Loved to see this entry in Xcode 16’s release notes:
Xcode 16 brings a new execution engine for Previews that supports a larger range of projects and configurations. Now with shared build products between Build and Run and Previews, switching between the two is instant. Performance between edits in the source code is also improved for many projects, with increases up to 30%.
It has been difficult at times to use SwiftUI previews when they sometimes just stop working with error messages leaving scratch head. Turns out, it comes with a hidden cost of Xcode 16 wrapping views with AnyView in debug builds which takes away performance. If you don’t know it only affects debug builds, one could end up on journey of trying to improve the performance for debug builds and making things worse for release builds. Not sure if this was ever mentioned in any of the WWDC videos, but feels like this kind of change should have been highlighted.
As of Xcode 16, every SwiftUI view is wrapped in an AnyView _in debug builds only_. This speeds switching between previews, simulator, and device, but subverts some List optimizations.
Add this custom build setting to the project to override the new behavior:
`SWIFT_ENABLE_OPAQUE_TYPE_ERASURE=NO`
Wrapping in Equatable is likely to make performance worse as it introduces an extra view in the hierarchy for every row.
Curt Clifton on Mastodon
Fortunately, one can turn off this if this becomes an issue in debug builds.
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