swift - 如何基于类中的其他属性创建变量属性

Question

class someClass
{
// This is a list of restaurant names
var restaurantNames = ["Cafe Deadend", "Homei", "Teakha", "Cafe Loisl", "Petite Oyster", "For Kee Restaurant", "Po's Atelier", "Bourke Street Bakery", "Haigh's Chocolate", "Palomino Espresso", "Upstate", "Traif", "Graham Avenue Meats", "Waffle & Wolf", "Five Leaves", "Cafe Lore", "Confessional", "Barrafina", "Donostia", "Royal Oak", "Thai Cafe"]

// This is an array to record the restaurants I have visited 
var restaurantIsVisited:[Bool] = [Bool](count: 21, repeatedValue: false)

// This is a function to set the restaurants I have visited
func setTheVisitedRestaurants(somenumber:Int)
{
self.restaurantIsVisited[somenumber] = true
}

}
let thisVisit = someClass()
thisVisit.setTheVisitedRestaurants(1)
let somearray = thisVisit.restaurantIsVisited

上面的代码在 playground 中没有错误。但是，如果我不知道餐厅总数是 21 家(可能有数百家，我不想数)怎么办？我尝试了以下代码来创建一个不可变的计算属性。但它不会工作，因为稍后该属性将被函数 setTheVisitedRestaurants 更改，并且它会返回一个错误。

// var restaurantIsVisited:[Bool]{
//        return [Bool](count: restaurantNames.count, repeatedValue: false)
//    }

简而言之，问题就像如何根据类中的其他属性创建一个可变属性。我是初学者，我真的尝试过。请帮忙!

Answer 1

使用 lazy 关键字声明 restaurantIsVisited。这将确保它在第一次访问之前不会被创建，到那时您将能够向 restaurantNames 询问其 count:

class someClass
{
    // This is a list of restaurant names
    var restaurantNames = ["Cafe Deadend", "Homei", "Teakha", "Cafe Loisl", "Petite Oyster", "For Kee Restaurant", "Po's Atelier", "Bourke Street Bakery", "Haigh's Chocolate", "Palomino Espresso", "Upstate", "Traif", "Graham Avenue Meats", "Waffle & Wolf", "Five Leaves", "Cafe Lore", "Confessional", "Barrafina", "Donostia", "Royal Oak", "Thai Cafe"]

    // This is an array to record the restaurants I have visited
    lazy var restaurantIsVisited:[Bool] = [Bool](count: self.restaurantNames.count, repeatedValue: false)

    // This is a function to set the restaurants I have visited
    func setTheVisitedRestaurants(somenumber:Int)
    {
        self.restaurantIsVisited[somenumber] = true
    }

}

let thisVisit = someClass()
thisVisit.setTheVisitedRestaurants(1)
let somearray = thisVisit.restaurantIsVisited
println(somearray.count)  // "21"