Controller.swift

protocol ThingDelegate: class {
    var fooString: String { get }
    func foo()
}

class Controller: ThingDelegate {
    let fooString: String
    func foo() {
        print("foo")
    }

    let myThing: Thing

    init () {
        fooString = ""
        myThing = Thing(delegate: self)    // ERROR: Constant 'self.myThing' used before being initialized
    }
}

class Thing {
    let delegate: ThingDelegate

    init(delegate: ThingDelegate) {
        self.delegate = delegate
    }
}