"oldValue"和"newValue"无法识别willSet/didSet中的默认参数名称

问题描述

我目前正在用Xcode 8编写Swift 3代码.

I am currently writing Swift 3 code in Xcode 8.

在willSet和didSet块中使用oldValue和newValue默认参数时，出现"unresolved identifier"编译器错误.

When using oldValue and newValue default parameters inside the willSet and didSet blocks, I am getting "unresolved identifier" compiler error.

我有一个非常基本的代码，如下所示

I have a very basic code as below

var vc:UIViewController? {
    willSet {
        print("Old value is \(oldValue)")
    }
    didSet(viewController) {
        print("New value is \(newValue)")
    }
}

Apple文档似乎仍支持这些功能.我希望我在这里不丢失任何东西吗?

Apple Documentation for Swift 3 still seems to support these feature. I hope I am not missing anything here?

推荐答案

您还可以使用vc:

var vc:UIViewController? {
    willSet {
        print("New value is \(newValue) and old is \(vc)")
    }
    didSet {
        print("Old value is \(oldValue) and new is \(vc)")
    }
}

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