An alternative combined fragment is used to specify an area of a group of lifelines/ actors to show conditional flow in a sequence diagram. It also models the if-then-else logic in the sequence diagram. In this article, we will demonstrate how to draw a simple sequence diagram by using an alternative combined fragment.
- Create a sequence diagram as follows:
- Select Alt. Combined Fragment in the diagram toolbar. Press on the blank area of the diagram and drag to cover the Person and ATM Machine lifeline.
- Draw a message from ATM Machine lifeline to Person lifeline in the upper compartment of the alternative combined fragment. Name it as Ask for password. Draw another message from ATM Machine lifeline to Person lifeline in the lower compartment of the alternative combined fragment afterwards. Name it as Display invalid card.
- Right-click on the combined fragment and select Open Specification… from the pop-up menu to open the Combined Fragment Specification dialog box.
- When the Combined Fragments Specification window appears, open the Interaction Operands tab, select Operand, and then click Open Specification… button. In the Interaction Operand Specification dialog box, open the Guard tab and enter Valid card in Constraint text field. Click OK button to confirm the changes.
- Similarly, enter Invalid card in Constraint text field for Operand2by following the previous step.
- Guard conditions are displayed in the alternative combined fragment as shown below:
How to try the sample
Download ATM machine and then unzip it to get the .vpp file.