How to pass a class type as a function parameter

Question

You are approaching it in the wrong way: in Swift, unlike Objective-C, classes have specific types and even have an inheritance hierarchy (that is, if class B inherits from A, then B.Type also inherits from A.Type):

class A {}
class B: A {}
class C {}

// B inherits from A
let object: A = B()

// B.Type also inherits from A.Type
let type: A.Type = B.self

// Error: 'C' is not a subtype of 'A'
let type2: A.Type = C.self

That's why you shouldn't use AnyClass, unless you really want to allow any class. In this case the right type would be T.Type, because it expresses the link between the returningClass parameter and the parameter of the closure.

In fact, using it instead of AnyClass allows the compiler to correctly infer the types in the method call:

class func invokeService<T>(service: String, withParams params: Dictionary<String, String>, returningClass: T.Type, completionHandler handler: ((T) -> ())) {
    // The compiler correctly infers that T is the class of the instances of returningClass
    handler(returningClass())
}

Now there's the problem of constructing an instance of T to pass to handler: if you try and run the code right now the compiler will complain that T is not constructible with (). And rightfully so: T has to be explicitly constrained to require that it implements a specific initializer.

This can be done with a protocol like the following one:

protocol Initable {
    init()
}

class CityInfo : NSObject, Initable {
    var cityName: String?
    var regionCode: String?
    var regionName: String?

    // Nothing to change here, CityInfo already implements init()
}

Then you only have to change the generic constraints of invokeService from <T> to <T: Initable>.

Tip

If you get strange errors like "Cannot convert the expression's type '()' to type 'String'", it is often useful to move every argument of the method call to its own variable. It helps narrowing down the code that is causing the error and uncovering type inference issues:

let service = "test"
let params = ["test" : "test"]
let returningClass = CityInfo.self

CastDAO.invokeService(service, withParams: params, returningClass: returningClass) { cityInfo in /*...*/

}

Now there are two possibilities: the error moves to one of the variables (which means that the wrong part is there) or you get a cryptic message like "Cannot convert the expression's type () to type ($T6) -> ($T6) -> $T5".

The cause of the latter error is that the compiler is not able to infer the types of what you wrote. In this case the problem is that T is only used in the parameter of the closure and the closure you passed doesn't indicate any particular type so the compiler doesn't know what type to infer. By changing the type of returningClass to include T you give the compiler a way to determine the generic parameter.

you can get the class of AnyObject via this way:

Swift 3.x

let myClass: AnyClass = type(of: self)

Swift 2.x

let myClass: AnyClass = object_getClass(self)

and you can pass it as paramater later, if you'd like.

I have a similar use case in swift5:

class PlistUtils {

    static let shared = PlistUtils()

    // write data
    func saveItem<T: Encodable>(url: URL, value: T) -> Bool{
        let encoder = PropertyListEncoder()
        do {
            let data = try encoder.encode(value)
            try data.write(to: url)
            return true
        }catch {
            print("encode error: \(error)")
            return false
        }
    }

    // read data

    func loadItem<T: Decodable>(url: URL, type: T.Type) -> Any?{
        if let data = try? Data(contentsOf: url) {
            let decoder = PropertyListDecoder()
            do {
                let result = try decoder.decode(type, from: data)
                return result
            }catch{
                print("items decode failed ")
                return nil
            }
        }
        return nil
    }

}

Simply copy paste each code here into swift file:

# save as: APICaller.swift

import Foundation

struct APICaller
{
    public static func get<T: Decodable>(url: String, receiveModel: T.Type, completion:@escaping (Decodable) -> ())
    {
        send(url: url, json: nil, receiveModel: receiveModel, completion: completion, httpMethod: "GET")
    }
    
    public static func post<T: Decodable>(url: String, json: [String: Any]?, receiveModel: T.Type, completion:@escaping (Decodable) -> ())
    {
        send(url: url, json: nil, receiveModel: receiveModel, completion: completion, httpMethod: "POST")
    }
    
    public static func delete<T: Decodable>(url: String, json: [String: Any]?, receiveModel: T.Type, completion:@escaping (Decodable) -> ())
    {
        send(url: url, json: nil, receiveModel: receiveModel, completion: completion, httpMethod: "DELETE")
   }

    private static func send<T: Decodable>(url: String, json: [String: Any]?, receiveModel: T.Type, completion:@escaping (Decodable) -> (), httpMethod: String)
    {
        // create post request
        let urlURL: URL = URL(string: url)!
        var httpRequest: URLRequest = URLRequest(url: urlURL)
        httpRequest.httpMethod = httpMethod
        
        if(json != nil)
        {
            // serialize map of strings to json object
            let jsonData: Data = try! JSONSerialization.data(withJSONObject: json!)
            // insert json data to the request
            httpRequest.httpBody = jsonData
            httpRequest.addValue("application/json", forHTTPHeaderField: "Content-Type")
        }
        
        // create an asynchronus task to post the request
        let task = URLSession.shared.dataTask(with: httpRequest)
        { jsonData, response, error in
            // on callback parse the json into the receiving model object
            let receivedModelFilled: Decodable = Bundle.main.decode(receiveModel, from: jsonData!)

            // cal the user callback with the constructed object from json
            DispatchQueue.main.async {
                completion(receivedModelFilled)
            }
        }
        task.resume()
    }
}

# save as: TestService.swift

import Foundation

struct TestService: Codable
{
    let test: String
}

then you can use it like this:

let urlString: String = "http://localhost/testService"  <--- replace with your actual service url

// call the API in post request
APICaller.post(url: urlString, json: ["test": "test"], receiveModel: TestService.self, completion: { testReponse in
    // when response is received - do something with it in this callback
    let testService: TestService = testReponse as! TestService
    print("testService: \(testService)")
})

Tip: i use online service to turn my JSONs into swift files, so all i have left is to write the call and handle the response i use this one: https://app.quicktype.io but you can search for the one you prefer