cache a complicated computed property using the following pattern

Abstract - CachedProperties.md

Cached Properties

Complicated, time expensive computed properties can be cached using the following pattern.

var a = Cached(0) {
    (a: Int) -> Int in
    print("did calculate")
    return a + 100
}
// Prints "did calculate"

print(a.calculated)
// Prints "100"

a.value = 10

print(a.calculated)
// Prints "did calculate\n110"

CachedProperties.swift

struct Cached <T, U> {
    var value : T { willSet { _calculated = nil } }
    private var _calculated : U? = nil
    private var function : (T) -> U
    
    init(_ value: T, _ f: (T) -> U) {
        self.function = f
        self.value = value
    }
    
    var calculated : U {
        mutating get {
            if _calculated == nil { _calculated = function(value) }
            return _calculated!
        }
    }
}