11 — Challenges

Rules of engagement: attempt each challenge cold, then open hints in order — 💡 nudge → 🔧 approach → 📜 the answer’s shape. Each challenge names the chapter that teaches its material. They stack: do them in order and you end with the real app.

Challenge 1 — The walking skeleton (Chapters 03, 05)

Empty project → a List of the last 14 days as "Jul 5 — 5h 39m", fetched live from Supabase. No cache, no goals — just proof the spine conducts electricity.

💡 Nudge

You need exactly three pieces: a Decodable struct pair (SummaryRow → DaySummary → grandTotal.totalSeconds), one URLSession GET with two headers, and a view with .task { }. Resist building more.

🔧 Approach

Build the PostgREST URL with URLComponents: /rest/v1/summaries?select=date,data&order=date.desc&limit=14. Set apikey and Authorization: Bearer headers to the anon key. Decode with .convertFromSnakeCase. Keep state as LoadState<[SummaryRow]> (the enum from Chapter 02) and switch in the view body.

📜 Shape of the answer

~80 lines total: SupabaseClient.rows(…) (Chapter 05’s listing, minus generics if you like), two DTO structs, one @Observable model with func load() async, one List. Formatting: Duration.seconds(total) .formatted(.units(allowed: [.hours, .minutes])) — don’t hand-roll “5h 39m”.

Challenge 2 — DateOnly and the midnight monster (Chapters 04, 06)

Write DateOnly (a Hashable, Comparable, Codable year-month-day type) plus func dayKey(for session: SessionRecord, calendar: Calendar) -> DateOnly that attributes epoch-timestamped sessions to calendar days — then make these tests pass: a 23:50 session lands on its start day; a session spanning midnight splits its seconds across both days; the answers hold in America/Denver across a DST transition.

💡 Nudge

The bug you’re defending against: Date → ISO8601 string conversions default to UTC, so a 22:00 Denver session becomes “tomorrow.” Never let a bare Date decide what day it is — only a Calendar with the right timeZone may answer that.

🔧 Approach

Store DateOnly as three Ints. Construct from Date via calendar.dateComponents([.year, .month, .day], from:). For the split: calendar.startOfDay(for: start.addingTimeInterval(seconds)) gives the boundary; seconds before/after it belong to different keys. Return [(DateOnly, Double)] from a splitByDay helper rather than forcing one key.

📜 Shape of the answer

~60 lines + tests. The DST test: construct March 8 2026 01:30 in Denver, add a 2-hour session, assert total attributed seconds still equal the session’s seconds (conservation law — the assertion that catches naive + 86400 math).

Challenge 3 — The GoalEngine, test-first (Chapter 06)

Implement verdict(for:sessions:), windowAdherence, and currentStreak — written test-first against Chapter 06’s three edge cases (straddling sessions, overnight windows, rest days as success).

💡 Nudge

Everything is interval intersection in minutes-since-midnight space. Write overlap(_ a: Range<Double>, _ b: Range<Double>) -> Double first; both window adherence and the overnight case are calls to it.

🔧 Approach

Overnight window (end < start) = union of two ranges: [start, 1440) and [0, end) — sum the two overlaps. For streaks, walk days.reversed(), counting while goalMet (remember: rest day + zero coding ⇒ goalMet == true), and decide your policy for today (it’s incomplete — most humane: today never breaks a streak, it can only extend it).

📜 Shape of the answer

GoalEngine stays a pure struct, ~120 lines; the test file is longer than the engine. That ratio is correct — this is the code invoices and rings both trust.

Challenge 4 — Rings that feel Apple (Chapter 06)

The three nested rings with spring animation, overshoot lap past 100%, rest-day leaf state, and a week strip of seven mini-rings underneath.

💡 Nudge

Circle().trim(from:to:) + rotationEffect(.degrees(-90)) is 90% of it. The animation comes free from .animation(.spring(duration: 0.8), value: progress) — animate the data, don’t orchestrate.

🔧 Approach

Chapter 06’s listing is the answer for one ring. Composition: ZStack + increasing .padding. Week strip: HStack(ForEach(verdicts)) reusing the same ActivityRing at lineWidth: 5. Rest day: if verdict.isRestDay swap in Image(systemName: "leaf.fill") — views are values; branching is free.

📜 Shape of the answer

One ActivityRing (~35 lines) reused at three sizes and seven mini sizes. If you wrote two ring implementations, refactor — the reuse is the lesson. Bonus polish: .sensoryFeedback(.success, trigger: goalJustMet).

Challenge 5 — The burnout fixtures (Chapter 07)

Implement BurnoutAnalyzer.report(days:sessions:config:) and make the three narrative fixtures pass: Steady Eddie stays sustainable, Crunch escalates watch → overreaching with both reasons named, Recovery steps back down and fires the celebration event exactly once.

💡 Nudge

Build the two rolling means first and chart them in a test playground before writing thresholds — you’ll immediately see why rest-day zeros must stay in the denominator.

🔧 Approach

acute = days.suffix(7), chronic = days.suffix(28) means over totalSeconds. Each pattern signal is its own tiny pure function returning Signal?; the verdict is a fold over [Signal] + ACWR using Chapter 07’s escalation rules. “Fires exactly once” = compare against the previous verdict (persist last verdict in GoalConfig or a tiny @Model).

📜 Shape of the answer

Analyzer ~150 lines, five pure signal functions, one mapping function. Fixtures generated by helpers like days(hours: 4, weekdaysOnly: true, weeks: 8) + days(hours: 9, includeWeekends: true, weeks: 3) — readable as the story they tell.

Challenge 6 — Invoice PDF round-trip (Chapter 09)

Timesheet → InvoicePage SwiftUI view → PDF bytes via ImageRenderer → in-app PDFView preview → ShareLink. Plus the rounding and immutability tests from Chapter 09’s “prove it.”

💡 Nudge

Design InvoicePage at fixed 612 × 792 and preview it as a normal SwiftUI view first — get it beautiful before involving CGContext.

🔧 Approach

Chapter 09’s renderInvoicePDF listing is complete — the work is the view: Grid { GridRow { … } } for line items, .monospacedDigit() on all numbers, Decimal.FormatStyle.Currency(code:) for money. Preview via UIViewRepresentable wrapping PDFKit.PDFView; share via a temp file URL named CS-2026-014.pdf.

📜 Shape of the answer

~200 lines: view (120), renderer (30, from the chapter), preview wrapper (20), share plumbing (10). Acceptance test: AirDrop it to your Mac and ask “would I send this to a client?” — that bar, not pixel-perfection.

Challenge 7 — The ring on the home screen (Chapters 03, 05, 06)

A WidgetKit extension showing today’s ring + hours, refreshed on a sensible timeline, sharing data with the app via an App Group.

💡 Nudge

The widget is a separate process — it cannot call your repository or hit the network on demand. It reads a pre-computed snapshot the app leaves behind. Decide the snapshot’s shape first; the rest is plumbing.

🔧 Approach

App Group container (group.com.cesarnml.codingstatsfitness) + a tiny JSON file (WidgetSnapshot: Codable — progress, hours, isRestDay, asOf). App writes it after every sync; widget’s TimelineProvider reads it and emits a few entries with .after(15 min) policy. Reuse ActivityRing by moving it (and WidgetSnapshot) into a shared framework target — your first real multi-target moment (Chapter 03’s vocabulary pays off here).

📜 Shape of the answer

New target (~80 lines) + shared framework + ~10 lines in the app’s sync path. Gotchas the hints already defused: placeholder entries for App Review (Chapter 10), and containerURL(forSecurityApplicationGroupIdentifier:) returning nil = the App Group capability isn’t on both targets.

Boss fight (no hints)

Ship it. TestFlight for a week of real use, fix what annoys you, write the privacy label, submit for review. When the “Ready for Sale” email arrives, you’ve earned the right to call Chapter 10 boring.