Inversion Of Control

• In traditional procedural programming, the custom-built code calls into the library code.
• With IoC, it’s the other way round: the library code calls the custom code
• So for instance, you might have a UI framework that listens for user events such as mouseclicks, and then custom event handlers, or callbacks, which define what should happen when a particular menu item is clicked (for instance)
• Or, with dependency injection, the parent object just holds an interface to the child object, which creates itself and is then given to the parent, rather than the parent asking for it.
  • Or, think about the interface to a repository which has two implementations: one is NHibernate, one is stored procs. The implementation has CONTROL over how it does things – it is not up to the caller to decide.
• Dependency injection is a way of using IoC.
  • There is an example of dependency injection in my solution to the Train routes kata.
  • Another example is the FileLoader class in Reconciliate, which has IInputOutput and IClock interfaces passed into its constructor.