IT工程師數位筆記本

    class MediaItem {
    var name: String
    init(name: String) {
    self.name = name
    }
    }

The next snippet defines two subclasses of MediaItem. The first subclass, Movie, encapsulates additional information about a movie or film. It adds a director property on top of the base MediaItem class, with a corresponding initializer. The second subclass, Song, adds an artist property and initializer on top of the base class:

接下來的片斷定義MediaItem的兩個子類。第一個子類，Movie，封裝了有關電影或影片的附加信息。它伴隨著相應的初始化構造器，在基類MediaItem的頂部增加了一個director屬性，。第二個子類Song在基類的頂部增加了一個artist屬性和初始化構造器：

    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)
    }
    }

The final snippet creates a constant array called library, which contains two Movie instances and three Song instances. The type of the library array is inferred by initializing it with the contents of an array literal. Swift’s type checker is able to deduce that Movie and Song have a common superclass of MediaItem, and so it infers a type of MediaItem[] for the library array:

The items stored in library are still Movie and Song instances behind the scenes. However, if you iterate over the contents of this array, the items you receive back are typed as MediaItem, and not as Movie or Song. In order to work with them as their native type, you need to check their type, or downcast them to a different type, as described below.

Checking Type

Use the type check operator (is) to check whether an instance is of a certain subclass type. The type check operator returns true if the instance is of that subclass type and false if it is not.

The example below defines two variables, movieCount and songCount, which count the number of Movie and Song instances in the library array:

This example iterates through all items in the library array. On each pass, the for-in loop sets the item constant to the next MediaItem in the array.

item is Movie returns true if the current MediaItem is a Movie instance and false if it is not. Similarly, item is Song checks whether the item is a Song instance. At the end of the for-in loop, the values of movieCount and songCount contain a count of how many MediaItem instances were found of each type.

Downcasting

A constant or variable of a certain class type may actually refer to an instance of a subclass behind the scenes. Where you believe this is the case, you can try to downcast to the subclass type with the type cast operator (as).

Because downcasting can fail, the type cast operator comes in two different forms. The optional form, as?, returns an optional value of the type you are trying to downcast to. The forced form, as, attempts the downcast and force-unwraps the result as a single compound action.

Use the optional form of the type cast operator (as?) when you are not sure if the downcast will succeed. This form of the operator will always return an optional value, and the value will be nil if the downcast was not possible. This enables you to check for a successful downcast.

Use the forced form of the type cast operator (as) only when you are sure that the downcast will always succeed. This form of the operator will trigger a runtime error if you try to downcast to an incorrect class type.

The example below iterates over each MediaItem in library, and prints an appropriate description for each item. To do this, it needs to access each item as a true Movie or Song, and not just as a MediaItem. This is necessary in order for it to be able to access the director or artist property of a Movie or Song for use in the description.

In this example, each item in the array might be a Movie, or it might be a Song. You don’t know in advance which actual class to use for each item, and so it is appropriate to use the optional form of the type cast operator (as?) to check the downcast each time through the loop:

The example starts by trying to downcast the current item as a Movie. Because item is a MediaItem instance, it’s possible that it might be a Movie; equally, it’s also possible that it might a Song, or even just a base MediaItem. Because of this uncertainty, the as? form of the type cast operator returns an optional value when attempting to downcast to a subclass type. The result of item as Movie is of type Movie?, or “optional Movie”.

Downcasting to Movie fails when applied to the two Song instances in the library array. To cope with this, the example above uses optional binding to check whether the optional Movie actually contains a value (that is, to find out whether the downcast succeeded.) This optional binding is written “if let movie = item as? Movie”, which can be read as:

“Try to access item as a Movie. If this is successful, set a new temporary constant called movie to the value stored in the returned optional Movie.”

If the downcasting succeeds, the properties of movie are then used to print a description for that Movie instance, including the name of its director. A similar principle is used to check for Song instances, and to print an appropriate description (including artist name) whenever a Song is found in the library.

Type Casting for Any and AnyObject

Swift provides two special type aliases for working with non-specific types:

• AnyObject can represent an instance of any class type.

• Any can represent an instance of any type at all, apart from function types.

AnyObject

When working with Cocoa APIs, it is common to receive an array with a type of AnyObject[], or “an array of values of any object type”. This is because Objective-C does not have explicitly typed arrays. However, you can often be confident about the type of objects contained in such an array just from the information you know about the API that provided the array.

In these situations, you can use the forced version of the type cast operator (as) to downcast each item in the array to a more specific class type than AnyObject, without the need for optional unwrapping.

The example below defines an array of type AnyObject[] and populates this array with three instances of the Movie class:

Because this array is known to contain only Movie instances, you can downcast and unwrap directly to a non-optional Movie with the forced version of the type cast operator (as):

For an even shorter form of this loop, downcast the someObjects array to a type of Movie[] instead of downcasting each item:

Any

Here’s an example of using Any to work with a mix of different types, including non-class types. The example creates an array called things, which can store values of type Any:

The things array contains two Int values, two Double values, a String value, a tuple of type (Double, Double), and the movie “Ghostbusters”, directed by Ivan Reitman.

You can use the is and as operators in a switch statement’s cases to discover the specific type of a constant or variable that is known only to be of type Any or AnyObject. The example below iterates over the items in the things array and queries the type of each item with a switch statement. Several of the switch statement’s cases bind their matched value to a constant of the specified type to enable its value to be printed: