Recently I have came across an excellent book on how to develop code with testing in mind, and I wanted to share it with you. The book is called Growing Object-Oriented Software, Guided by Tests and is written by Steve Freeman and Nat Pryce.
The book starts right at the beggining as to why we want to develop test first and covers advanced topics such as testing persistance, threads, and asynchronous code. I particulary like the style with which the book delivers the message. They start by building a simple application and add new requirements to it, morphing the codebase in the process. As they do so they introduce new classes and walk the reader through the thought process considering alternatives finally choosing a solution. This gives the reader a good understanding as to what to think about when looking at code. Of course all of these changes are driven by tests, and a lot of discussion is spent on explaining why a test was written in a particular way and how to refactor it as it grows with the applications, so that tests do not become a liability.
Reading the book I sometimes felt that I was listening to myself, especially when the authors warned about global state, singletons, overusing mocks, and doing work in constructors among other things. But unlike myself, who draws sharp lines between right and wrong, the authors did a good job of presenting things on the gray scale of benefits and drawbacks. The book shows what a typical code most people will write, and then show how tests point a way towards refactoring.
If you are newbie, or an intermediate to developing with tests than this book is a must for your library!