activeChild should be Equatable

你说得对，NSViewController是从NSObject继承的，NSObject和Equatable是一致的. 所以，NSViewController的每个子类仍然是Equatable.

因此，无论其他一致性要求如何，类型为NSViewController的变量仍然是Equatable.

事实上，这段代码编译得很好.

protocol SpinnerHosting: AnyObject { }
class MyViewController: NSViewController, SpinnerHosting { }
var a: (NSViewController & SpinnerHosting) = MyViewController()
var b: (NSViewController & SpinnerHosting) = a
print(a == b)

❌当我们将a%和b%设置为可选项时，问题就出现了.

protocol SpinnerHosting: AnyObject { }
class MyViewController: NSViewController, SpinnerHosting { }
var a: (NSViewController & SpinnerHosting)? = MyViewController()
var b: (NSViewController & SpinnerHosting)? = a
print(a == b) // Type 'any NSViewController & SpinnerHosting' cannot conform to 'Equatable'

现在我们得到你的错误👆

Wait, but Swift Conditional Conformance allows us to compare optional!

条件一致性最显著的好处是，存储其他类型(如数组或可选)的类型能够符合Equatable协议.

https://www.swift.org/blog/conditional-conformance/个

对，如果2个可选项包含2个可以相等的泛型元素，则可以将它们等同

~

protocol SpinnerHosting: AnyObject { }
class MyViewController: NSViewController, SpinnerHosting { }
var a: NSViewController? = MyViewController()
var b: NSViewController? = a
print(a == b)

然而，当涉及Existential Types和Conditional Conformance时，SWIFT编译器似乎无法推断Equatable符合性.

✅您可以帮助SWIFT编译器添加显式强制转换

protocol SpinnerHosting: AnyObject { }
class MyViewController: NSViewController, SpinnerHosting { }
var a: (NSViewController & SpinnerHosting)? = MyViewController()
var b: (NSViewController & SpinnerHosting)? = a
print(a as NSViewController? == b as NSViewController?)

现在它起作用了.

Here's another example

我们可以做一个测试，用Array代替Optional(这与Conditional Conformance有类似的好处).

✅将编译此代码.

let list: [NSViewController] = []
list == list

但是，当我们把存在主义类型加入到聚会中时，

let list: [NSViewController & Codable] = []
list == list // Type 'any NSViewController & Codable' (aka 'any NSViewController & Decodable & Encodable') cannot conform to 'Equatable'

希望能有所帮助.