Constraint a generic type to be @NSManaged or @objc

Woody · February 3, 2022, 7:19pm

Question

Hi !
I have been looking for a way to constraint a generic type in a function or type to be a property marked with @NSManaged or at least @objc . Unfortunately I could not find anything tangible. Thus I would like to ask here if anyone knows if such a solution exists.

Here is an exemple of what I would like to achieve.

class User: NSManageObject {
    @NSManaged var isAdmin

    var isAuthenticated: Bool { !isAdmin }

    func request<V>(for keyPath: KeyPath<User, V>) where V: NSManaged {
        // ...
    }
}

user.request(for: \.isAdmin) // should compile
user.request(for: \.isAuthenticated) // should not compile

Context and motivation

I am working on a library named SafeFetching to build NSPredicates and NSFetchRequests with a DSL.
The goal is to offer a safe way to build such predicates to avoid a a runtime crash and to use types in the expression while being as expressive as possible.
I am satisfied with the results. For instance, here is an imaginary example of what I was able to achieve so far.

User.request()
    .all(after: 10)
    .where(\.isAdmin && \.score == 10 
        || \.name * .hasPrefix("Jo")
    )
    .nsValue // NSFetchRequest

But there is catch. I use NSExpression(forKeyPath:) to get the label: String property from the NSExpression. But it only works on types that can be used with Objective-C. This is fine as long as the targeted property is an attribute of the entity - which is often the case. But I would prefer to enforce the @NSManaged mark to avoid runtime crash to be able to really call this library "Safe".

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Constraint a generic type to be @NSManaged or @objc

Question

Context and motivation

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