Swift 5+
None of the answers really cover in detail the default built in local storage capabilities. It can do far more than just strings.
You have the following options straight from the apple documentation for 'getting' data from the defaults.
func object(forKey: String) -> Any?
//Returns the object associated with the specified key.
func url(forKey: String) -> URL?
//Returns the URL associated with the specified key.
func array(forKey: String) -> [Any]?
//Returns the array associated with the specified key.
func dictionary(forKey: String) -> [String : Any]?
//Returns the dictionary object associated with the specified key.
func string(forKey: String) -> String?
//Returns the string associated with the specified key.
func stringArray(forKey: String) -> [String]?
//Returns the array of strings associated with the specified key.
func data(forKey: String) -> Data?
//Returns the data object associated with the specified key.
func bool(forKey: String) -> Bool
//Returns the Boolean value associated with the specified key.
func integer(forKey: String) -> Int
//Returns the integer value associated with the specified key.
func float(forKey: String) -> Float
//Returns the float value associated with the specified key.
func double(forKey: String) -> Double
//Returns the double value associated with the specified key.
func dictionaryRepresentation() -> [String : Any]
//Returns a dictionary that contains a union of all key-value pairs in the domains in the search list.
Here are the options for 'setting'
func set(Any?, forKey: String)
//Sets the value of the specified default key.
func set(Float, forKey: String)
//Sets the value of the specified default key to the specified float value.
func set(Double, forKey: String)
//Sets the value of the specified default key to the double value.
func set(Int, forKey: String)
//Sets the value of the specified default key to the specified integer value.
func set(Bool, forKey: String)
//Sets the value of the specified default key to the specified Boolean value.
func set(URL?, forKey: String)
//Sets the value of the specified default key to the specified URL.
If are storing things like preferences and not a large data set these are perfectly fine options.
Double Example:
Setting:
let defaults = UserDefaults.standard
var someDouble:Double = 0.5
defaults.set(someDouble, forKey: "someDouble")
Getting:
let defaults = UserDefaults.standard
var someDouble:Double = 0.0
someDouble = defaults.double(forKey: "someDouble")
What is interesting about one of the getters is dictionaryRepresentation, this handy getter will take all your data types regardless what they are and put them into a nice dictionary that you can access by it's string name and give the correct corresponding data type when you ask for it back since it's of type 'any'.
You can store your own classes and objects also using the func set(Any?, forKey: String) and func object(forKey: String) -> Any? setter and getter accordingly.
Hope this clarifies more the power of the UserDefaults class for storing local data.
On the note of how much you should store and how often, Hardy_Germany gave a good answer on that on this post, here is a quote from it
As many already mentioned: I'm not aware of any SIZE limitation (except physical memory) to store data in a .plist (e.g. UserDefaults). So it's not a question of HOW MUCH.
The real question should be HOW OFTEN you write new / changed values... And this is related to the battery drain this writes will cause.
IOS has no chance to avoid a physical write to "disk" if a single value changed, just to keep data integrity. Regarding UserDefaults this cause the whole file rewritten to disk.
This powers up the "disk" and keep it powered up for a longer time and prevent IOS to go to low power state.
Something else to note as mentioned by user Mohammad Reza Farahani from this post is the asynchronous and synchronous nature of userDefaults.
When you set a default value, it’s changed synchronously within your process, and asynchronously to persistent storage and other processes.
For example if you save and quickly close the program you may notice it does not save the results, this is because it's persisting asynchronously. You might not notice this all the time so if you plan on saving before quitting the program you may want to account for this by giving it some time to finish.
Maybe someone has some nice solutions for this they can share in the comments?