Hello,

To retrieve information from Core Data we have to create fetch requests. In this project we have made a generic fetch request in the FilteredList struct as shown below

  init(filterKey: String, filterValue: String, sortDescriptor: NSSortDescriptor, @ViewBuilder content: @escaping (T) -> Content) {
    fetchRequest = FetchRequest<T>(entity: T.entity(), sortDescriptors: [sortDescriptor], predicate: NSPredicate(format: "%K BEGINSWITH %@", filterKey, filterValue))
    self.sortDescriptor = sortDescriptor
    self.content = content
  }

In my project I have 3 entities: Candy, Country, and Singer.

In the main view I call this struct in order to display a list of Singers. But I am not sure how Core Data knows that I want Singers (as opposed to Candies or Countries). It is either due to the key path or argument label provided to the closure. Originally I thought both of these served different roles, but after inspection it feels like it must be one of them.

FilteredList(filterKey: "lastName", filterValue: lastNameFilter, sortDescriptor: NSSortDescriptor(keyPath: \Singer.firstName, ascending: true)) { (singer: Singer) in
              Text("\(singer.wrappedFirstName) \(singer.wrappedLastName)")
          }

Can someone shed some light on this?

It knows because of the closure you pass as the content parameter. That closure is of type (T) -> Content, with T and Content being generic placeholders. It's the placeholder T that tells Core Data which entity you want.

So since in this example the parameter to the content closure is (singer: Singer), that means T is a Singer. Core Data then can use that when you build the FetchRequest:

FetchRequest<T>(entity: T.entity(), sortDescriptors: [], predicate: NSPredicate(format: "%K BEGINSWITH %@", filterKey, filterValue))

Which in this example becomes:

FetchRequest<Singer>(entity: Singer.entity(), sortDescriptors: [], predicate: NSPredicate(format: "%K BEGINSWITH %@", filterKey, filterValue))