Same Feature, Three Bodies

Architecture arguments are usually tribal. This bench makes them countable.

Same feature: bump Harbor’s build by one. Three ways to hold the state. A body-run counter on each panel — the ch6 meter, reused.

Midway — the split appears

After 1 bump (build 401):

Settled — five bumps

-mode settle stops at five. Build 405 everywhere.

MVVM and the reducer match on the meter. The difference between them is where the next change goes (a bump() method vs dispatch(.increment) through shipGateReduce), not how often SwiftUI redraws a silent label.

What each column is

MV (control). @State var build and the READY label sit in one body. Every +1 re-runs the whole panel — chrome has nowhere else to live.

MVVM. @Observable ShipGateVM with bump(). A digit child reads vm.build. A chrome child reads nothing. Observation invalidates only the digit.

Reducer. @Observable ShipGateStore holds ShipGateState. dispatch(.increment) runs a pure shipGateReduce. Same child split as MVVM — unidirectional write path, same body-run story.

Mini-exercise

Add a CREW label in the MVVM chrome that reads vm.build. Watch CHROME start climbing with DIGIT. Remove the read; it freezes again.

Challenges

1. Port the MV column to a UIKit UIViewController that mutates a model property — count viewDidLayoutSubviews instead of body runs.
2. Add a fourth column: VIPER-style presenter that pushes build into the view via a protocol; meter the view body only.
3. Make the reducer log every action to an on-screen tape (ch12 style) without bumping chrome.
4. Replace @Observable MVVM with ObservableObject + @Published and compare chrome runs to ch6’s legacy row.
5. Drive all three columns from one shared ticker without letting the bench parent own per-bump @State.

Key Points

• Observed mid (1 bump): MV BODY 2; MVVM/REDUCER DIGIT 2 / CHROME 1.
• Observed settle (5 bumps): MV BODY 6; MVVM/REDUCER DIGIT 6 / CHROME 1.
• MV is the control: chrome shares the body that reads build.
• MVVM and reducer can match on body runs when children isolate reads.
• Reducer uniqueness is the write path (dispatch → pure reduce), not the redraw count.
• Parent @State bump counters can erase the split — tickers belong on leaves that read them.
• Measure architecture with the same meter you use for observation (ch6).

Next up: images and memory — full-resolution vs downsampled thumbnails with a live memory readout.