Getting string from Swift 4 new key path syntax?

Question

How can you get a string value from Swift 4 smart keypaths syntax (e.g., \\Foo.bar)? At this point I\'m curious about any way at all, does not matter if it

Answer 1

Expanding on @Andy Heard's answer we could extend KeyPath to have a computed property, like this:

extension KeyPath where Root: NSObject {
    var stringValue: String {
        NSExpression(forKeyPath: self).keyPath
    }
}

// Usage
let stringValue = (\Foo.bar).stringValue
print(stringValue) // prints "bar"

Answer 2

Short answer: you can't. The KeyPath abstraction is designed to encapsulate a potentially nested property key path from a given root type. As such, exporting a single String value might not make sense in the general case.

For instance, should the hypothetically exported string be interpreted as a property of the root type or a member of one of its properties? At the very least a string array would need to be exported to address such scenarios...

Per type workaround. Having said that, given that KeyPath conforms to the Equatable protocol, you can provide a custom, per type solution yourself. For instance:

struct Auth {
    var email: String
    var password: String
}
struct User {
    var name: String
    var auth: Auth
}

provide an extension for User-based key paths:

extension PartialKeyPath where Root == User {
    var stringValue: String {
        switch self {
        case \User.name: return "name"
        case \User.auth: return "auth"
        case \User.auth.email: return "auth.email"
        case \User.auth.password: return "auth.password"
        default: fatalError("Unexpected key path")
    }
}

usage:

let name:  KeyPath<User, String> = \User.name
let email: KeyPath<User, String> = \User.auth.email
print(name.stringValue)  /* name */
print(email.stringValue) /* auth.email */

I wouldn't really recommend this solution for production code, given the somewhat high maintenance, etc. But since you were curious this, at least, gives you a way forward ;)

Answer 3

A bit late to the party, but I've stumbled upon a way of getting a key path string from NSObject subclasses at least:

NSExpression(forKeyPath: \UIView.bounds).keyPath

Answer 4

For Objective-C properties on Objective-C classes, you can use the _kvcKeyPathString property to get it.

However, Swift key paths may not have String equivalents. It is a stated objective of Swift key paths that they do not require field names to be included in the executable. It's possible that a key path could be represented as a sequence of offsets of fields to get, or closures to call on an object.

Of course, this directly conflicts with your own objective of avoiding to declare properties @objc. I believe that there is no built-in facility to do what you want to do.