Activity diagrams can also implement logic that is based on conditions. You can branch a control flow using the Decision element.
The following example shows an activity diagram that checks if a related REST service is up and running, or not.
The diagram performs a GET request on the service. Directly after this request, any thrown errors are caught and used to identify the status of the service.
The branching conditions must evaluate to a Boolean value (boolean expression) and be defined on the outgoing control flows of the decision node as a Guard Expression. Page Logical Operators provides a list of all possible logical operators that can be used within a guard expression. You can also use Boolean operators (and, or) as described in Boolean Operators.
In the example above, there are two outgoing control flows:
| Nr. | Control Flow | Condition | Guard Expression |
|---|---|---|---|
| 1 | Decision → setNotAvailable | REST service not available | catch errorCode = "*" errorType = "*" |
| 2 | Decision → setReturn | REST service up and running | else |
The one or more of the control flows contain the actual condition(s) (1). Exactly one control flow must contain an else expression (2): This control flow is followed if all other expressions evaluate to false.
The boolean operators of the xUML Runtime support short-circuiting . This means that the second operand is evaluated only when the result is not fully determined by the first operand.
Adding a Guard Expression
To add a guard expression to a control flow proceed as follows:
| Select the control flow you want to add a guard expression to from the activity diagram and open the Attributes panel. |
| Enter a boolean expression into the field Guard Expression. |
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