Observer Pattern

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

Let objects propagate information without depending on each other much.

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

• publish-subscribe / constraints / broadcast / change-update

Purpose:

Mainly used to implement distributed event handling systems.

3 Main Ideas:

Registration

• Observer registers itself as a dependent of subject.

  subject addDependent: observer

Notification

• Object notifies dependents when information changes by sending itself a "changed" message.

  changed: anArg
  {
  
    For each dependent { update: anArg }
  
  }

Update

• Dependent can define what it does in response to changes by defining the update method
  
  
  update: anArg                                                            
  {                                                                                
        do whatever                                                         
  }