Hands-On Design Patterns and Best Practices with Julia

The observer pattern

The observer pattern is useful for registering observers to an object so that all state changes in that object trigger the sending of notifications to the observers. In a language that supports first-class functions—for example, Julia—such functionality can be implemented easily by maintaining a list of functions that can be called before or after the state changes of an object. Sometimes, these functions are called hooks.

The implementation of the observer pattern in Julia may consist of two parts:

1. Extend the setproperty! function of an object to monitor state changes and notify observers.
2. Maintain a dictionary that can be used to look up the functions to call.

For this demonstration, we will bring up the bank account ...