Swift function 返回多种类型

Question

I'm trying to build a utility function called makePostBuilder which looks something like this.

fileprivate func makePostRequest(apiUrl: String, params: [String: String]) -> URLRequest? {
        // build url
        let urlString = "\(apiUrl)"
        guard let serviceUrl = URL(string: urlString) else { return nil }
        
        // build url request
        var request = URLRequest(url: serviceUrl)
        request.httpMethod = "POST"
        request.setValue("Application/json", forHTTPHeaderField: "Content-Type")
        
        //set http body for url request with incoming params
        guard let httpBody = try? JSONSerialization.data(withJSONObject: params, options: []) else {
            return nil 
        }
        request.httpBody = httpBody
        return request
    }

The return type of this function is obviously incorrect. I would like it to either return a URLRequest instance OR an Error instance. The error instance is mainly so the optional unwrapping can return a valuable message instead of just nil (as per the current implementation.

I was thinking along the lines of a typealias , but I am not sure if that is the right approach.

//not sure if this is right
typealias CustomRequestType = (URLRequest, Error) 

At the end of the appropriate type definitions, I would like the function to look something like this

fileprivate func makePostRequest(apiUrl: String, params: [String:String]) -> CustomType {
  let urlString = apiUrl
  guard let serviceUrl = URL(string: urlString) else { return //error based on customtype? }

  // build url request
  var request = URLRequest(url: serviceUrl)
  request.httpMethod = "POST"
  request.setValue("Application/json", forHTTPHeaderField: "Content-Type")

  //set http body for url request with incoming params
  guard let httpBody = try? JSONSerialization.data(withJSONObject: params, options: []) else {
    return //error type 
  }

  //return success type
  request.httpBody = httpBody
  return request
}

I think I'm getting close, but not quite there yet. I'd also love if the community could point me to some docs!

UPDATE: Possible Solution?

//does this seem plausible?

enum DCError: String, Error {
    case invalidUrl = "the url seems to be invalid"
}

typealias DCUrlRequestType = Result<URLRequest, Error>

fileprivate func makePostRequest(apiUrl: String, params: Dictionary<String, String>) -> DCUrlRequestType {
    let urlString = apiUrl
    guard let serviceUrl = URL(string: urlString) else {
        return DCUrlRequestType.failure(DCError.invalidUrl)
    }
    
    var request = URLRequest(url: serviceUrl)
    request.httpMethod = "POST"
    request.setValue("Application/json", forHTTPHeaderField: "Content-Type")
    
    return DCUrlRequestType.success(request)
}

Answer 1

There is a built-in enum Result you can use as your return type. And you can use URLError(code: .badURL) (provided by Foundation) in the case where you can't create the URL . Thus:

fileprivate func makePostRequest(apiUrl: String, params: [String: String]) -> Result<URLRequest, Error> {
    let urlString = "\(apiUrl)"
    guard let serviceUrl = URL(string: urlString) else {
        return .failure(URLError(.badURL))
    }

    var request = URLRequest(url: serviceUrl)
    request.httpMethod = "POST"
    request.setValue("application/json", forHTTPHeaderField: "Content-Type")
    do {
        request.httpBody = try JSONSerialization.data(withJSONObject: params, options: [])
    } catch {
        return .failure(error)
    }
    return .success(request)
}

BUT…

The natural way to write this function is to declare that it throws , like this:

fileprivate func makePostRequest(apiUrl: String, params: [String: String]) throws -> URLRequest? {
    let urlString = "\(apiUrl)"
    guard let serviceUrl = URL(string: urlString) else {
        throw URLError(.badURL)
    }

    var request = URLRequest(url: serviceUrl)
    request.httpMethod = "POST"
    request.setValue("application/json", forHTTPHeaderField: "Content-Type")
    request.httpBody = try JSONSerialization.data(withJSONObject: params, options: [])
    return request
}

and then let the caller use do/catch if she wants to handle the error, instead of making the caller switch over the Result cases.

If the caller really wants a Result , she can use the Result(catching:) initializer, which (with trailing closure syntax) looks like this:

let requestResult = Result { try makePostRequest(apiUrl: urlString, params: [:]) }