code:

protocol KVO(properties: String[]) { // this protocol takes an array of properties to observe
    // conceptually, the body of the protocol is executed for each class that consumes the protocol
    // so we can loop over the parameters and generate hooks for each one
    // it's not *that* different from Swift's enum with associated values

    for property in properties {
        // generate before and after hook methods for each property in the array
        // these methods names of course can't be hardcoded,
        // since each consumer of the protocol chooses its own properties

        before "set\(property)" { publish("will-set-\(property)") }
        after "set\(property)" { publish("did-set-\(property)") }
    }
}


// this creates a one-off subclass of myView
// wraps the setBounds and setCenter methods with before- and after-hooks
// then changes the class of myView to that subclass
myView.meta.applyProtocol(KVO(["bounds", "center"]))
// note that this is happening at runtime


// could also inspect all objects that are, or descend from, a particular class
UIView.meta.apply(KVO["bounds", "center"]))


// this instance of the KVO protocol observes these different properties instead
myModel.meta.applyProtocol(KVO(["firstName", "lastName", "email"])