Swift lazy instantiating using self

问题

I have something that really puzzles me, specifically the following code triggers a compiler error "unresolved identifier self", and I am not sure why this is happening, as lazy means that at the time the property will be used, the class is already instantiated. Am I missing something?

Many thanks in advance.

Here is the code

class FirstClass {
    unowned var second: SecondClass

    init(second:SecondClass) {
        self.second = second
        print("First reporting for duty")
    }

    func aMethod() {
        print("First's method reporting for duty")
    }
}


class SecondClass {

    lazy var first = FirstClass(second: self)

    func aMethod() {
        first.aMethod()
    }
}

回答1:

For some reason, a lazy property needs an explicit type annotation if its initial value refers to self. This is mentioned on the swift-evolution mailing list, however I cannot explain why that is necessary.

With

lazy var first: FirstClass = FirstClass(second: self)
//            ^^^^^^^^^^^^

your code compiles and runs as expected.

Here is another example which demonstrates that the problem occurs also with structs, i.e. it is unrelated to subclassing:

func foo(x: Int) -> Int { return x + 1 }

struct MyClass {
    let x = 1

    lazy var y = foo(0)            // No compiler error
    lazy var z1 = foo(self.x)      // error: use of unresolved identifier 'self'
    lazy var z2: Int = foo(self.x) // No compiler error
}

The initial value of y does not depend on self and does not need a type annotation. The initial values of z1/z2 depend on self, and it compiles only with an explicit type annotation.

Update: This has been fixed in Swift 4/Xcode 9 beta 3, lazy property initializers can now reference instance members without explicit self, and without explicit type annotation. (Thanks to @hamish for the update.)

来源：https://stackoverflow.com/questions/38118429/swift-lazy-instantiating-using-self