decodeJSON.swift

// Based on https://twitter.com/ravibastola/status/1249555595285291008?s=20

extension Bundle {
    // This is synchronous, which is bad if called on the main queue.
    func decodeJSON<T: Decodable>(_ type: T.Type = T.self, from filename: String) throws -> T {
        guard let path = self.url(forResource: filename, withExtension: nil) else {
            throw NSError(domain: NSCocoaErrorDomain,
                          code: CocoaError.fileNoSuchFile.rawValue,
                          userInfo: [NSFilePathErrorKey: filename])
        }

        let data = try Data(contentsOf: path)
        return try JSONDecoder().decode(T.self, from: data)
    }

    // An async version
    func decodeJSON<T: Decodable>(_ type: T.Type = T.self,
                                  from filename: String,
                                  on queue: DispatchQueue = .global(),
                                  completion: @escaping (Result<T, Error>) -> ()) {

        queue.async {
            completion(Result { try self.decodeJSON(type, from: filename) })
        }
    }
}

Bundle.main.decodeJSON(String.self, from: "x") { strResult in print(strResult) }


// If you *always* want this to be on Bundle.main, then I'd just push up to a free function. No
// need for an instance that has no state.

func decodeJSONFile<T: Decodable>(_ type: T.Type = T.self,
                                  from filename: String,
                                  on queue: DispatchQueue = .global(),
                                  completion: @escaping (Result<T, Error>) -> ()) {

    queue.async {
        completion(Result { try Bundle.main.decodeJSON(type, from: filename) })
    }
}

// Or if you want a namespace, we typically use an Enum for that:

enum Loader {
    function json( ... )
}

let result = Loader.json(...) // etc.