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:

无法在属性内使用实例成员'getEventCalendar' 初始值设定项；属性初始值设定项在自我"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来初始化lazy实例变量，其值取决于self的其他对象类型instance variables(不仅仅是self

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

您可以使用仅执行一次的闭包来捕获self的属性，并在执行时使用它们(=首次使用lazy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.自行尝试:您可以以同样的方式在此仅执行一次的闭包中使用self的帮助(实例)功能.

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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