向下转型多态和赋值

Question

我创建了一个玩具问题来理解为什么我无法在向下转换后设置属性的值。问题出在视图控制器 #3 中：我想为属性 artist 赋值。

import Foundation

protocol MediaType {
    var year: String? { get set }
}

struct Song: MediaType {
    var artist: String?
    var musicians: [String]?

    // Implement Protocol `MediaType`
    var year: String?
}

class MediaStateController {
  var media: MediaType?
}

//
// view controller 1: controller does not know the media type just yet.
//
let stateController = MediaStateController() // media remains nil for now
print(stateController.media is Song) // false

//
// view controller 2: knows the media type so it can assign the media type.
//
stateController.media = Song()

print(stateController.media is Song) // true
print(stateController.media as? Song) // Optional(Song(artist: nil, musicians: nil, year: nil))
print(stateController.media) // Optional(Song(artist: nil, musicians: nil, year: nil))
print(type(of: stateController.media)) // Optional<MediaType>

// view controller 3: knows only the artist name but not the musicians. 
if var song = stateController.media as? Song {
    song.artist = "SomeArtist" // ISSUE: this does not set the value (see print-out below)
}

print(stateController.media as? Song) // Optional(Song(artist: nil, musicians: nil, year: nil))


if var song = stateController.media as? Song {
    // makes no sense to not use song, but let's do it anyway.
    // (stateController.media as? Song)?.artist = "SomeArtist" // error: cannot assign to immutable expression of type 'String?'
}
print(stateController.media as? Song) // Optional(=Song(artist: nil, musicians: nil, year: nil))

if var song = stateController.media as? Song {
      // THIS DOES SET THE VALUE, but why?
      song.artist = "SomeArtist" 
      stateController.media = song
    }
print(stateControllaer.media as? Song) // Optional(Song(artist: Optional("SomeArtist"), musicians: nil, year: nil))

Answer 1

Song是一个struct，在Swift中是一个值类型。在这个例子中：

// view controller 3: knows only the artist name but not the musicians. 
if var song = stateController.media as? Song {
    song.artist = "SomeArtist" // ISSUE: this does not set the value (see print-out below)
}

您正在将原始 Song 实例复制到新变量中，然后更新该副本的 artist 属性。一旦您退出 if var.

的范围，该副本就会被丢弃

向下转型多态和赋值

Downcasting Polymorphism and assignments

swift

downcast