Extending property wrappers to function and closure parameters
Extending property wrappers to function and closure parameters 관련
Available from Swift 5.5
SE-0293 (apple/swift-evolution) 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.
Tips
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.
Other Changes in Swift 5.5