Common Mechanisms of UML

Activity Diagram 

• The main reason to use activity diagrams is to model the workflow behind the system being designed
• Activity diagrams are also useful for analyzing a use case by describing what actions need to take place and when they should occur 
• An activity diagram shows the flow from activity to activity 
• An activity is an ongoing non-atomic execution within a state machine

Activity diagram contains 

• Activity states and action states 
• Transitions 
• Objects 

Action States 

• The executable, atomic computations are called action states because; they are states of the system, each representing the execution of an action 
• They cannot be decomposed and atomic, meaning that events may occur, but the work of the action state is not interrupted 

Activity States 

• Activity states can be further decomposed 
• Activity states are not atomic, i.e., they may be interrupted
• They take some duration to complete 
• Activity states are also special kinds of states in a state machine

Transitions

• Transitions show the path from one action or activity state to the next action or activity state 
• A transition is represented as a simple directed line 
• Triggerless control passes immediately once the work of the source state is done 

Branching 

• It represents a branch as a diamond 
• A branch, represented as a diamond, may have one incoming transition and two oi more outgoing ones
• Each outgoing transition contains a guard expression, which is evaluated only once on entering the branch