Declare empty dictionary in codable class

Question

I want to send empty Dictionary like this

“visitor_attrs”: {}

I try to implement empty dictionary in a class. In the decoder I get the warning:

No 'decode' candidates produce the expected contextual result type 'Dictionary'

How can I do this?

var data: String
var event: String
var visitorAttrs: Dictionary<String, Any>

init(data: String, event: String) {
    self.data = data
    self.event = event
    self.visitorAttrs = [:]
}

private enum CodingKeys: String, CodingKey {
    case data
    case event
    case visitorAttrs = "visitor_attrs"
}

required public init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    self.data = try container.decode(String.self, forKey: .data)
    self.event = try container.decode(String.self, forKey: .event)
    self.visitorAttrs = try container.decode(Dictionary<String:Any>.self, forKey: .visitorAttrs)
}

public func encode(to encoder: Encoder) throws {
    var container = encoder.container(keyedBy: CodingKeys.self)
    try container.encode(self.data, forKey: .data)
    try container.encode(self.event, forKey: .event)
    try container.encode(self.visitorAttrs, forKey: .visitorAttrs)
}

Joakim Danielson Joakim Danielson · Accepted Answer · 2019-03-11T11:14:28

The error is caused by having Any as value in your dictionary and Any isn't decodable, replace it with String (or whatever data type you have) and this should work.

var visitorAttrs: Dictionary<String, String>

Fix this issue and you will get the expected behaviour

let item = Item(data: "data", event: "event") //I gave your class the name Item
let encoder = JSONEncoder()

do {
    let data = try encoder.encode(item)
    if let str = String(data: data, encoding: .utf8) {
        print(str)
    }
} catch {
    print(error)
}

Output is

{"event":"event","visitor_attrs":{},"data":"data"}

Declare empty dictionary in codable class

2 Answers