Generic function with structs - ios

Question

I try to create reusable function. It select data only for current week. But I need use this function few times in my app, so I've decided to do it generic.
I have two structs
struct BodyWeightCalendarModel {
let weight: Float
let date: Date
}
struct RetrievedWorkoutsByExercise {
let exerciseTitle: String
let sets: Int
let maxWeight: Int
let maxReps: Int
let date: Date
let volume: Int
}
func loopForWeek<T> (data: [T], completionHandler: ([T]) -> Void) {
let arrayForPeriod2: [T] = []
for value in data where value.date >= (calendar.currentWeekBoundary()?.startOfWeek)! && value.date <= (calendar.currentWeekBoundary()?.endOfWeek)! {
arrayForPeriod.append(value)
}
completionHandler(arrayForPeriod2)
}
How to get access to data values? I can't get access through "value.data".
So I want to use this function for different struct (but all this structs needs to have field "date").

Answer 1

As mentioned in other answers using a protocol with date property makes most sense in your case. However theoretically you could also use keypaths to achieve that. You could make a function that takes any instances and gets dates from them in a similar way as below:
func printDates(data: [Any], keyPath:AnyKeyPath) {
for value in data {
if let date = value[keyPath: keyPath] as? Date {
print("date = \(date)")
}
}
}
and then pass for example your BodyWeightCalendarModel instances as below:
let one = BodyWeightCalendarModel(date: Date(), weight: 1)
let two = BodyWeightCalendarModel(date: Date(), weight: 2)
printDates(data: [one, two], keyPath: \BodyWeightCalendarModel.date)
But still a protocol makes more sense in your case.

Answer 2

The reason you can't access value.date is that your function knows nothing about T. Your function declares T but it doesn't constrain it. To the compiler, T can be anything.
You need to create a protocol that tells your function what to expect, and make your structs conform to it:
protocol Timed { // You might find a better name
var date: Date { get }
}
struct BodyWeightCalendarModel: Timed {
let date: Date
...
}
struct RetrievedWorkoutsByExercise: Timed {
let date: Date
...
}
Now you can constrain T to Timed and your function will know that value has a date property.
func loopForWeek<T: Timed> (data: [T], completionHandler: ([T]) -> Void) {

Answer 3

What you're trying to do is close. A Generic value doesn't necessarily have a value on it called date so this won't work. What you can do instead is create a protocol that has a variable of date on it. With that protocol you can do something neat with an extension.
protocol Dated {
var date: Date { get set }
}
extension Dated {
func loopForWeek() -> [Dated] {
let arrayForPeriod2: [Dated] = []
for value in date >= (calendar.currentWeekBoundary()?.startOfWeek)! && value.date <= (calendar.currentWeekBoundary()?.endOfWeek)! {
arrayForPeriod.append(value)
}
return arrayForPeriod2
}
}
Then we can make your two structs use that protocol like:
struct BodyWeightCalendarModel: Dated {
let date: Date
}
struct RetrievedWorkoutsByExercise: Dated {
let date: Date
}
And they can call the function like:
let workouts = RetrievedWorkoutsByExercise()
let result = workouts.loopForWeek()