SE-0293 extends property wrappers so they can be applied to parameters for functions and closures. Parameters passed this way are still immutable unless you take a copy of them, and you are still able to access the underlying property wrapper type using a leading underscore if you want.
As an example, we could write a function that accepts an integer and prints it out:
func setScore1(to score: Int) {
print("Setting score to \(score)")
}
When that’s called we can pass it any range of values, like this:
setScore1(to: 50)
setScore1(to: -50)
setScore1(to: 500)
If we wanted our scores to lie only within the range 0...100 we could write a simple property wrapper that clamps numbers as they are created:
@propertyWrapper
struct Clamped<T: Comparable> {
let wrappedValue: T
init(wrappedValue: T, range: ClosedRange<T>) {
self.wrappedValue = min(max(wrappedValue, range.lowerBound), range.upperBound)
}
}
Now we can write and call a new function using that wrapper:
func setScore2(@Clamped(range: 0...100) to score: Int) {
print("Setting score to \(score)")
}
setScore2(to: 50)
setScore2(to: -50)
setScore2(to: 500)
Calling setScore2()
with the same input values as before will print different output, because the numbers will get clamped to 50, 0, 100.
Tip: Our property wrapper is trivial because parameters passed into a function are immutable – we don’t need to handle re-clamping the wrapped value when it changes because it won’t change. However, you can make your property wrappers as complex as you need; they work just as they would with properties or local variables.
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