The Hollywood Principle

As you might know if you read this blog, I've been getting very excited by Inversion of Control containers recently. One of the things I keep coming across is 'The Hollywood Principle':

"don't call us, we'll call you."

Basically the idea is that a class says what it does by implementing an interface and what it needs by requesting interfaces, then a framework decides when to create it and what concrete instances to give it.