What function does "as" in the Swift syntax have?

Question

Recently I stumbled upon a syntax I cannot find a reference to: What does as mean in the Swift syntax?

Like in:

var touch = touches.anyObject() as UITouch!

Unfortunately, it's hard to search for a word like as, so I didn't find it in the Swift Programming Language handbook by Apple. Maybe someone can guide me to the right passage?

And why does the element after as always have an ! to denote to unwrap an Optional?

Thanks!

Answer 1

The as keyword is used for casting an object as another type of object. For this to work, the class must be convertible to that type.

For example, this works:

let myInt: Int = 0.5 as Int // Double is convertible to Int

This, however, doesn't:

let myStr String = 0.5 as String // Double is not convertible to String

You can also perform optional casting (commonly used in if-let statements) with the ? operator:

if let myStr: String = myDict.valueForKey("theString") as? String {
    // Successful cast
} else {
    // Unsuccessful cast
}

---

In your case, touches is (I'm assuming from the anyObject() call) an NSSet. Because NSSet.anyObject() returns an AnyObject?, you have to cast the result as a UITouch to be able to use it.

In that example, if anyObject() returns nil, the app will crash, because you are forcing a cast to UITouch! (explicitly unwrapping). A safer way would be something like this:

if let touch: UITouch = touches.anyObject() as? UITouch {
  // Continue
}