使用依赖于其他实例变量的值初始化惰性实例变量 [英] Initialize lazy instance variable with value that depends on other instance variables

问题描述

以下初始化当前在调用 getEventCalendar 的行中产生此错误:

The following initialization currently produces this error in the line that calls getEventCalendar:

不能在属性中使用实例成员getEventCalendar"初始化程序；属性初始值设定项在 'self' 可用之前运行.

Cannot use instance member 'getEventCalendar' within property initializer; property initializers run before 'self' is available.

是否有任何合适的方法可以使用依赖于 self 的其他对象类型 instance variables 的值来初始化 lazy 实例变量(不只是 self 单独)?我已经例如尝试将 getEventCalendar 从方法转换为函数，但这也无济于事.

Is there any suitable way for initializing the lazy instance variable with a value that depends on other object-type instance variables of self (not just self alone) ? I've e.g. tried turning getEventCalendar from a method into a function, but this does not help either.

class AbstractEventCalendarClient {

  let eventStore: EKEventStore
  let entityType: EKEntityType

  lazy var eventCalendar = getEventCalendar()

  init(eventStore: EKEventStore, entityType: EKEntityType) {
    self.eventStore = eventStore
    self.entityType = entityType
  }

  func getEventCalendar() -> EKCalendar? {
    // ...
  }
}

推荐答案

您可以使用只执行一次的闭包，它捕获 self 的属性并在执行时使用这些(= 第一次使用 <代码>懒惰属性).例如

You can use a once-only executed closure which captures properties of self and use these at execution (= first use of the lazy property). E.g.

class Foo {
    var foo: Int
    var bar: Int
    lazy var lazyFoobarSum: Int = { return self.foo + self.bar }()

    init(foo: Int, bar: Int) { 
        self.foo = foo 
        self.bar = bar
    }
}

let foo = Foo(foo: 2, bar: 3)
foo.foo = 7
print(foo.lazyFoobarSum) // 10

W.r.t.到你自己的尝试:你可以在这个只执行一次的闭包中以同样的方式使用 self 的帮助(实例)函数.

W.r.t. to your own attempt: you may, in the same way, make use of help (instance) functions of self in this once-only executed closure.

class Foo {
    var foo: Int
    var bar: Int
    lazy var lazyFoobarSum: Int = { return self.getFooBarSum() }()

    init(foo: Int, bar: Int) { 
        self.foo = foo 
        self.bar = bar
    }

    func getFooBarSum() -> Int { return foo + bar }
}

let foo = Foo(foo: 2, bar: 3)
foo.foo = 7
print(foo.lazyFoobarSum) // 10

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