How do I use the update function in Swift Playground

Using SpriteKit in Xcode projects, I am able to use the update function to update everything I need to in between frames.

e.g.

override func update(currentTime: CFTimeInterval) {
    /* Called before each frame is rendered */
}

How can I do the same thing in Xcode Playground?

I do not think that this is a duplication of NSTimer.scheduledTimerWithTimeInterval in Swift Playground