Observer Pattern is an important Pattern backed by an important idea in software engineering. And it is usually a key part of MVC architecture and its variants as well.

If you have ever written an application with a rich user interface without a framework like Angular or a solution with React, you might probably have struggled with changing class names and other properties of UI elements. More specifically, the code that controls those properties of the same group of elements lies every branch related to the elements in related event listeners, just to keep the elements being correctly updated.

Consider a "Do" button of which the disabled property should be determined by the status of a WebSocket connection to a server and whether the currently active item is done. Every time the status of either the connection or the active item gets updated, we'll need to update the button correspondingly. The most "handy" way could be two somewhat identical groups of code being put in two...