Initialize lazy instance variable with value that depends on other instance variables

Question

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

Cannot use instance member \'getEvent

Answer 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

Answer 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()