playground.swift

import Foundation

enum Provider: String {
    case Vimeo = "Vimeo"
    case SoundCloud = "SoundCloud"
}

class Item {
    let title: String
    let provider: Provider
    var width: Int!
    
    init(data: [String: AnyObject]) {
        provider = Provider(rawValue: data["provider"] as String)!
        title = data["title"] as String
        
        if let width = data["width"] as? Int {
            self.width = width
        }
    }
}

class Vimeo: Item {
    let video: String!
    
    override init(data: [String: AnyObject]) {
        super.init(data: data)
        
        video = data["video"] as String
    }
}

class SoundCloud: Item {
    let sound: String!
    
    override init(data: [String : AnyObject]) {
        super.init(data: data)
        
        width = data["_width"] as Int
        sound = data["sound"] as String
    }
}

let dataSet: [[String: AnyObject]] = [
    ["title": "Vimeo title", "provider": "Vimeo", "video": "blah!", "width": 42],
    ["title": "SoundCloud title", "provider": "SoundCloud", "sound": "boom", "_width": 38]
]

func allItems() -> [Item] {
    return dataSet.map { (data: [String: AnyObject]) -> Item in
        let provider = Provider(rawValue: data["provider"] as String)!
        switch provider {
        case .Vimeo:
            return Vimeo(data: data)
        case .SoundCloud:
            return SoundCloud(data: data)
        }
    }
}

for item in allItems() {
    // Accessing item's attribute
    
    println(item.title)
    
    println(item.width)
    
    // Accessing downcasted item
    
    switch item.provider {
    case .Vimeo:
        println((item as Vimeo).video)
    case .SoundCloud:
        println((item as SoundCloud).sound)
    }
}

weird.swift

class Item {
    var title: String!
}

class SoundCloud: Item {
}

class Vimeo: Item {
    override var title: String! {
        get {
            return super.title
        }
        
        set {
            super.title = "Vimeo setter \(newValue)"
        }
    }
}

let item = Vimeo() as Item
item.title = "blah!"

if let item = item as? SoundCloud {
    println("Yep")
} else {
    println("Nope")
}

item.title