ReferenceWritableKeyPath to a struct property (like SwiftUI’s @State)

Using Swift
,
3

Ah, thanks for the hint about nonmutating set. I can confirm that:

@propertyWrapper struct MyWrapper {
    var wrappedValue: Int {
        get { 0 }
        nonmutating set { }
    }
}

struct MyStruct {
    @MyWrapper var i: Int
}

print(type(of: \MyStruct.i)) // ReferenceWritableKeyPath<MyStruct, Int>

Removing just the nonmutating keyword results in a WritableKeyPath again.

I’m sure somebody put a whole lot of thought into this and there are good reasons why it is the way it is. Now I just need to figure out these reasons :thinking:

Edit: What I’d come up with if put on the spot is: A struct property that can be set without mutating the struct itself must be something that “delegates” the mutation to some place behind a pointer, like the example by Rob Mayoff with the UnsafeMutablePointer in this post. Another option would be a struct that delegates its storage to a singleton like UserDefaults.
The crucial thing is that there must be a pointer involved somewhere, i.e. a reference to something else.

Therefore, key paths to properties of structs that have nonmutating setters are ReferenceWritableKeyPaths.

I’m not sure this is convincing, but that’s what I’d guess.

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