An optional in Swift is basically a constant or variable that can hold a value OR no value. The value can or cannot be nil. It is denoted by appending a “?” after the type declaration.
An optional in Swift is basically a constant or variable that can hold a value OR no value. The value can or cannot be nil. It is denoted by appending a “?” after the type declaration.
Optional : A type that represents either a wrapped value or nil, the absence of a value.
Declaration : @frozen enum Optional<Wrapped>Optional Binding
To conditionally bind the wrapped value of an Optional instance to a new variable, use one of the optional binding control structures, including if let, guard let, and switch.
if let starPath = imagePaths["star"] {
print("The star image is at '\(starPath)'")
} else {
print("Couldn't find the star image")
}
// Prints "The star image is at '/glyphs/star.png'"
optional Integer declaration − var perhapsInt: Int?
optional String declaration − var perhapsStr: String?Optional Chaining
To safely access the properties and methods of a wrapped instance, use the postfix optional chaining operator (postfix ?). The following example uses optional chaining to access the hasSuffix(_:) method on a String? instance.
if imagePaths["star"]?.hasSuffix(".png") == true {
print("The star image is in PNG format")
}
// Prints "The star image is in PNG format"Using the Nil-Coalescing Operator
Use the nil-coalescing operator (??) to supply a default value in case the Optional instance is nil. Here a default path is supplied for an image that is missing from imagePaths.
let defaultImagePath = "/images/default.png"
let heartPath = imagePaths["heart"] ?? defaultImagePath
print(heartPath)
// Prints "/images/default.png"
The ?? operator also works with another Optional instance on the right-hand side. As a result, you can chain multiple ?? operators together.
let shapePath = imagePaths["cir"] ?? imagePaths["squ"] ?? defaultImagePath
print(shapePath)
// Prints "/images/default.png"Unconditional Unwrapping
When you’re certain that an instance of Optional contains a value, you can unconditionally unwrap the value by using the forced unwrap operator (postfix !). For example, the result of the failable Int initializer is unconditionally unwrapped in the example below.
let number = Int("42")!
print(number)
// Prints "42"
You can also perform unconditional optional chaining by using the postfix ! operator.
let isPNG = imagePaths["star"]!.hasSuffix(".png")
print(isPNG)
// Prints "true"
Unconditionally unwrapping a nil instance with ! triggers a runtime error.
Forced Unwrapping
If you defined a variable as optional, then to get the value from this variable, you will have to unwrap it. This just means putting an exclamation mark at the end of the variable.
var myString:String?
myString = "Hello, Swift 4!"
if myString != nil {
print(myString)
} else {
print("myString has nil value")
}
Automatic Unwrapping
You can declare optional variables using exclamation mark instead of a question mark. Such optional variables will unwrap automatically and you do not need to use any further exclamation mark at the end of the variable to get the assigned value.
var myString:String!
myString = "Hello, Swift 4!"
if myString != nil {
print(myString)
} else {
print("myString has nil value")
}
Creating an Optional Value
case some(Wrapped) : The presence of a value, stored as Wrapped.
init(Wrapped) : Creates an instance that stores the given value.Creating a Nil Value
case none : The absence of a value.
init(nilLiteral: ()) : Creates an instance initialized with nil.Transforming an Optional Value
func map<U>((Wrapped) -> U) -> U? : Evaluates the given closure when this Optional instance is not nil, passing the unwrapped value as a parameter.
func flatMap<U>((Wrapped) -> U?) -> U? : Evaluates the given closure when this Optional instance is not nil, passing the unwrapped value as a parameter.Coalescing Nil Values
func ?? <T>(T?, () -> T) -> T : Performs a nil-coalescing operation, returning the wrapped value of an Optional instance or a default value.
func ?? <T>(T?, () -> T?) -> T? : Performs a nil-coalescing operation, returning the wrapped value of an Optional instance or a default Optional value.Comparing Optional Values
static func == (Wrapped?, Wrapped?) -> Bool : Returns a Boolean value indicating whether two optional instances are equal.
static func != (Optional<Wrapped>, Optional<Wrapped>) -> Bool : Returns a Boolean value indicating whether two values are not equal.Encoding and Decoding
func encode(to: Encoder) : Encodes this optional value into the given encoder.
init(from: Decoder) : Creates a new instance by decoding from the given decoder.Publishing an Optional
var publisher: Optional<Wrapped>.Publisher : A Combine publisher that publishes this instance’s value to each subscriber exactly once, if it has a value at all.
struct Optional.Publisher : The type of a Combine publisher that publishes the value of a Swift optional instance to each subscriber exactly once, if the instance has any value at all.Type Aliases
typealias Optional.Body
typealias Optional.DecodingConfiguration
typealias Optional.EncodingConfiguration
typealias Optional.Value
-Initializers : init(from: Decoder, configuration: Wrapped.DecodingConfiguration)
-Instance Methods : func encode(to: Encoder, configuration: Wrapped.EncodingConfiguration)Happy Swift Coding 🙂