Swift 错误:无法转换类型 'B -> 的值(A) ->()' 到预期的参数类型 '(A) ->空白' [英] Swift error: cannot convert value of type 'B -> (A) -> ()' to expected argument type '(A) -> Void'

问题描述

我有一个 Swift 程序，它产生一个我无法理解的编译器错误.它归结为以下代码片段.

I have a Swift program that produces a compiler error I cannot understand. It boils down to the following code fragment.

Class A 是一个辅助类，它在初始化时提供一个处理程序函数，稍后将被调用.这个函数有一个 A 对象作为参数.

Class Ais a helper class that is upon initialization provided with a handler function that is to be called later. This function has an A object as an argument.

Class B 初始化一个 A 类型的对象，并有一个函数 f 应该用作处理程序.或者，可以提供匿名代码块(闭包).

Class B initializes an object of type A and has a function f that should be used as the handler. Alternatively, an anonymous code block (closure) could be provided.

class A {
    init(_: (A) -> Void) {}
}

class B {
    let a = A(f)
    func f(a: A) {}
}

这会产生以下编译器消息:

This produces the following compiler message:

error: cannot convert value of type 'B -> (A) -> ()' to expected argument type '(A) -> Void'
let a = A(f)
          ^

Void 和 () 是等价的，我想.

Void and () are equivalent, I suppose.

我什至不理解双 -> 的类型表达式.这是什么类型的?

I do not even understand the type expression with the double ->. What kind of type is this?

谁能解释一下这里出了什么问题?还有，怎么做才对?

Can anyone explain what's wrong here? And, how to do it right?

推荐答案

在初始化实例属性(如 a)时不能引用实例方法(如 f)code>)，因为实例正是我们正在创建的.一种解决方案是声明您的实例属性 lazy，如下所示:

You can't refer to an instance method (like f) while initializing an instance property (like a), because the instance is exactly what we are in the middle of creating. One solution is to declare your instance property lazy, like this:

class A {
    init(_: (A) -> Void) {}
}

class B {
    func f(a: A) {}
    lazy var a : A = A(self.f)
}

这是合法的，因为 lazy 保证 a 的初始化器不会被评估，直到 later，当实例 已创建.请注意，在此上下文中绝对需要 self(通常我建议您始终在可以使用的地方使用它).

That is legal because lazy guarantees that the initializer for a won't be evaluated until later, when the instance has been created. Note that self is absolutely required in this context (and in general I recommend you always use it wherever it can be used).

另一种解决方案是将a声明为A!(这样它就有一个初始值，即nil)并初始化它"真正的"稍后.例如:

Another solution is to declare a as an A! (so that it has an initial value, namely nil) and initialize it "for real" later. For example:

class A {
    init(_: (A) -> Void) {}
}

class B {
    func f(a: A) {}
    var a : A!
    init() {
        self.a = A(self.f)
    }
}

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