Programming Groovy 2

19.10 Categories and DSLs

Using categories, we can intercept method calls in a controlled fashion (see Section 13.1, ​Injecting Methods Using Categories​). We can put that to use in creating a DSL. Let’s figure out ways to implement the following fluent call: ​2.days.ago.at(4.30)​.

​2​ is an instance of ​Integer​, and we know that ​days​ is not a property on it. We’ll inject that, using categories, as a property (the ​getDays​ method). The ​days​ is just noise here, but in another context it may be useful to differentiate between five days ago and five minutes ago. It provides connectivity in the sentence “two days ago at 4.30.” We can implement the method ​getDays​ that accepts ​Integer​ and returns the received instance. In the ​getAgo