Writing swift dictionary to file

Question

There are limitations with writing NSDictionaries into files in swift. Based on what I have learned from api docs and this stackoverflow answer, key types should be NSString, and value types also should be NSx type, and Int, String, and other swift types might not work. The question is that if I have a dictionary like: Dictionary<Int, Dictionary<Int, MyOwnType>>, how can I write/read it to/from a plist file in swift?

Comp

Answer 1

Anyway, when you want to store MyOwnType to file, MyOwnType must be a subclass of NSObject and conforms to NSCoding protocol. like this:

class MyOwnType: NSObject, NSCoding {

    var name: String

    init(name: String) {
        self.name = name
    }

    required init(coder aDecoder: NSCoder) {
        name = aDecoder.decodeObjectForKey("name") as? String ?? ""
    }

    func encodeWithCoder(aCoder: NSCoder) {
        aCoder.encodeObject(name, forKey: "name")
    }
}

Then, here is the Dictionary:

var dict = [Int : [Int : MyOwnType]]()
dict[1] = [
    1: MyOwnType(name: "foobar"),
    2: MyOwnType(name: "bazqux")
]

So, here comes your question:

Writing swift dictionary to file

You can use NSKeyedArchiver to write, and NSKeyedUnarchiver to read:

func getFileURL(fileName: String) -> NSURL {
    let manager = NSFileManager.defaultManager()
    let dirURL = manager.URLForDirectory(.DocumentDirectory, inDomain: .UserDomainMask, appropriateForURL: nil, create: false, error: nil)
    return dirURL!.URLByAppendingPathComponent(fileName)
}

let filePath = getFileURL("data.dat").path!

// write to file
NSKeyedArchiver.archiveRootObject(dict, toFile: filePath)

// read from file
let dict2 = NSKeyedUnarchiver.unarchiveObjectWithFile(filePath) as [Int : [Int : MyOwnType]]

// here `dict2` is a copy of `dict`

But in the body of your question:

how can I write/read it to/from a plist file in swift?

In fact, NSKeyedArchiver format is binary plist. But if you want that dictionary as a value of plist, you can serialize Dictionary to NSData with NSKeyedArchiver:

// archive to data
let dat:NSData = NSKeyedArchiver.archivedDataWithRootObject(dict)

// unarchive from data
let dict2 = NSKeyedUnarchiver.unarchiveObjectWithData(data) as [Int : [Int : MyOwnType]]

Poe

Thanks a lot for your detailed and great response. I am new to swift and iOS dev and it helped me to understand related stuff too :) – Comp

this writes a lot of entities to the plist – Thrasonical

Answer 2

Response for Swift 5

private func getFileURL(fileName: String) throws -> URL {
        let manager = FileManager.default
        let dirURL = try manager.url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: false)
        return dirURL.appendingPathComponent(fileName)
 }

 if let filePath = try? getFileURL(fileName: "data.dat").path {
        NSKeyedArchiver.archiveRootObject(data, toFile: filePath)
 }

Clairvoyance