Routing Problem

Routing.swift

import Foundation

// This is the equivalent to `CellDescriptor` https://talk.objc.io/episodes/S01E26-generic-table-view-controllers-part-2
struct Handler {
  // This is the problem... What I really need is `type` to be a concrete type that conforms to Decodable.
  // I don't want to force users to have to conform to some arbitrary class of mine.
  // In the case of UITableViewCell this worked out fine bc of the already-required inheritance. Is there a work around?
  let type: Decodable.Type
  let handle: (Decodable) -> Void
  
  init<T: Decodable>(handler: @escaping (T) -> Void) {
    type = T.self
    handle = { codable in
      handler(codable as! T)
    }
  }
}

// Here's where I set up the (simplified) routing aka try to store functions in a dictionary keyed by "endpoints":

var handlers = [String: Handler]()

func register<T: Decodable>(
  endpoint: String,
  with handle: @escaping (T) -> Void)
{
  handlers[endpoint] = Handler(handler: handle)
}

// I'd like to do something like this at the callsite:

register(endpoint: "hello") { (object: String) in
  // handle receiving messages for this endpoint here
}

// Here's what I'd like to do once I have data and know the endpoint:

func didReceive(data: Data, for endpoint: String) throws {
  guard let handler = handlers[endpoint] else { return }
  let object = try JSONDecoder().decode(handler.type, from: data) // Cannot invoke 'decode' with an argument list of type '(Decodable.Type, from: Data)'. Expected an argument list of type '(T.Type, from: Data)'
  handler.handle(object)
}