15 Callbacks, hooks, and runtime introspection

Ruby provides a rich set of runtime hooks and reflective tools that let programs react to events as they happen and inspect their own structure while running. Callbacks such as method_missing, included, extended, inherited, const_missing, method_added, and singleton_method_added can be used to intercept missing methods, module mix-ins, extensions, subclassing, unresolved constants, and newly defined methods. These hooks are per class or per object, making them precise tools for shaping behavior without adding a lot of boilerplate.

The chapter then explores how Ruby lets you query capabilities at runtime. Using methods like methods, private_methods, protected_methods, instance_methods, public_instance_methods, singleton_methods, and their variants, you can inspect what an object, class, or module can do, and even compare method lists to discover overrides or inheritance relationships. The text emphasizes filtering and combining these lists to answer practical questions, such as which bang methods lack non-bang counterparts or which classes override methods from Enumerable.

Finally, the chapter shows how Ruby can reveal what variables are in scope and how execution arrived at a given point. You can inspect local, global, instance, class, and constant-related state, and use caller or caller_locations to work with stack traces in either string form or as structured location objects. The chapter culminates in a small test framework that uses callbacks, method inspection, and stack introspection to automatically run test methods, execute setup code, and report assertion failures with useful location information, demonstrating how these features can be combined into practical metaprogramming.