Read JSON file with Swift 3

Question

I have a JSON file called points.json, and a read function like:

private func readJson() {
    let file = Bundle.main.path(forResource: "points", ofType: "json")
    let data = try? Data(contentsOf: URL(fileURLWithPath: file!))
    let jsonData = try? JSONSerialization.jsonObject(with: data!, options: []) as! [String:Any]
    print(jsonData)
}

It does not work, any help?

Answer 1

Your problem here is that you force unwrap the values and in case of an error you can't know where it comes from.

Instead, you should handle errors and safely unwrap your optionals.

And as @vadian rightly notes in his comment, you should use Bundle.main.url.

private func readJson() {
    do {
        if let file = Bundle.main.url(forResource: "points", withExtension: "json") {
            let data = try Data(contentsOf: file)
            let json = try JSONSerialization.jsonObject(with: data, options: [])
            if let object = json as? [String: Any] {
                // json is a dictionary
                print(object)
            } else if let object = json as? [Any] {
                // json is an array
                print(object)
            } else {
                print("JSON is invalid")
            }
        } else {
            print("no file")
        }
    } catch {
        print(error.localizedDescription)
    }
}

When coding in Swift, usually, ! is a code smell. Of course there's exceptions (IBOutlets and others) but try to not use force unwrapping with ! yourself and always unwrap safely instead.

Answer 2

The Swift 5 / iOS 12.3 code below shows a possible rewrite of your method that avoids force unwrap on optional values and handles gently potential errors:

import Foundation

func readJson() {
    // Get url for file
    guard let fileUrl = Bundle.main.url(forResource: "Data", withExtension: "json") else {
        print("File could not be located at the given url")
        return
    }

    do {
        // Get data from file
        let data = try Data(contentsOf: fileUrl)

        // Decode data to a Dictionary<String, Any> object
        guard let dictionary = try JSONSerialization.jsonObject(with: data, options: []) as? [String: Any] else {
            print("Could not cast JSON content as a Dictionary<String, Any>")
            return
        }

        // Print result
        print(dictionary)
    } catch {
        // Print error if something went wrong
        print("Error: \(error)")
    }
}