One of the biggest differentiators of popular frameworks is how easily they can be extended with third-party libraries. Up until now, all the classes we have developed have belonged either to the framework or to the application.


  • We need to understand how we can create our own libraries.
  • We need to understand how we can use libraries from other developers.
  • We need to understand what the Observer design pattern is and why it’s relevant to our application.
  • We need to build a means of listening for and emitting events within our applications.


The first kind of library we will look at is the kind we create ourselves. ...

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