Question

我有一些类用“后向指针”创建图形。我尝试制作后向指针unowned（否则会创建周期）并在我的init()中初始化它们，但这会导致self引用出现问题。我怎么能绕过这个？

class Parent {
    var child1:Child
    var child2:Child
    unowned var myOwner:Thing

    init(myOwner:Thing) {
        child1 = Child(parent: self) // Compiler errors here
        child2 = Child(parent: self) // and here
        self.myOwner = myOwner
    }
}

class Child {
    unowned var parent:Parent

    init(parent:Parent) {
        self.parent = parent
    }
}

我得到的错误是

'self' used before all all stored properties are initialized

Answer 1

You have that error because Swift enforces that all variables must have a value before you can use self to avoid your instance not being fully initialised. In your case you have two options:

1. Mark child1 and child2 as implicitly unwrapped optionals:

 class Parent {
    var child1: Child!
    var child2: Child!
    unowned var myOwner:Thing

    init(myOwner:Thing) {
        // Note - myOwner is being set before the children.
        self.myOwner = myOwner

        child1 = Child(parent: self) 
        child2 = Child(parent: self) 
    }
}

This results in no errors because both child1 and child2 have a default value of nil, which you are then changing in init.

2. Use lazy instantiation:

class Parent {
    lazy var child1: Child = Child(parent: self)
    lazy var child2: Child = Child(parent: self)
    unowned var myOwner:Thing

    init(myOwner:Thing) {
        self.myOwner = myOwner
    }
}

This results in no errors because a lazy property's value is only calculated when it is first used, therefore self is guaranteed to have been fully initialised.

Hope that helps.

如何在Swift中初始化双向图？

1 个答案: