Downcasting with protocols and classes

The ComicCharacter protocol defines one of the method requirements for the drawSpeechBalloon method with destination as an argument of the ComicCharacter type, which is the same type that the protocol defines. The following is the first line in our sample code that called this method:

    brian.drawSpeechBalloon(destination: merlin, message: "How do you do?") 

We called the method defined within the AngryDog class because brian is an instance of AngryDog. We passed an AngryDog instance, merlin, to the destination argument. The method works with the destination argument as an instance that conforms to the ComicCharacter protocol; therefore, whenever we reference the destination variable, we will only be able to see what the ComicCharacter type defines.

We can easily understand what happens under the hood when Swift downcasts a type from its original type to a target type, such as a protocol to which the class conforms. In this case, AngryDog is downcasted...