Editing Observer pattern (section)

===UML class and sequence diagram===

[[File:w3sDesign Observer Design Pattern UML.jpg|frame|none|A sample UML class and sequence diagram for the observer design pattern. <ref>{{cite web|title=The Observer design pattern - Structure and Collaboration|url=http://w3sdesign.com/?gr=b07&ugr=struct|website=w3sDesign.com|access-date=2017-08-12}}</ref>]]

In this [[Unified Modeling Language|UML]] [[class diagram]], the <code>Subject</code> class does not update the state of dependent objects directly. Instead, <code>Subject</code> refers to the <code>Observer</code> interface (<code>update()</code>) for updating state, which makes the <code>Subject</code> independent of how the state of dependent objects is updated. The <code>Observer1</code> and <code>Observer2</code> classes implement the <code>Observer</code> interface by synchronizing their state with subject's state.

The [[Unified Modeling Language|UML]] [[sequence diagram]] shows the runtime interactions: The <code>Observer1</code> and <code>Observer2</code> objects call <code>attach(this)</code> on <code>Subject1</code> to register themselves. Assuming that the state of <code>Subject1</code> changes, <code>Subject1</code> calls <code>notify()</code> on itself. <code>notify()</code> calls <code>update()</code> on the registered <code>Observer1</code> and <code>Observer2</code>objects, which request the changed data (<code>getState()</code>) from <code>Subject1</code> to update (synchronize) their state.