How can a operate with value of setter in structs?

Question

I have this code of swift struct with getter and setter. How that value of newValue got inside this setter? I don't know how it was defined.

struct Program {
var allResults : [String] = []
var items: Int {
    get {
        return allResults.count
    }
    set {            
        let result = String(newValue * 100) // what's that newValue, how did it get there?
        allResults.append(result)    
    }
}

Answer 1

It is called Shorthand Setter Declaration.

Cited from Swift 3 book:

If a computed property’s setter does not define a name for the new value to be set, a default name of newValue is used.

If you would like to have a better readable format, you could use this:

...
set (newItems) { //add your own variable named as you like
    let result = String(newItems * 100)
    allResults.append(result)
}    
...