What does 'finish in' mean in Swift? - ios

Question

I'm a noob and have been learning from Apple's Playgrounds and random books doing tutorials. I'm working on a tutorial where it deals with a closure. I've seen this 'finish in' before in another tutorial but I don't know what it means precisely in layman terms.
What is it finishing, what is being finished, and inside of what? Or is there an idea of order of operation?
Here is the function where it was used:
func playSequence(index: Int, highlightTime: Double){
currentPlayer = .Computer
if index == inputs.count{
currentPlayer = .Human
return
}
var button: UIButton = buttonByColor(color: inputs[index])
var originalColor: UIColor? = button.backgroundColor
var highlightColor: UIColor = UIColor.white
UIView.animate(withDuration: highlightTime, delay: 0.0, options: [.curveLinear, .allowUserInteraction, .beginFromCurrentState], animations: {
button.backgroundColor = highlightColor
}, completion: {
finished in button.backgroundColor = originalColor
var newIndex: Int = index + 1
self.playSequence(index: newIndex, highlightTime: highlightTime)
})
}

Answer 1

finished is the parameter to the completion closure. The in is simply part of Swift's closure syntax.
The full signature of the UIView animate method is:
class func animate(withDuration duration: TimeInterval, delay: TimeInterval, options: UIViewAnimationOptions = [], animations: #escaping () -> Void, completion: ((Bool) -> Void)? = nil)
Note the Bool parameter to the completion closure. The finished in your code is the name given to that parameter.
An excerpt from the documentation about the completion parameter states:
This block has no return value and takes a single Boolean argument that indicates whether or not the animations actually finished before the completion handler was called.
A more typical way to write the code is as:
UIView.animate(withDuration: highlightTime, delay: 0.0, options: [.curveLinear, .allowUserInteraction, .beginFromCurrentState], animations: {
// animation code
}) { (finished) in
// completion code
}
This syntax makes it clearer than the syntax you are using. This is also using the "trailing closure" syntax.
Another way, closer to your usage, would be:
UIView.animate(withDuration: highlightTime, delay: 0.0, options: [.curveLinear, .allowUserInteraction, .beginFromCurrentState], animations: {
// animation code
}, completion: { (finished) in
// completion code
})
Your usage simply omits the parentheses around the parameter and it leaves out a line break. Adding those back in makes the code clearer.