使用 self 进行 Swift 惰性实例化

问题描述

我有一些让我很困惑的事情，特别是以下代码触发了编译器错误未解析的标识符自身"，我不确定为什么会发生这种情况，因为惰性意味着在使用该属性时，该类已经实例化了.我错过了什么吗?

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?

提前非常感谢.

这里是代码

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()
    }
}

推荐答案

由于某种原因，如果惰性属性需要显式类型注释初始值是指self.swift-evolution 邮件列表，但是我无法解释 为什么必要的.

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.

有

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

您的代码按预期编译和运行.

your code compiles and runs as expected.

这是另一个例子，它证明了问题的发生也与 structs，即它与子类无关:

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
}

y的初始值不依赖于self，不需要a类型注释.z1/z2的初始值依赖于self，并且只能使用显式类型注释进行编译.

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.

更新:这个已在 Swift 4 中修复/Xcode 9 测试版 3，惰性属性初始化器现在可以在没有显式 self 和显式类型注释的情况下引用实例成员.(感谢@hamish 的更新.)

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.)

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