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April 5, 2022 09:38 am GMT
Original Link: https://dev.to/naveenragul/swift-type-casting-4ifn
Swift - Type Casting
Type casting is a way to treat instance of a type as a different superclass or subclass and to check the type of an instance, implemented with the as and is operators.
It can be also used to check whether a type conforms to a protocol.
Checking Type
- is operator is used to check whether an instance is of a certain type or it's subclass type.
example :
class MediaItem { var name: String init(name: String) { self.name = name }}class Movie: MediaItem { var director: String init(name: String, director: String) { self.director = director super.init(name: name) }}class Song: MediaItem { var artist: String init(name: String, artist: String) { self.artist = artist super.init(name: name) }}let library = [ Movie(name: "Casablanca", director: "Michael Curtiz"), Song(name: "Blue Suede Shoes", artist: "Elvis Presley"), Movie(name: "Citizen Kane", director: "Orson Welles"), Song(name: "The One And Only", artist: "Chesney Hawkes"), Song(name: "Never Gonna Give You Up", artist: "Rick Astley")]var movieCount = 0var songCount = 0for item in library { if item is Movie { movieCount += 1 } else if item is Song { songCount += 1 }}Downcasting
A constant or variable of a certain class type may actually refer to an instance of a subclass using type cast operator (as? or as!).
- as?, returns an optional value
- as!, attempts the downcast and force-unwraps the result
Use as! when you are sure that downcast will succeed and as? when you are not sure.
example :
for item in library { if let movie = item as? Movie { print("Movie: \(movie.name), dir. \(movie.director)") } else if let song = item as? Song { print("Song: \(song.name), by \(song.artist)") }}Type Casting for Any and AnyObject
Swift provides two special types for working with nonspecific types:
- Any can represent an instance of any type at all, including function types.
- AnyObject can represent an instance of any class type.
var things: [Any] = []things.append(0)things.append(0.0)things.append(3.14159)things.append("hello")things.append(Movie(name: "Ghostbusters", director: "Ivan Reitman"))- The Any type represents values of any type, including optional types. Swift gives you a warning if you use an optional value where a value of type Any is expected. If you really do need to use an optional value as an Any value, you can use the as operator to explicitly cast the optional to Any.
example :
let optionalNumber: Int? = 3things.append(optionalNumber) // Warningthings.append(optionalNumber as Any) // No warningTo discover the specific type of a constant or variable thats known only to be of type Any or AnyObject, you can use an is or as pattern in a switch statements cases.
example :
for thing in things { switch thing { case 0 as Int: print("zero as an Int") case 0 as Double: print("zero as a Double") case let someInt as Int: print("an integer value of \(someInt)") case let someDouble as Double where someDouble > 0: print("a positive double value of \(someDouble)") case is Double: print("some other double value that I don't want to print") case let someString as String: print("a string value of \"\(someString)\"") case let (x, y) as (Double, Double): print("an (x, y) point at \(x), \(y)") case let movie as Movie: print("a movie called \(movie.name), dir. \(movie.director)") case let stringConverter as (String) -> String: print(stringConverter("Michael")) default: print("something else") }}Original Link: https://dev.to/naveenragul/swift-type-casting-4ifn
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