The subject-based style of event programming is a natural extension of unicast callback-based event handling. In this style of programming, event emitters (subjects) allow other objects to subscribe/register for notifications about their events. In practice, this is very similar to callback-based style, as event emitters usually store a list of functions or methods to call when some new event happens.
In subject-based event programming, the focus moves from the event to the subject (event emitter). The most common emanation of that style is the Observer design pattern. We will discuss the Observer design pattern in detail in Chapter 17, Useful Design Patterns, but it is so important that we can't discuss subject-based ...
