Swift：为多种类型的变量建模

Question

I'm building an alarm app where each alarm has a sound associated with it.

The sound can either be a song, artist([song]), or playlist([song]); all of which have a title and other specific properties.

I'd like to be able to do things like alarm.sound.title and have the title return regardless of if it's a song, artist, or playlist. I also need to implement logic where the app behaves differently depending on whether it's a song, artist, or playlist.

I'm not sure if I should be creating a sound class with a type property and subclasses song, artist, playlist or if there is a better way to structure the data. I read about typecasting but I'm not sure if that's the right direction.

Any advice on how to model this would be appreciated. Thanks

Answer 1

Since you want to use Swift I'd suggest using protocols. That would give what you need. Simple example:

protocol Sound {
    let title: String { get }
    let length: TimeInterval { get }
}

The protocol would define the interface. Then you can just create various classes which would conform to the protocol - the underlaying implementation can be completely different:

class Song: Sound {
    var title: String
    var length: TimeInterval

    init(title: String, length: TimeInterval) {
        self.title = title
        self.length = length
    }
}

class Artist: Sound {
    var title: String {
        return "Artist"
    }
    var length: TimeInterval {
        return 11.1
    }
}

Then you can simply access title on any object conforming to the Sound protocol:

let sound1: Sound = Song(title: "Song", length: 1)
let sound2: Sound = Artist()

print("\(sound1.title)")  // prints: Song
print("\(sound2.title)")  // prints: Artist