Initialize lazy instance variable with value that

问题:

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

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

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? {
    // ...
  }
}

回答1:

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. 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

回答2:

It's a confusing error message (which you may well want to file a bug report on). The problem is just a quirk of lazy properties – they currently require an explicit use of self in order to access instance members, as well as an explicit type annotation when doing so (which has been previously noted in this Q&A).

Therefore you need to say:

lazy var eventCalendar: EKCalendar? = self.getEventCalendar()