Key-value coding is a really cool function that works well with key-value observing. It allows you to code less and create very elegant solutions and code modules. There are many cases in a real application when something changes and another part of the application should be affected. The thing is that you can do anything when a property of an instance or class changes, including but not limited to checking whether its value is valid, sending a message to someone when something changes to a certain value, and so on. The options are unlimited.
We will cover the following topics in this chapter: