1. Observer Design Pattern
observing changes

2. Observer Motivation
objects (observers) need to synchronize behavior with some other object (subject): observe changing subject’s state
ex: bar chart and bar graph need to know when original spreadsheet values change

3. Observer Implementation
observers subscribe to change state notifications, while subjects publish changes by notifying observers or sending them messages (synonymous terms)
another name for pattern – publish-subscribe
common way to implement GUI interfaces (foundation of so-called model-view-controller architectural pattern)
the “business logic” is in subject hierarchy
the “presentation logic” is in observer hierarchy
if using abstract/concrete classes
abstract observer/subject – implement registration/notification functionality
concrete observer/subject – implement subject state and state acquisition by observer
there may be a registry of subjects for observers to subscribe to
behavioral pattern

4. Communication Methods: Push & Pull
observer needs to communicate state change information to subject
two communication methods
push – state change is in message itself
may require large message
not all concrete subjects need all the data
subject may push a reference to itself
pull – observer queries the state of subject after receiving notification
observer needs to keep reference to subject
subject needs to implement getters

5. Observer UML Diagram (Push Method)
+notify()

6. Observer UML Diagram (Pull Method)

7. (Forward) Class Declaration
in C++, any named construct must be declared before use
a class may be declared by
class definition
(forward) class declaration
syntax
class NameOfClass;
after forward declaration, can declare pointers/references (but not objects) of this class
used when classes mutually refer to each other, or for better program structure
Class A; // declaration
Class B{
B(A*p):p_(p){}
private:
A *p_; };
// definition
Class A{
... };