Quality Code: Software Testing Principles, Practices, and Patterns

Callbacks, Observers, Listeners, and Notifiers

Pretty much any system that requires a low-latency response to asynchronous events uses some form of callbacks, observers, or notifiers. They can be found in user interface toolkits, in which they are often called event listeners, and distributed systems. Callbacks are a generic mechanism allowing you to supply code that the system invokes when certain well-known events happen. They have a variety of forms, some making creative use of language-specific features. Languages like C, Perl, JavaScript, and others with a procedural or functional flavor use function pointers (Listing 12-6) or function references (Listing 12-7). Object-oriented languages often implement callbacks through implementations ...