How to save and load objects with NSKeyedArchiver and NSKeyedUnarchiver

Swift version: 5.10

Paul Hudson @twostraws May 28th 2019

You can write any kind of object to disk as long as it supports the NSCoding protocol – which includes strings, arrays, dictionaries, UIView, UIColor and more right out of the box. To write to disk, use this:

let randomFilename = UUID().uuidString
let fullPath = getDocumentsDirectory().appendingPathComponent(randomFilename)

do {
    let data = try NSKeyedArchiver.archivedData(withRootObject: somethingToSave, requiringSecureCoding: false)
    try data.write(to: fullPath)
} catch {
    print("Couldn't write file")
}

That call to getDocumentsDirectory() is a small helper function I frequently use to write files to disk:

func getDocumentsDirectory() -> URL {
    let paths = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)
    return paths[0]
}

When you want to read your object back you need to use unarchiveObject(withFile:), but be warned: the file might not exist or might not be unarchivable, so this method returns an optional value that you need to unwrap carefully.

For example, if you want to unarchive an array of strings, you would use this code:

do {
    if let loadedStrings = try NSKeyedUnarchiver.unarchiveTopLevelObjectWithData(data) as? [String] {
        savedArray = loadedStrings
    }
} catch {
    print("Couldn't read file.")
}