Gantt-Observer.uml

@startuml Gantt-Observer.png

class Observer {
   void receiveEvent()
}

class Subscription {
   void addObserver(Observer o)
   void delObserver(Observer o)
   void notifyObservers()
}

class Task implements Observer {
   DateTime start_time
   DateTime end_time
   DateTime::Duration duration
   ArrayRef[task] dependents
   ArrayRef[task] depends_on
   
   void calculateStart()
   void calculateEnd()
   void calculateDuration()
   void addDependent(Task t)
   void addDependsOn(Task t)
   
   void receiveEvent()

   void notifyStartChange()
   void notifyEndChange()
   
   void display()
}

class Task implements Subscription
@enduml 

@startuml Gantt-Observer-seq.png
   [-> task1 : start_time(12:00)
   [-> task1 : duration(0:30)
   [-> task2 : addDependsOn(task1)
   task2 -> task1 : end_time
   return end_time1
   task2 -> task2 : start_time(end_time1)
   task2 -> task1 : addObserver(task2)
   [-> task1 : addDependent(task2)
   [-> task2 : duration(0:30)
   task2 -> task2 : calculateEnd()
 
   [-> task1 : display()
   [-> task2 : display()
   
   [-> task1 : duration(0:45)
   task1 -> task1 : calculateEnd()
   task1 -> task1 : notifyEndChange()
   task1 -> task1 : notifyObservers()
   task1 -> task2 : receiveEvent()
   task2 -> task1 : end_time
   return end_time1
   task2 -> task2 : start_time(end_time1)
   task2 -> task2 : calculateEnd()

   [-> task1 : display()
   [-> task2 : display()
@enduml