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The Body Runs Every Frame

August 05, 2026

The Problem

Hello Weather’s watch app has an hourly strip - 24 hour columns you drag horizontally. On hardware, the drag was choppy, and it was choppy for a reason that generalizes to almost any SwiftUI list: during a drag, the body runs every frame, and computed properties don’t know they’re in a loop.

The strip’s drag is a @GestureState driving an .offset through a computed property, so every touch event re-evaluates the whole body. Inside that body is a ForEach over 24 hours, and inside the ForEach are reads of computed properties that are invariant across the loop:

// In a shared extension - every one of these is a computed property.
extension HourlyView {
    var hourlyData: [Forecast.Hour] {
        viewModel.weather?.forecast?.hourly?.data ?? Forecast.Fallback.hourlyData
    }

    var hourly: [Forecast.Hour] {
        Array(hourlyData.prefix(min(hourCount, hourlyData.count)))
    }

    var maxHourlyTemp: Int {
        let safeCount = min(hourCount + 2, hourlyData.count)
        return (hourlyData.prefix(safeCount).map { $0.temperature ?? 0 }.max() ?? 0).toInt
    }
    // ...minHourlyTemp and showHourlyPrecips, same shape
}

Each looks innocent alone. But hourly[index] materializes a fresh array per subscript, each hour column reads maxHourlyTemp, minHourlyTemp, and showHourlyPrecips (each a prefix-and-map over the data), and the day-boundary check between adjacent hours constructed a Calendar per comparison - 23 of them for 24 hours. Totaled up: roughly 142 array allocations and 23 Calendar constructions per body evaluation, which during a drag means per frame, on a watch.

Nobody wrote that. It’s what accretes when “add a computed var to the shared extension” is the path of least resistance for four years, and it’s invisible until a gesture makes the body hot.

The Fix Is a let

SwiftUI evaluates a computed property every time it’s read; it has no memoization. But a let at the top of a @ViewBuilder evaluates exactly once per body pass. So the fix is almost embarrassingly small - snapshot the invariants above the loop:

private var hourlyChart: some View {
    let hours = hourly
    let maxTemp = maxHourlyTemp
    let minTemp = minHourlyTemp
    let showPrecips = showHourlyPrecips
    let dayCalendar = settingsManager.hourlyIsGrouped
        ? viewModel.weather?.forecast?.currentCalendar
        : nil

    return HStack(spacing: 2) {
        ForEach(hours.indices, id: \.self) { index in
            let hour = hours[index]

            if index > 0,
               let calendar = dayCalendar,
               let currentTime = hour.time,
               let previousTime = hours[index - 1].time,
               calendar.startOfDay(for: currentTime) != calendar.startOfDay(for: previousTime) {
                DaySeparatorView(day: currentTime)
            }

            Hour(max: maxTemp, min: minTemp, bar: hour, showPrecips: showPrecips)
        }
    }
}

Per body pass: one array, one max, one min, one precip scan, one calendar. Two details worth noting because they bit us:

One ForEach, one afternoon. Except a three-agent audit then went looking for the same class across the app, watch, and widget targets - and found it in 51 files. Same shape everywhere: settings pickers rebuilding localized name dictionaries per option, a radar legend measuring label widths per label per animation frame, a locations list running three JSON decodes per row, stats charts re-deriving ranges per data point.

Hoisting Rules, Learned the Hard Way

The sweep that fixed all 51 files went through two adversarial review rounds (seven independent reviewers, run per the practice in /adversarial-review-rounds/), and the reviewers’ confirmed findings turned “hoist the invariants” from an instinct into a ruleset. This is the part worth stealing, because every one of these rules exists because the naive hoist was wrong:

Hoist only what is provably invariant AND unconditionally evaluated. These are two separate tests. A sparkline view read its precip range only behind if showsCurve - hoisting that read above the guard made it run for every daily row, including rows where the server’s min...max bounds could be inverted, which traps ClosedRange at construction. The hoist widened both the performance cost and the crash surface. If the original read sat behind a condition, the hoist keeps the same condition (with placeholder values for the other branch), or it doesn’t happen.

Hoist INTO deferred builders, never out of them. Menu and Picker content closures don’t evaluate at body time - they evaluate at presentation time. A names dictionary hoisted from inside a Menu builder up to body scope changes freshness: the user now sees a snapshot from whenever the body last ran, not from when they opened the menu. Round two of review moved one of ours back inside each menu builder for exactly this reason. The flip side is free wins: work already inside a deferred builder that runs per option can be hoisted to the top of that builder, and it still only costs anything when the menu opens.

Leave onAppear and action-closure reads live. Same principle, other direction: those closures run later than body, and they should see the world as it is when they fire, not a body-time snapshot.

Pass optionals through; don’t ?? "". Several hoists turned force-unwraps and dictionary lookups into parameters. Where the receiving view’s parameter is Optional, pass the optional: nil makes SwiftUI skip the subview entirely, while "" renders an empty shell that occupies layout. They are not the same view tree, and the diff that “just adds a fallback” is a visible behavior change.

Harden server-fed ranges while you’re in there. Any min...max built from network data gets clamped (lower...max(lower, upper)), and any stride gets a > 0 step guard - review found an empirically verified stride-by-zero trap sitting next to one of the hoists.

The meta-rule: a “mechanical” perf sweep isn’t mechanical. Every one of those findings came from a reviewer with no stake in the sweep being simple.

Green Everywhere, Merged Nowhere

Here’s the process half, and the more transferable lesson.

The all-at-once sweep PR was, by every formal measure, done. Build and unit-test gates green at every commit. Two adversarial review rounds survived, every finding adjudicated and fixed. Fifty-one files of consistent, pattern-applying change.

We closed it anyway.

Not because anything was wrong with it - because 51 files is beyond confident human review, and each review round had surfaced new issues, which is the strongest possible evidence that another round would too. “All checks passed” measures what the checks measure. The honest question at the merge call is “can a human vouch for this diff?”, and for 51 files of changes that are 90% mechanical and 10% judgment, the answer was no - the 10% hides in the 90%.

So the PR became a patch source instead of a merge candidate: closed, with its branch kept on origin under a stable name, and a plan doc decomposing it into 11 slices - one to four files each, ordered, with per-slice notes carrying exactly what an implementer needs. The two slices that carry deliberate behavior deltas (the range-clamping hardening, and a rewrite that stops force-unwrapping a nullable timestamp so a missing time skips a separator instead of crashing) are flagged in the table and must disclose those deltas in their PR bodies. In a sliced re-land, “mostly mechanical” is no longer an acceptable summary - each slice is either behavior-identical or it says what changed.

The extraction recipe matters more than it looks:

git diff main...fix/foreach-hoisting -- <this slice's paths>

applying the latest state of each file from the banked branch - never cherry-picking the original sweep commit alone. The review-fix commits amend the sweep, so commit one by itself contains exactly the bugs the reviewers caught. And since main drifts while slices land one at a time, each slice re-verifies its files against fresh main before applying. Each slice PR cites the banked branch and the reviews it already survived, so slice review is cheap: confirm the extraction is faithful and the slice is self-contained, not re-litigate the pattern.

Landed So Far

The watch strip went first, promoted out of the slice sequence into its own release-blocker lane. One file, byte-identical to the banked version, verified against branch history that the review-fix commits never touched it. Its PR states the honest expectation up front: finger-tracked dragging improves markedly, but the momentum glide after release stays juddery - the deceleration loop mutates state from a Task.sleep(16ms) loop, and watchOS has no display-link to align ticks to vsync, so cheaper frames can’t fix irregular presentation. That expectation is written down before the hands-on device check by the project owner, which is the decision gate for whether a separate paged-navigation rewrite (also built, also banked) ships or closes. Writing the expected outcome down before the check is what keeps the check honest.

The widget strips went second: pure hoists in the hourly and minutely widget views, where body cost matters most because widget rendering runs in a time-constrained context. A read-only mini-review returned zero findings and one good nit - with the hoists, an empty data array now evaluates the three getters once instead of zero times, which is safe but is exactly the kind of edge a per-slice review can actually hold in its head. That nit is the sliced approach working as designed.

The Audit Found More Than the Sweep Fixed

The same audit surfaced a Phase-2 list that the mechanical sweep deliberately excludes, because these need design rather than a let:

That’s the payoff of auditing a bug class instead of fixing a bug: the sweep repaired the sites, but the audit found the systems. Hoisting is the floor. The ceiling is making the expensive thing cheap once, for everyone.

Lessons Learned

How This Post Was Made

Prompt 1: “see recent work in ~/Code/helloweather, perhaps a blog post about our opus 4.8 agents and why we decided to do that? perhaps something about the swift testing + snapshots inspired by minitest-snapshots? anything else? bring me a list of potential post ideas for review.”

Prompt 2: “skip 4, 5, 6, 9 but create posts for each of the others in the 1-9 list. also add Four Answers to One Question, and Write the Rule, Not the Story – show me a concise version of your plan and then I can approve” — then “proceed, one pr per post”

Research by one Claude agent per repo mining git history since the previous post; this draft was written by a dedicated agent from that research plus the underlying commits and skill files, then reviewed before publishing.