Hey folks!
Recently I’ve been working on a small library which leverages the Swift 4
Codable protocol
and KeyPaths to provide a Swift-y interface to CoreData. (It maps back and
forth between
native, immutable Swift structs and NSManagedObjects). In doing so I found a
couple of
frustrating limitations to the KeyPath API. Firstly, KeyPath does not provide
the name of the
property on the type it indexes. For example, if I have a struct:
struct Person {
let firstName: String
let lastName: String
}
let keyPath = \Person.firstName
But once I have a keyPath, I can’t actually figure out what property it
accesses.
So, I wind up having to make a wrapper:
struct Attribute {
let keyPath: AnyKeyPath
let propertyName: String
}
let firstNameAttribute = Attribute(keyPath: \Person.firstName, propertyName:
“firstName”)
This forces me to write out the property name myself as a string which is very
error prone.
All I want is to be able to access:
keyPath.propertyName // “firstName”
It would also be nice if we provided the full path as a string as well:
keyPath.fullPath // “Person.firstName"
Also, if I want to get all of the attributes from a given Swift type, my
options are to try to hack
something together with Mirrors, or forcing the type to declare a function /
computed property
returning an array of all of its key path / property name pairings. I would
really like to be able to
retrieve a type-erased array of any type’s key paths with:
let person = Person(firstName: “John”, lastName: “Doe”)
let keyPaths = Person.keyPaths
let firstNameKeyPath = keyPaths.first { $0.propertyName = “firstName” } as!
KeyPath<Person, String>
let firstName = person[keypath: firstNameKeyPath] // “John"
And finally, without straying too far into Objective-C land, it would be nice
if we could initialize key paths
with a throwing initializer.
let keyPath = try Person.keyPath(“firstName”) // KeyPath<Person, String> type
erased to AnyKeyPath
let keyPath = AnyKeyPath(“Person.firstName”)
Let me know what you think about any / all of these suggestions!
Thanks,
Logan
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