generic type from string - json

Question

EDIT: (initial example was too simplified, so I rewrote the code to be more specific)
based on http://holko.pl/2016/01/05/typed-table-view-controller/
I am trying to see if it is possible to set the generic parameter of a type from a string..
Say we have this code
protocol Updatable
{
associatedtype ViewModel
func updateWith(viewModel: ViewModel)
}
class ToasterCell: UITableViewCell
{
var toast: String?
func updateWith(viewModel: String) {
toast = viewModel
//Additional config...
}
}
extension ToasterCell: Updatable
{
typealias ViewModel = String
}
class PriceCell: UITableViewCell
{
var tagPrice: Float?
func updateWith(viewModel: Float) {
tagPrice = viewModel
//Additional config
}
}
extension PriceCell: Updatable
{
typealias ViewModel = Float
}
protocol CellConfiguratorType {
var reuseIdentifier: String { get }
var cellClass: AnyClass { get }
func updateCell(_ cell: UITableViewCell)
}
class MyTypeTest<Cell> where Cell: Updatable , Cell: UITableViewCell
{
let viewModel: Cell.ViewModel
let reuseIdentifier: String = String(describing: Cell.self)
let cellClass: AnyClass = Cell.self
init(viewModel: Cell.ViewModel) {
self.viewModel = viewModel
}
func updateCell(_ cell: UITableViewCell)
{
if let c = cell as? Cell
{
c.updateWith(viewModel: viewModel)
}
}
}
extension MyTypeTest: CellConfiguratorType{
}
let myTT1 = MyTypeTest<PriceCell>(viewModel: 3.76)
let myTT2 = MyTypeTest<ToasterCell>(viewModel: "Carpe Diem")
let data = [myTT1, myTT2] as [CellConfiguratorType] // data for the tableView
//register Cell calss ...
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell
{
let cellConf = data[indexPath.row]
let cell = tableView.dequeueReusableCell(withIdentifier: cellConf.reuseIdentifier)
cellConf.updateCell(cell)
return cell
}
And we want to make it so that the type T is set from a string we get from a JSON response.
//Some JSON {"list":[{"k":"Price","v":368.0},"{"k":"ToasterCell","v":"YOLO"},"{"k":"Toaster","v":"Space"},{"k":"PriceCell","v":1999}]}
the JSON value does not map directly to any object/class, So I need to use that key "k" to know witch class to use.
I tried using the string from the "k" value to setup the cell configurator.
(short example)
//for now let skip any logic in decoding the value / viewModel.
let myTT1 = MyTypeTest<NSClassFromString(list.first.k + "Cell")>(viewModel: list.first.v as Any)
All I got was following errors:
Cannot assign value of type 'T' to type 'AnyClass' (aka 'AnyObject.Type')
Use of undeclared type 'myTypeOBJ'
Is there a way to do this via a string, or do I really need to create a huge "if-else" structure for whatever type I could get from my JSON responses?
EDIT:
I tried to add an init to the CellConfigurator with a param of type Cell so it can infer the Type from the param it self.
init(viewModel: Cell.ViewModel, inferUsing: Cell){....}
where I could attempt to use this (but is does not work as the PATs is getting in the way)
func getSafeBundelName() -> String
{
if let namespace = Bundle.main.infoDictionary!["CFBundleExecutable"] as? String
{
return namespace
}
return ""
}
let cellClass = NSClassFromString("\(getSafeBundelName()).PriceCell") as? UITableViewCell.Type
let cell = cellClass?.init()
let myTT1 = MyTypeTest(viewModel: list.first.v as Any, inferUsing: cell)
I get the error that the Cell type can not be infered. If I try to use the cellClass in the <>
ex: MyTypeTest<cellType>(viewModel: 3.76) all it gives me is that "cellClass" is not declared. Looks to me I am hitting a dead end where PATs become impossible to infer in any way that I can see. I find this limitation very very sad.

Answer 1

You can do this if you downcast the result from NSClassFromString to a type with a known initializer that you can call. If it will always be Toaster or a subclass then you can do:
if let myTypeOBJ = NSClassFromString("Toaster") as? Toaster.Type {
let test = myTypeTest(someOBJ: myTypeOBJ.init())
// test.someThing will be of type Toaster
}
In order for NSClassFromString to work you will also need to specify how you want the type name to be represented in objc, otherwise there will be some other stuff prepended to the type name:
#objc(Toaster)
class Toaster: NSObject