ios willSet / didSet 中的“oldValue”和“newValue”默认参数名称无法识别
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"oldValue" and "newValue" default parameters names inside willSet / didSet unrecognized
提问by Adithya
I am currently writing Swift 3 code in Xcode 8.
我目前正在 Xcode 8 中编写 Swift 3 代码。
When using oldValueand newValuedefault parameters inside the willSetand didSetblocks, I am getting "unresolved identifier"compiler error.
在and块中使用oldValue和newValue默认参数时，出现编译器错误。willSetdidSet"unresolved identifier"
I have a very basic code as below
我有一个非常基本的代码如下
var vc:UIViewController? {
willSet {
print("Old value is \(oldValue)")
}
didSet(viewController) {
print("New value is \(newValue)")
}
}
Apple Documentationfor Swift 3 still seems to support these feature. I hope I am not missing anything here?
Swift 3 的Apple 文档似乎仍然支持这些功能。我希望我在这里没有遗漏任何东西？
回答by FranMowinckel
You can also use vc:
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)")
}
}
回答by Robin Stewart
The special variable newValueonly works within willSet, while oldValueonly works within didSet.
特殊变量newValue仅在 内有效willSet，而oldValue仅在 内有效didSet。
The property as referenced by its name (in this example vc) is still bound to the old value within willSetand is bound to the new value within didSet.
由其名称（在此示例中vc）引用的属性仍绑定到 中的旧值，willSet并绑定到 中的新值didSet。
回答by Dmytro Shvetsov
var vc:UIViewController? {
willSet {
print("New value is \(newValue)")
}
didSet {
print("Old value is \(oldValue)")
}
}
回答by Wissa
It's important to know that the special variable newValueonly works within willSet, while oldValueonly works within didSet.
重要的是要知道特殊变量newValue仅适用于willSet，而oldValue仅适用于didSet。
var vc: UIViewController? {
willSet {
// Here you can use vc as the old value since it's not changed yet
print("New value is \(newValue)")
}
didSet {
// Here you can use vc as the new value since it's already DID set
print("Old value is \(oldValue)")
}
}
回答by gomozor
You didn't try to initialize the variable with a new object by which you can set your clojure:
您没有尝试使用可以设置 clojure 的新对象来初始化变量：
var vc:UIViewController? = UIViewController(){
willSet {
print("Old value is \(oldValue)")
}
didSet(viewController) {
print("New value is \(newValue)")
}
}
回答by HymanMao
That's why we use NSKeyValueObservation to monitor class object
这就是为什么我们使用 NSKeyValueObservation 来监控类对象
