swift - 属性(property)观察员应该什么时候运行？有哪些异常(exception)情况？

Question

在以下情况下，属性观察者的行为让我感到惊讶。我知道观察者不会递归地调用自己，但这种行为似乎会转移到同一类的不同实例，甚至转移到不同子类的实例。

据我了解，属性观察器会在任何时候设置属性时运行，即使值没有改变，初始化时除外。该规则的异常(exception)情况到底是什么？如下所示，究竟什么时候会忽视属性(property)观察员？

var observersCalled = 0

class ClassOne {
    var relatedOne: ClassOne?
    var relatedTwo: ClassTwo?
    var property: String = "Initial" {
        didSet {
            observersCalled += 1
            relatedOne?.property = property
            relatedTwo?.property = property
        }
    }
}

class ClassTwo {
    var property: String = "Initial" {
        didSet {
            observersCalled += 1
        }
    }
}

class Subclass: ClassOne {
    override var property: String {
        didSet {
            observersCalled += 1
        }
    }
}

let thing = ClassOne()
thing.relatedOne = ClassOne()
thing.property = "New Value"
print(observersCalled) //1 (really?)

observersCalled = 0
thing.relatedOne = nil
thing.relatedTwo = ClassTwo()
thing.property = "Another Value"
print(observersCalled) //2 (makes sense)

observersCalled = 0
thing.relatedOne = Subclass()
thing.relatedTwo = nil
thing.property = "Yet Another Value"
print(observersCalled) //1 (really!?)

Answer 1

没有异常(exception)。我询问的行为是一个错误。我提交了a bug report对于那些跟踪其进展的人。