//: Playground - noun: a place where people can play
import Foundation
struct Foo {
var bar = 0
}
class A {
var foo = Foo() {
didSet {
print("foo changed")
}
}
}
let a = A()
for i in 1...5 {
a.foo.bar = i
}
// prints five times
// foo changed
// foo changed
// foo changed
// foo changed
// foo changed
If I change `struct Foo` to `class Foo`, nothing will happen.
My code depends on the class version didSet but I used the struct version.
It took me a long time to debug this out. Is this a knowledge that has been
well known already?
In your example, when `Foo` is a class, then `A` does not see a mutation since the storage is out-of-line. When `Foo` is a struct, it is stored in-line and a mutation does mutate `A`.
In other words, when what’s stored in `A` is a reference, the reference itself does not get mutated when you mutate your `Foo`. If you change the statement in your loop to `a.foo = Foo()`, you’ll see that the `didSet` gets invoked again.
My code depends on the class version didSet but I used the struct version. It took me a long time to debug this out. Is this a knowledge that has been well known already?
Yes, that’s how it’s supposed to be.
It’s a feature of struct which isn’t available with classes (so considering the Objective-C past, the „surprise“ is actually that didSet is called when you do that change to a struct property).