Understanding the observer pattern
When we need to update a group of objects when the state of another object changes, a popular solution is offered by the Model-View-Controller (MVC) pattern. Let's assume that we are using the data of the same model in two views; for instance, in a pie chart and a spreadsheet. Whenever the model is modified, both views need to be updated. That's the role of the observer pattern.
The observer pattern describes a publish-subscribe relationship between a single object – the publisher, which is also known as the subject or observable – and one or more objects – the subscribers, also known as observers.
In the case of MVC, the publisher is the model, while the subscribers are the views. There are other examples that we will discuss throughout this chapter.
The ideas behind the observer pattern are the same as those behind the separation of concerns principle; that is, to increase decoupling between the publisher...
