The problem
While working on a Swift/iOS project, I had to fetch data via SwiftData and ended up with this predicated:
let predicate = #Predicate<Entry> {
$0.processingState != .processed && $0.processingState != .cancelled
&& ($0.processingState == .failed && $0.failure != nil && $0.failure.count < 5) // Failed but has only failed less than 5 times
}
The next build I ran froze and I cancelled it, cleared the build cache and tried again, that did not work. Eventually, I let it run for however long it wanted and ended up with this strange error message:
Take 1
The expression looks simple enough to not cause problems but turns out #Predicate doesn’t like enums and the compiler just has no idea how to handle that (uh?). This is the first workaround I tried to fix this stupid issue:
let processed = ProcessingState.processed
let cancelled = ProcessingState.cancelled
let failed = ProcessingState.failed
let predicate = #Predicate<Entry> {
$0.processingState != processed
&& $0.processingState != cancelled
&& ($0.processingState == failed && $0.failure != nil && $0.failure.count < 5) // Failed but has only failed less than 5 times
}
If you are wondering, no, the code below will not work, it requires you to explicitly “freeze” the values OUTSIDE the predicate (as we did above).
let predicate = #Predicate<Entry> {
$0.processingState != ProcessingState.processed
&& $0.processingState != ProcessingState.cancelled
&& ($0.processingState == ProcessingState.failed && $0.failure != nil && $0.failure.count < 5) // Failed but has only failed less than 5 times
}
Take 2
There’s another problem here, Entry.failure is optional, the compiler also doesn’t like optionals in #Predicate, so this in fact still won’t compile! Here’s how I fixed it:
- Make
failurenot optional but give it a default value
@Model
class Entry {
...
- var failure: Failure?
+ var failure: Failure = Failure(reason: .none, message: "", count: 0)
...
}
NOTE: SwiftData doesn’t like this either! SwiftData/CoreData will most likely fail to initialize if you don’t handle the migrations manually now (or if you already have data in there). Also, this is an absolute mess now,
failureshould be nil by default, but nope, the compiler does not like it and I have to handle trivial things like uniqueness and nil-ability in code (even SQLite handles these things fine, I probably should have just used that but I wanted free iCloud sync :eye_roll:).
- Remove the nil check (because I suppose this counts as an
Optionalcheck and it doesn’t like it still?)
let predicate = #Predicate<Entry> {
$0.processingState != processed
&& $0.processingState != cancelled
- && ($0.processingState == failed && $0.failure != nil && $0.failure.count < 5) // Failed but has only failed less than 5 times
+ && ($0.processingState == failed && $0.failure.count < 5) // Failed but has only failed less than 5 times
}
TLDR;
SwiftData has a lot of annoying limitations, especially when used with CloudKit (you literally cannot have unique or non-optional values), the compile errors are not very useful and you have to write hacky code to get it to work! The compiler does not like a lot of things and you WILL be forced to write horrible code to get around these senseless limitations; oh yeah, there are like a thousand ways to do almost every single thing too it seems, and endless syntax.
I might resort to just dealing with the devil I know (React Native) or Tauri Mobile for the tiny mobile apps I make for myself. I definitely have a lot of skill issues when it comes to Swift and the whole shebang surrounding it but this time, it wasn’t my fault.