Design patterns which I would like to present in this post are well described in the book Design Patterns. Elements of Reusable Object-Oriented Software  written by The Gang of Four (Gramma, Helm, Johnson, Vlissides). In my opinion this book is must read for every developer, regardless what programming language you are using. Besides the fact that this book was written more than 20 years ago, it still contains a lot of useful details for developers of all levels. I often revisit this book to keep in touc...