Swift 3-具有静态方法的协议，具有泛型类型的实例方法，可选的'.Type'参数

Question

I'm trying do implement some networking code that will get json at an endpoint, then parse the result into CoreData objects.

I defined a protocol JSONParsable that some of my CoreData object classes conform to.

import SwiftyJSON
import CoreData

protocol JSONParsable {

// you validate the json response from the server to make sure it has everything you expect to function properly
static func validate(_ jsonResponse: JSON) throws

// now parse the jsonResponse
static func parseAll<T: NSManagedObject>(_ jsonResponse:JSON, into context:NSManagedObjectContext) -> [T]!

// describes how to parse one data object in a json response
static func parse<T: NSManagedObject>(_ jsonObject:JSON, into context:NSManagedObjectContext) -> T?

}

Why? Because they are utility methods for objects of that type. So the "GET Objects" method on my networking client "Manager" looks basically like this:

private func get<T:JSONParsable>(_ urlString: URLConvertible, expectedObjectType:T.Type?, completion: @escaping (_ success: Bool) -> Void)

The problem is, if expectedObjectType is ever nil, the compiler complains with an error:

Generic parameter 'T' could not be inferred

I'm not sure how to solve this. I guess I've taken a wrong approach somewhere, being still from the Objective-C world. Would appreciate some help, including a different approach if necessary.

I am basically saying "get json at a specific URL and use a specific class extension to parse it."

I'm also thinking I could implement a 'Dummy Class' and provide that? Seems a little dirty, but aren't we all a little dirty from time to time? :)

Answer 1

The answer is not exactly an answer, only to say I changed the approach. I made an explicit parser object, changed the protocol definition to instance methods, and the parser object remains a member of a 'service' object, that belongs to the networking client.

It works well this way because json parsing relates to the endpoint that provides that JSON.