Protocol extension doesn't seem to enforce variable in consumer?

Question

Say you

protocol Able: class {
    var v:UIView? { get set }
    var x:CGFloat { get set }
}

then of course, when you use Able,

if you forget "v" or "x"...

it is an error. That's good.

So do this:

class ScreenThing: UIViewController, Able {
    @IBOutlet var v: UIView?
    var x: CGFloat = 0.0
}

All's well. That's great.

It is enforced that you specify "v" and "x" and indeed initialize them.

But. Try this...

var _H: UInt8 = 0

protocol Able: class {
}

extension Able where Self:UIViewController {

    var p:P {
    get {
        return objc_getAssociatedObject(self, &_H) as! P
        }
    set {
        objc_setAssociatedObject(self, &_H, newValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        __setter()
        }
    }
}

Able now has a property p.

You can use p perfectly either in functions in Able, or, in functions in ScreenThing. That's great.

However.....

When you do this.....

class ScreenThing: UIViewController, Able {
}

you do not get an error.

You can forget to initialize "p" (it will crash).

Indeed you don't have to specify "p" as a variable (as you must with "v" and "x").

Why is it so?

This seems like a huge problem.

Is there something different I have to do, to, make the compiler enforce "p", just as it of course normally enforces variables in protocols?

---

Another way to look at this question:

Given exactly my code above:

is there a way to enforce the compiler to need an initializer for "p" in the consumer class?

For example.

I tried this...

class ScreenThing: UIViewController, Able {
    var p:P
}

But that doesn't work.

(Strangely, that compiles - actually I don't know what the hell it's doing! It seems to be a "different" p from the p in the Extension. But in any event it doesn't enforce the need for an initializer.)

In short, again, is there something I could do or add, above, that would make the compiler enforce me initializing the pseudo-property-thing, just as of course it normally does when I put a property in the protocol such as "x" or "v".

?

• maybe I have to add some ordinary property in the protocol (like "pp"), and somehow make p related to that in some way?? Just a thought.

---

(Footnote -- see this to understand the ": class" needed in the protocol above.)

---

Answering my own question:

My confusion above is that there is nothing to initialize. The var p:P in the extension (with the get and set code blocks) is simply two functions.

There's nothing to initialize.

So for example: in my extra question, I ask "how to force conforming classes initailize it on wake up?" That is meaningless. If anything, one could ask: "how to force conforming classes be sure to 'use those functions' on wake up?" - which has nothing to do with initialization.

Note too that my specific example code in the computed variable, happens to (unrelatedly) use a variable that doesn't get initialized - leading to confusion.

Answer 1

You don't have to implement p in the protocol's adopter, because the protocol extension has supplied an implementation. That is what a protocol extension is.

Simpler example:

protocol P {}
extension P {
    func greet() {print("hello")}
}
class C : P {}
C().greet()

Note that (1) that compiles even though C does not declare greet and (2) it runs even though C does not contain an implementation of greet. That's because that's the job of the protocol extension.