Reading and Writing Dictionaries

When you need to store a dictionary, there are a few different options for doing so.

Info.plist

First, a quick note about Info.plist. You don’t need to manually read in a file to access the contents of this file as a dictionary: you can just call Bundle.main.infoDictionary.

URLs to the docs directory

First, let’s talk about how to refer to file locations. The URL is the main way locations are referred to, even though they’re local URLs on the device.

To write files, you use the documents directory. The base documents directory is a bit more complex to calculate, because it uses the same API as on macOS, where there are more options:

let docsBaseURL = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!

From there, you can get the path to a specific file by adding a path component again:

let customPlistURL = docsBaseURL.appendingPathComponent("custom.plist")

Reading and Writing Plists

To read a property list file, you can use a constructor built in to NSDictionary:

let customDict = NSDictionary(contentsOf: customPlistURL) as? [String : AnyObject]

NSDictionary is the old Objective-C dictionary class; if you’re working in swift, you probably want a Swift-style dictionary, which is why it’s cast to [String : AnyObject].

To write a property list, there is also a method built in to NSDictionary:

NSDictionary(dictionary: customDict).write(to: customPlistURL, atomically: true)

We need to use a NSDictionary constructor again to turn the Swift dictionary back into an NSDictionary. If you used the NSDictionary as-is, you wouldn’t need to translate it.

Note that the value type of the Swift array is AnyObject, not Any. this means that the values of the dictionary have to be objects, so Ints and Doubles can’t be used. Instead, you need to wrap them in a NSNumber. If you would rather be able to use Ints and Doubles directly, there’s another option: archives.

Reading and Writing Archives

Archives can be used to store a wide variety of types, including Swift dictionaries. To read an archive, you use the NSKeyedUnarchiver class:

let customDict = NSKeyedUnarchiver.unarchiveObject(withFile: customArchiveURL.path) as? [String: Any]

Note that unarchiveObject() takes a path String instead of a URL, but all you need to do differently is access the path attribute of the URL object.

Note, as well, that the type of the object we unarchived is a Swift dictionary with a value of type Any, so Ints and Doubles can be used directly.

To save an archive, you use NSKeyedArchiver:

NSKeyedArchiver.archiveRootObject(dictionary, toFile: url.path)