keypath-hashable.swift

struct Point: Hashable, KeyPathSchema {
  var x: Double, y: Double

  // We can describe the schema of our type as an array of key paths, one for each property
  // that makes up the value
  static let schema: [PartialKeyPath<Point>] = [\Point.x, \Point.y]
}

// By defining a protocol for type schemas...
protocol KeyPathSchema {
  static var schema: [PartialKeyPath<Self>] { get }
}

// ...we can provide default implementations of other protocols for reflectiony things that
// want to walk all of the fields of a type, such as equality, hashing, comparison, printing,
// etc. Here's an example of == and hashValue:
extension Hashable where Self: KeyPathSchema {
  static func ==(a: Self, b: Self) -> Bool {
    for key in Self.schema {
      guard let aValue = a[keyPath: key] as? AnyHashable,
            let bValue = b[keyPath: key] as? AnyHashable,
            aValue == bValue
      else {
        return false
      }
    }
    return true
  }
  
  var hashValue: Int {
    var hash = unsafeBitCast(Self.self, to: Int.self)
    for key in Self.schema {
      if let value = self[keyPath: key] as? AnyHashable {
        // You should use a real hash combining function here
        hash = hash &* 17 ^ value.hashValue
      }
    }
    return hash
  }
}

For those who's want that auto-generated for your own types, you could even use Sourcery to generate that automatically 😉

See AliSoftware/SourceryTemplates#4