Is there a way to get didSet to work when changing a property in a class?

问题

I have a class as property with a property observer. If I change something in that class, is there a way to trigger didSet as shown in the example:

class Foo {
    var items = [1,2,3,4,5]
    var number: Int = 0 {
        didSet {
            items += [number]
        }
    }
}

var test: Foo = Foo() {
    didSet {
        println("I want this to be printed after changing number in test")
    }
}

test.number = 1 // Nothing happens

回答1:

Nothing happens because the observer is on test, which is a Foo instance. But you changed test.number, not test itself. Foo is a class, and a class is a reference type, so its instances are mutable in place.

If you want to see the log message, set test itself to a different value (e.g. a different Foo()).

Or, add the println statement to the other didSet, the one you've already got on Foo's number property.

Or, make Foo a struct instead of a class; changing a struct property does replace the struct, because a struct is a value type, not a reference type.

来源：https://stackoverflow.com/questions/29453735/is-there-a-way-to-get-didset-to-work-when-changing-a-property-in-a-class