At the beginning of Chapter 4, Decorators, Generics, and Asynchronous Features, we discussed the use of decorators, and how they are invoked when a class is defined. Decorators are not invoked when a class is instantiated, so their usage is limited to interrogating and manipulating class definitions. Decorators, as we know, can be applied to classes, properties, functions, and parameters. Let's build a simple class decorator, and see what information it gives us about a class.

Remember that there are three things about a class that we are interested in during this exercise. Firstly, we need to find the definition of the class constructor. Once we know what the constructor looks like, we need to find the list of parameters that the constructor uses. Each of these parameters will then become getters that use our service locator to resolve dependencies. The...