feat(core): Freeze UIActivityIndicatorView spinners during snapshots - #279
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Disabling UIView and SwiftUI animations does not stop UIActivityIndicatorView, which drives its own layer animation. Indeterminate spinners, including the default SwiftUI ProgressView, were captured mid-rotation and produced flaky diffs. Swizzle startAnimating to a no-op from RenderingStrategy.setup so every rendering path skips spinners. Because hidesWhenStopped defaults to true, the indicator is hidden rather than frozen at a fixed frame. This matches the behavior the hosted Emerge runner previously injected outside the library.
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Sentry Snapshot Testing
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Set EMERGE_DISABLE_FREEZE_SPINNERS=1 to keep UIActivityIndicatorView animating, mirroring the existing EMERGE_DISABLE_FIX_TIME opt-out for the clock hook. watchOS ships UIKit without UIActivityIndicatorView, so guard the swizzle the same way other UIKit-only code in this target does. Mark the replacement method dynamic to follow the standard swizzling idiom.
Add a ModuleA preview containing an indeterminate ProgressView so the CI snapshot upload renders a spinner case. With spinner freezing in place the image contains only the label, and any regression shows up as a diff.
jamieQ
reviewed
Sep 15, 2026
Replace company-branded names in the new spinner opt-out and swizzled selector. Keep the Objective-C hook product-specific to avoid collisions, and update the environment-variable test and documentation together.
jamieQ
reviewed
Sep 15, 2026
| let original = class_getInstanceMethod(UIActivityIndicatorView.self, #selector(UIActivityIndicatorView.startAnimating)), | ||
| let replacement = class_getInstanceMethod(UIActivityIndicatorView.self, #selector(UIActivityIndicatorView.snapshotPreviews_frozenStartAnimating)) | ||
| else { return } | ||
| method_exchangeImplementations(original, replacement) |
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Maybe you all tried this in the past, but could/should we just disable core animation 'actions' around the point where the snapshots are rendered? Or does that not work for some reason?
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cc @noahsmartin
(not going to treat this as a blocker btw)
trevor-e
approved these changes
Sep 15, 2026
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Disabling UIView and SwiftUI animations does not stop
UIActivityIndicatorView, which drives its own layer animation. Indeterminate spinners, including the default SwiftUIProgressView, get captured mid-rotation and produce flaky diffs.This swizzles
startAnimatingto a no-op fromRenderingStrategy.setup(), so every rendering path gets it. SincehidesWhenStoppeddefaults to true, indicators are hidden rather than frozen at a fixed frame, which matches what the hosted runner produced and keeps existing baselines valid. iOS and tvOS only; AppKit has no equivalent view and watchOS has no activity indicator.Opt-out: set
SNAPSHOTS_DISABLE_FREEZE_SPINNERS=1in the test environment to keep spinners animating. Existing baselines that currently show a spinner will change once on upgrade, so this flag is the escape hatch for anyone who needs the old output.Verified end to end with a new
ModuleA Loading Indicatorpreview in MultiModuleDemo, which CI now renders and uploads. Locally, two runs with freezing on produced byte-identical images with no spinner, and two runs with the opt-out set rendered the spinner. Also covered by unit tests for the swizzle and the opt-out, the full package suite on iOS simulator, and builds of all schemes on macOS, Mac Catalyst, iOS, and watchOS.