什么时候应该运行属性观察器？有哪些例外？

Question

在以下情况下，财产观察者的行为让我感到惊讶。我知道观察者不会递归调用自己，但这种行为似乎会延续到同一类的不同实例，甚至会延续到不同子类的实例。

据我了解，属性观察者会在任何时候设置属性时运行，即使值没有改变，初始化除外。这条规则到底有哪些例外？如下所示，究竟什么时候会忽略属性观察者？

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

没有例外。我问的行为是一个错误。我已提交a bug report 供那些跟踪其进度的人使用。