Difference between associated and raw values in swift enumerations

Swift Enum raw vs associated values

[Swift types]

Enumerations or Enum allows you to create a finite set of values and enum variable references on a single value from the set

In Swift, an enum cannot have both raw values and associated values at the same time.

Raw Values

enum Color: String {
    case white = "#ffffff"
    case black = "#000000"
}

A "raw value" is an unique identifier of a type. It means that you are able to construct your type by ID. For example:

XCTAssertEqual(Color.white, Color(rawValue: "#ffffff"))

To get raw value use

Color.white.rawValue

Associated Values

enum Color {
    case white
    case black
    
    case custom(hex: String)
}

Swift's "associated values" allows you to add additional information into enum that can be defined dynamically. Please note when we introduce "associated values", we omit the "raw values" and add a type annotation. This makes it impossible to use the "raw value" to reconstruct your type, because it is now set up dynamically.

You can read the "associated value" as follows:

let myColor = Color.custom(hex: "#ff00ff")
    
switch myColor {
case .custom(let hex):
    print("custom color hex:\(hex)") //#ff00ff
case .white:
    print("white color")
case .black:
    print("black color")
}

Good example of Associated Value enum is Result^[About]

Please note that Objective-C does not support Swift's enum(except Int-bound)

Compare value in enum

raw enum

if case .error = someResult {
    //some logic
}

associated enum

if case .success(let result) = someResult, result == "ok" {
    //some logic
}

[Swift change Enum associated value]

[Swift indirect enum]

Raw values are for when every case in the enumeration is represented by a compile-time-set value. The are akin to constants, i.e.

let A = 0
let B = 1

is similar to:

enum E: Int {
    case A  // if you don't specify, IntegerLiteralConvertible-based enums start at 0
    case B
}

So, A has a fixed raw value of 0, B of 1 etc set at compile time. They all have to be the same type (the type of the raw value is for the whole enum, not each individual case). They can only be literal-convertible strings, characters or numbers. And they all have to be distinct (no two enums can have the same raw value).

Associated values are more like variables, associated with one of the enumeration cases:

enum E {
    case A(Int)
    case B
    case C(String)
}

Here, A now has an associated Int that can hold any integer value. B on the other hand, has no associated value. And C has an associated String. Associated types can be of any type, not just strings or numbers.

Any given value of type E will only ever hold one of the associated types, i.e. either an Int if the enum is an A, or a String if the enum is a C. It only needs enough space for the bigger of the two. Types like this are sometimes referred to as "discriminated unions" – a union being a variable that can hold multiple different types, but you know (from the enum case) which one it is holding.

They can even be generic. The most common example of which is Optional, which is defined like this:

enum Optional<T> {
    case .Some(T)
    case .None
}