Plist encoding and decoding back to Dictionary [String:Decodable]

Question

I want to be able to save Dictionary of type [String:Codable] to plist and recover back as same. I tried this but it throws errors:

  let dictionary:[String:Any] = ["point":CGPoint(1,1), "value": 10, "key" : "testKey"] 

   do { 
        let url = FileManager.default.temporaryDirectory.appendingPathComponent("test.plist")
        try savePropertyList(dictionary, toURL: url)
        buildFromPlist(url)
      } catch {
        print(error)
    }
  


    private func savePropertyList(_ plist: Any, toURL url:URL) throws
   {
    let plistData = try PropertyListSerialization.data(fromPropertyList: plist, format: .xml, options: 0)
    try plistData.write(to: url)
   }

  private func buildFromPlist(_ url:URL)
  {
       do {
          let data = try Data(contentsOf: url)
          let decoder = PropertyListDecoder()
          let dictionary = try decoder.decode([String:Decodable], from: data)
          NSLog("\(dictionary)")
      } catch {
           NSLog("Error decoding \(error)")
      }
   
    
   }

But I get build errors in decode function:

  Value of protocol type 'Decodable' cannot conform to 'Decodable'; only struct/enum/class types can conform to protocols

I wonder how I read back the dictionary I saved to plist file?

EDIT: Even savePropertyList fails at runtime with objects such as CGPoint and CGAffineTransform with the error -

 "Property list invalid for format: 100 (property lists cannot contain objects of type 'CFType')" UserInfo={NSDebugDescription=Property list invalid for format: 100 (property lists cannot contain objects of type 'CFType')}

I wonder how can we write Codable objects to plist and recover back?

Answer 1

This cannot work because the type in the decoder.decode line must be a concrete type. And [String:Decodable] without trailing .self will throw another error.

The goal of the Codable protocol is to serialize custom structs or classes so make your dictionary a struct

struct MyType : Codable {
    let point : CGPoint
    let value : Int
    let key : String
}

and encode this. In the decoding part write

let item = try decoder.decode(MyType.self, from: data)