Function for reading in JSON files - json

Question

I am both new to stackoverflow as well as Swift programming (coming from statistics/R), so please bear with me in case I'm doing something super wrong. Ok, lets get to my problem: I want to read in multiple JSON files, e.g. metrics.json and accounts.json. After having read various posts on that topic, I have now written the following function and created one struct for each of the JSON files, replicating its structure:
func readJsonFile(fileName: String) -> [metric]? {
var result: [metric]?
if let path = Bundle.main.path(forResource: fileName, ofType: "json") {
do {
let jsonData = try Data(contentsOf: URL(fileURLWithPath: path))
result = try JSONDecoder().decode([metric].self, from: jsonData)
} catch {
fatalError("Unresolved error when loading JSON file \(fileName): \(error)")
}
}
return result
}
struct Metric: Codable {
let METRIC_ID: Int
let METRIC_NAME: String
}
struct Account: Codable {
let ACCOUNT_ID: Int
let ACCOUNT_NAME: String
}
Now what I was trying to achieve is somehow provide not just the filename to the function but also somehow the respective struct as a blueprint. Also, I would probably need to dynamically change the function output. It then should like somehow like that:
let metrics = readJsonFile("metrics", Metric)
let accounts = readJsonFile("accounts", Account)
I have a feeling that might be an easy thing if one knows his way around. Unfortunately, I do not. Can someone help me with any suggestions? Or should I in general take a different approach? Also, if there is anything else odd or wrong in the code, happy to receive any constructive feedback. Thanks guys.

Answer 1

I think what I would do is make the function generic over Codable, like this:
enum MyError:Error { case noSuchFile }
func readJsonFile<T:Codable>(fileName: String, type:T.Type) throws -> [T] {
if let path = Bundle.main.path(forResource: fileName, ofType: "json") {
let jsonData = try Data(contentsOf: URL(fileURLWithPath: path))
return try JSONDecoder().decode([T].self, from: jsonData)
}
throw MyError.noSuchFile
}
Now you can call it for either file and either struct:
let x = try? self.readJsonFile(fileName:"metric", type:Metric.self)
let y = try? self.readJsonFile(fileName:"account", type:Account.self)
I've revised your function to throw when there's an error, and I count the lack of the appropriate file as an error. The outcome of using try? in the call is that you get an Optional which is nil if things went badly and is an array of the correct struct type if things went well; if you don't like that, you can use do/catch instead of course, or even try! if you're dead sure this will always work (you should be, they are your files after all).