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iOS Build a Playlist Browser with Swift Building the Music Library and Playlist Models Struct Initialization

Posted by zhengyangdong
zhengyangdong
2,623 Points

In Swift 2.0, shall I use as! or as?

The course was recorded when Swift 2.0 was not released yet. I noticed that in Swift 2.0 you have to choose between as! and as? for down casting. Which one shall I use?

init(index: Int) {
    let musicLibrary = MusicLibrary().library
    let playlistDictionary = musicLibrary[index]

    title = playlistDictionary["title"] as! String!
    description = playlistDictionary["description"] as! String!

    let iconName = playlistDictionary["icon"] as! String!
    icon = UIImage(named: iconName)

    let largeIconName = playlistDictionary["largeIcon"] as! String!
    largeIcon = UIImage(named: largeIconName)

    artists += playlistDictionary["artists"] as! [String]  
}

In this code for example, I tried using as? for each down casts but the compiler gives me an error:

While these two lines are perfectly okay: title = playlistDictionary["title"] as? String! description = playlistDictionary["description"] as? String!

These lines give me errors: let iconName = playlistDictionary["icon"] as? String! icon = UIImage(named: iconName) let largeIconName = playlistDictionary["largeIcon"] as? String! largeIcon = UIImage(named: largeIconName) artists += playlistDictionary["artists"] as? [String]

The compiler says that "Value of optional type String!? not unwrapped; did you mean to use ! or ?"

I really don't get the difference between as? and as!. As for the concept of down casting, I believe it is also unfamiliar for many people. I really hope that treehouse can make a separate course talking about these.

1 Answer

Richard Lu
Richard Lu
20,185 Points
Richard Lu
Richard Lu
20,185 Points

Hey Zhengyang,

So the question, from what I understand, is when to use as! and as?. In order to find out which of the as to use, you'll need to know what type of value you're assigning. I've provided some examples below.

as!

  • as! is used when you're sure that the value is of that type, for example:
var aString: String = "Some string value"  // This is a String for sure
var anotherString = aString as! String     // Can be cast to a String for sure

as?

  • as? is used to safely check the value type. If the value is not what it is, then the rest of that line is ignored. Here's an example
var x: String = "Zhengyang"    // This is a String
var y = x as? Int              // Can not be cast to an Int, therefore the value is nil

For the example for as?, if we replaced it with as!:

var x: String = "Zhengyang"    // This is a String
var y = x as! Int              // The as! in this case is stating I KNOW FOR SURE X IS AN INT, which really isn't and would cause the program to crash.

Let me know if there are any additional questions :)

- Rich