| section: < previous | next > | | | |
chapter: << previous | next >> |
3.1 Creating a navigation expression
A navigation expression consists of one or more Classifiers (ClassifierRoles and SetClassifierRoles) which are connected by AssociationRoles.
Navigations start at objects (ClassifierRoles or SetClassifierRoles) , go along AssociationEnds (the end roles of AssociationRoles) and end at properties that can be an attribute, a associationend or a operation or mehtod that is a query. You are able to draw different kinds of VOCL diagrams with this editor that are explained below.
The navigation starts with the ClassifierRole that has same context as the underlying Constraint. If no name is given for the context the navigation starts with the ClassifierRole whose name is self and the type corresponds with context type of the Constraint.
 |
Fig. 3.1.1 - Starting the
converter in the menu
You can start the converter by selecting the menu entry Convert to OCL
string in the Eclipse menu Visual OCL. If the VOCL expression can not
be converted then a dialog with an empty message appear otherwise the
OCL String corresponding to the VOCL expression is displayed.
In the user
handbook you can several examples for navigation expression in
appendix B.
In the next subsections several kinds of navigations are presented.
3.1.1 Attribute navigation
An attribute navigation ends at a ClassifierRole / SetClassifiier which
has attributes. In the following example we only have one
ClassifierRole c:Company with
its attributes name:String = GMC
and profitPerYear:Integer = x (see
Fig. 3.1.2). The first attribute has a constant value and the other one
has a variable which is bound to a Condition.
The starting point for the navigation is in that case the
ClassifierRole because its rolename and roletype correspond to the
context name and context type. It is always necessary to have such a
start point.
 |
Fig. 3.1.2 - attribute navigation
Now, the converter can be started. The head of the Constraint is overtaken to the OCL-String exactly. The variable being bound to one attribute is replaced by the operator and the right side of the Condition.
If there are several attributes, they are connected by a logical "and".
 |
Fig. 3.1.3 - OCL - String of attribute navigation example
3.1.2 Round trip
Another kind of navigations are round trips, that means that you can
reach one target ClassifierRole from a source ClassifierRole via
different targets. In the example in Fig. 3.1.4 you see the source
ClassifierRole (c:Company) and
the target ClassifierRole (manager:Person).
You can reach the target ClassifierRole either via manager or via the ClassifierRole emp:Person.
In Fig. 3.1.5 you can see this fact also in the OCL-String. Furthermore
the attribute navigation of the starting ClassifierRole are shown in
the OCL-String.
 |
Fig. 3.1.4- round trip navigation
 |
Fig. 3.1.5- OCL-String of round trip navigation
3.1.3 A more complex example
In Fig. 3.1.6 you can see a more complex example. First the navigation
starts with the ClassifierRole c:Company
because the context of the Constraint is of the type Company and has as its name c. Then you can navigate to the
SetClassifierRole Person via
the AssociationRole with the end employee.
The ClassifierRole Person has
an attribute salary whose
variable y is used in sum
operation and the result of the this operation is assigned to a
variable cost that is used in
the condition. The other variable profit
is a result of a method call in the ClassifierRole Company.
The resulting OCL-String can be seen in Fig. 3.1.7.
 |
Fig. 3.1.6 - a complex example
 |
Fig. 3.1.7 - OCL-String of the complex example