Swift 6 and Strict Concurrency Checking

[leandroalonso]

This is a summary of an audit trying to build the project using Xcode 16.0 Beta 1 and Swift 6 enabled.

Given Swift 6 has Complete Strict Concurrency Checking the app doesn't build. These are the build errors organized by the number of occurrences:

• [ ] 79 occurrences: Reference to class property 'appSettings' is not concurrency-safe because it involves shared mutable state
• [ ] 19 occurrences: Static property 'X' is not concurrency-safe because it is non-isolated global shared mutable state
• [ ] 12 occurrences: Capture of 'self' with non-sendable type '?' in a @Sendable closure
• [ ] 12 occurrences: Main actor-isolated property '?' can not be referenced from a non-isolated context
• [ ] 11 occurrences: Call to main actor-isolated instance method '?' in a synchronous nonisolated context
• [ ] 7 occurrences: Main actor-isolated class property '?' can not be referenced from a non-isolated context
• [ ] 6 occurrences: Call to main actor-isolated initializer '?' in a synchronous nonisolated context
• [ ] 4 occurrences: Class property '?' is not concurrency-safe because non-'Sendable' type '?' may have shared mutable state
• [ ] 3 occurrences: Main actor-isolated property '?' can not be mutated from a non-isolated context

Discussion

The build errors related to appSettings are somewhat straightforward to deal with: we can just delete it as it's part of a feature we abandoned.

The big issues are on the usage of static properties (and yes, Singletons): we'll have to ensure each and every one of them is concurrent safe. PodcastDataManager is an example of a concurrent-safe class, which updates the cachedPodcasts in a safe way.