变量在它自己的初始值中使用，而变量在 init 之后的闭包中使用 [英] Variable used within its own initial value while variable is used inside a closure past init

问题描述

typealias CBType = () -> Void

class A {

    let b = B()

    func test() {
        let token = b.register { CBType in
            self.b.waitFor([token]) // ERROR: Variable used within its own initial value
        }

        b.dispatch()
    }
}

class B {
    private var _callbacks = [String:CBType]()

    func register(callback: CBType) -> String {
        let id = "1234"
        _callbacks[id] = callback
        return id
    }

    func dispatch() {
        for (_, cb) in self._callbacks {
            cb()
        }
    }

    func waitFor(tokens: [String]) {
    }
}

A().test()

当我修改测试函数以使用实例变量时，事情又恢复了，但语法感觉有点沉重.

When I modify the test function to use a instance variable, things are working again but that syntax feels a bit heavy.

class A {

    let b = B()
    var token: String?

    func test() {
        token = b.register { CBType in
            self.b.waitFor([self.token!])
        }

        b.dispatch()
    }
}

为什么我不能在闭包中使用局部变量，因为在最终调用闭包时它会超过初始化时间?

Why can't I use a local variable in the closure since it will be way past initialization when the closure is finally called?

推荐答案

常量 token 在被闭包捕获时没有值.

The constant token doesn't have a value at the time it is captured by the closure.

您可以改用可变变量，闭包将捕获该变量而不是其值.

You can use a mutable variable instead, and the closure will capture the variable rather the its value.

func test() {
    var token = ""
    token = b.register {
        self.b.waitFor([token])
    }

    b.dispatch()
}

或者，您可以将令牌作为参数传递给闭包:

Alternatively, you can pass the token as a parameter into the closure:

typealias CBType = (String) -> Void

class A {
    let b = B()

    func test() {
        let token = b.register { theToken in
            self.b.waitFor([theToken])
        }

        b.dispatch()
    }
}

class B {
    private var _callbacks = [String:CBType]()

    func register(callback: CBType) -> String {
        let id = "1234"
        _callbacks[id] = callback
        return id
    }

    func dispatch() {
        for (id, cb) in self._callbacks {
            cb(id)
        }
    }

    func waitFor(tokens: [String]) {
        println("Wait for \(tokens)")
    }
}

A().test()

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