Does Xcode's SwiftUI Preview automatic reload reliably reflect source changes across complex view hierarchies?

SwiftUI Previews run in an isolated preview process with distinct build settings from the main app target, and preview invalidation is triggered by file saves, dependency changes, and manual actions. However, the exact conditions for automatic reload are not fully specified and can appear inconsistent across projects, especially when complex view hierarchies or external resources are involved.

Developers face an unresolved decision about trusting automatic preview updates versus forcing manual refreshes to ensure fidelity. Build settings such as optimization level and conditional compilation flags can differ between the main target and the preview build, leading to divergent runtime behavior that is not always obvious. This behavior is version-sensitive and may change between Xcode releases, while preview reliability also depends on project structure, package dependencies, and system resources.

Which specific file-change events guarantee a preview rebuild versus those that may be silently ignored? Does the presence of Swift Package dependencies alter the invalidation logic in a predictable way? Can developers programmatically detect when a preview is stale relative to the current source state?