chapter02_observer

Chapter 2: Observer design pattern

Observer: defines a one-to-many dependency between objects so that when one object changes state, all of its dependents are notified and updated automatically.

A very useful design pattern, often used in user interfaces and as part of the popular Model-View-Controller (MVC) pattern used, for instance, in Django. As I wrote out the code I found it very appealing that I did not need to change the subject at all to add new observers.

Class Diagram

classDiagram

    Subject --> Observer : observers
    Subject1 <-- Observer1 : subject
    Subject1 <-- Observer2 : subject
    Subject <|-- Subject1
    Observer <|-- Observer1
    Observer <|-- Observer2
    Subject : attach(o)
    Subject : detach(o)
    Subject: notify()
    class Observer{
      update()
    }
    class Subject1{
      state
      get_state()
      set_state()
    }
    class Observer1{
        state
        update()
    }
        class Observer2{
        state
        update()
    }

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Use in Python

Django signals allow other parts of the code to sign up to receive updates, for instance when settings are changed
or model instances are created and / or updated.

Running the code

python weather.py