remix + make it lived-in

1. Remix. The user names what they liked by preview name and element — "the standings board from Club Hub + the current-book cover from Cozy." Generate new hybrid variations from only those elements, each its own new named preview. Go wide → remix → repeat until one direction feels right.
2. Lived-in content. Empty scaffolding lies about how a screen feels. Generate reusable sample models in their own file (delegate to generators/preview-data-generator) so you can edit them, and make the content plausible for the audience — a book club's discussions are about books, not lorem ipsum. Plausible also means messy, not idealized: prototype with real data ranges, because the layout that flatters a hilly San Francisco route must also survive a pancake-flat New York one (WWDC23). Ideal placeholders hide exactly the cases that break a design.
3. Edge-case previews. Think the states through yourself; don't let the agent silently pick the happy path. Cover at minimum:

• Empty / blank-slate — no meeting scheduled yet, zero items. Is there a call-to-action and an account/management entry, or does it just look broken?
• Unbounded growth — many members, long message threads, dozens of past items, long titles. Truncate, or an expand control? (The leaderboard that pushes the discussion off-screen is the classic bug.)
• Long input — does text truncate with .help()/detail access, or wrap to multiple lines?

Delegate the state-matrix generation to the swiftui-builder agent (it emits a #Preview per state); you decide which states matter.

Stage 3 — Tune key moments (a tuning panel)

Signature interactions — a cover-to-detail transition, a staggered list entrance — are where an app reads as considered or cheap. Don't hand-edit constants scattered across files; have the agent build a tuning panel.

1. Be specific about what you're tuning. Ease (duration) vs. spring (stiffness / damping / mass)? A single element, or a transition where views enter and leave the hierarchy?
2. Break the animation into named phases — "Phase 1: cover transitions to detail. Phase 2: rows stagger in." Phases give you and the agent a shared vocabulary; ask for controls per phase so you can inspect and retune one in isolation.
3. Lay the panel out side-by-side on a wide window, not a modal that obstructs the content — toggle a parameter and see the effect without context-switching. Ask for a resize control that moves the panel beside the UI on a larger canvas.
4. Tuning panels generalize beyond animation: swap app states, colors, fonts, visual offsets. Use design/animation-patterns for the curve knowledge; the panel is its runnable counterpart.
5. Prototype beyond static layout. Friction, inertia, device-motion effects (gyroscope parallax), and haptics are all promptable and panel-tunable the same way — and they're feel, only judgeable running on a device, not in a screenshot (WWDC26).
6. Guard it behind DEBUG / a launch arg so it never ships — same discipline as walkthrough's -uiTestSeed.
7. On device early; review as a demo app. Don't wait for "done" to leave the canvas — feel parameters (Digital Crown sensitivity, spring values) only tune while running live, so the earlier a prototype is on hardware the cheaper every iteration. And when it's time for a design review, bundle the competing prototypes into one on-device demo app rather than exporting stills: "no slide deck in the world can compare with one spectacular demo" (WWDC23).

Output: .planning/PROTOTYPE.md + real Swift

Record the decisions so the next session — and /apple:plan — inherits them:

markdown
# UI Prototype — [Screen], Phase [N]

## Variations explored (Stage 1)
| Preview name | Organizing idea | Verdict |
|--------------|-----------------|---------|
| Cozy | single-scroll, warm serif, current-book hero | ✅ base direction |
| Club Hub | tab bar + standings board | remix: take the standings board |
| Blueprint Atelier | grid → detail | ✗ too cold |

## Chosen direction & remix (Stage 2)
- Base: *Cozy*; grafted the standings board from *Club Hub*.
- Sample data: `PreviewData/BookClubSamples.swift` (editable, reused across previews).
- Edge cases covered: empty (added CTA + account entry), long member list (rank pinned + expand), long titles (truncate).

## Tuned moments (Stage 3)
- Cover→detail: spring(stiffness: …, damping: …); rows stagger 0.04s. Panel: `Debug/TransitionTuner.swift` (DEBUG only).

## Carry-forward
- Winning layout → `/apple:plan` `<flows>` + `<apple-patterns>`; keep the Swift files.

Plus the actual files in the project: the variation views, the sample-data file, and any tuning panel (DEBUG-guarded).

Honest limits

• Agents propose; you decide. Don't delegate "which is best" — that judgment is the design work. A pile of variations with no chosen direction is not a result.
• Divergent ≠ good. Some variations will flop; that's the method working, not failing.
• Nothing beats real users. Lived-in sample content is a head start on the feedback stage, not a substitute for people using your app with their own content.
• The tuning panel is scaffolding — DEBUG-only, guarded, and removed or gated before release.

Cadence & integration

• Run early, once per signature screen — after /apple:new-app, or at the start of a UI-heavy phase, before /apple:plan locks a layout. Exploration is cheapest before the "real" screen exists.
• Feeds forward: the winning variation is real SwiftUI — hand its structure to /apple:plan (<flows> / <apple-patterns>) and /apple:build; don't rebuild it.
• Bookends with flow-walkthrough / /apple:walkthrough: prototype the screen (diverge, choose, tune), then walkthrough the flow (verify transitions and dead-ends) once it's wired up. Explore → build → verify.
• Complements product/ux-spec: ux-spec formalizes one spec; this skill is how you find the one worth formalizing. Feed the winner in, or skip ux-spec for small apps.
• Reuses: generators/preview-data-generator (sample content), the swiftui-builder agent (edge-case #Preview matrix), design/animation-patterns (curve reference).