@swiftbot Key Path 経由だと無限再帰・・・。

struct Foo { var count: Int = 0 var a: Int = 42 { didSet { print("Foo.a didSet: \(count)") count += 1 } } } struct Bar { var count: Int = 0 var foo: Foo = Foo() { didSet { print("Bar.foo didSet: \(count), \(foo.count)") count += 1 // foo.count += 100 self[keyPath: \Bar.foo].count += 100 } } } var bar = Bar() bar.foo.a = -1

(edited)