Objects, relationships and roles are reasonably strong mechanisms for presenting the semantics between various model elements. However, sometimes even more precise semantics or behavioral rules are required for bindings. For this purpose, the GOPPRR metamodeling language defines a concept of Port. Port is a concept that can be thought of as part of an object to which roles connect. Using ports we can provide additional information or constraints on how roles and objects are connected.

While ports are mostly intended for use on the conceptual level, they can also be utilized for advanced representational structures. For example, one visualization-related requirement for our Family Tree language could be that each Child role must be connected only to the top edge of the ‘Person’ symbol and each Parent role only to the side edge (right for Male, left for Female). This would constrain users to draw diagrams in the way we did in Figure 1-1, with the father on the left and mother on the right, a line between them as parents, and from the midpoint of that line descending lines to children. With our current binding definition such precision is not possible, so let us see what kinds of additions would make it possible.

What we need to do is to define two new connectables for our symbols and give them respective Parent and Child ports. First of all, though, we must define a new port type. Start the Port Tool by clicking Port Tool toolbar button or selecting Metamodel | Port Tool. Enter ‘Family Port’ in the top-most field and add a new property type called ‘Port type’ (with data type of String and widget of Input Field). The Port Tool should now look like in Figure 5-8. Accept the generation of the new port type and quit the Port Tool by pressing Save and Close.

Open an Object Tool then the Symbol Editor for the ‘Male’ object. To make the drawing of new connectables easier, remove the default one by selecting it (click the red cross-hair in the middle of the symbol) and choosing Delete from its pop-up menu. Activate the Connectable tool from the toolbar (the red polygon with a cross-hair in it on the right). Move the cursor over the top-left corner of the symbol, click the left mouse button, move the cursor to the top-right corner and double-click the left mouse button. A new connectable should now appear as in Figure 5-9 (blue parts of the symbol are shown as light-grey in the figure to emphasize the new connectable).

Keep the new connectable selected and select Format... from its pop-up menu. In the Port(s) list, press the right mouse button and select Add... from the pop-up menu that opens. Enter ‘Child’ in the Port type field and press OK. The Child port now appears in the Port(s) list as in Figure 5-10. Press OK to accept and close the format dialog.

Now create another connectable in a similar manner for the right edge of the symbol and give it a new Family port called Parent. The symbol definition should now look like Figure 5-11. Save the symbol and quit the Symbol Editor.

Next, open the Symbol Editor for the ‘Female’ object. Create a connectable along the top edge as you did for the ‘Male’ symbol, but do not create a new port for it. Instead, add the existing Child port to it by selecting Add Existing... from the Port(s) list’s pop-up menu in the connectable format dialog and choose the existing Child port from the list that opens. Next, make a new connectable down the side like for Male, but this time along the left edge of the symbol, and give it the existing Parent port. It is important to use the existing ports rather than creating new ones: otherwise you will not be able to create relationships between these objects. The symbol for the ‘Female’ object should now look like in Figure 5-12.

Once you have made these additions to the symbols, open the Graph Tool for the ‘Family Tree’ graph type and go to the Bindings page. Select the Family relationship from the Relationships list and the Parent role from the Roles list, go to the Ports list, press the right mouse button and select Add... from the pop-up menu. You will now see the following dialog:

As we want to create a static port in this case, choose Static. In the list dialog that opens next, double-click the ‘Family Port’ type and then select its instance, ‘Parent: Family Port’ from the next list dialog that appears. The Graph Tool should now appear as in Figure 5-14.

Similarly, attach the Parent port to the other Parent role, and the Child port to the Child role. You can then close the Graph bindings definer and accept the changes to the graph type by pressing Save in the Graph Tool.

Now let’s try out this latest version of the Family Tree modeling language. Delete the objects you created before, and recreate them as in Figure 5-15. Note that when clicking an object while creating a relationship, that port is selected whose connectable’s cross-hair is closest to the cursor. To make sure that the correct port is selected, you should thus click near to the edge you want: the right edge for a father, left for a mother, and top for a child. You will notice that unlike before, when role lines could move freely around the symbol while you moved related objects, all Child roles now remain connected to the top edge of the symbols. Similarly, Parent roles stick to the left or right edge of the parent symbols.

If you notice that although you are using the grid, role lines are no longer precisely horizontal or vertical, the probable cause is the position of the connectables’ cross-hairs. These form the target points to which role lines point. If, for example, the Child cross-hair is not directly above the symbol’s center, as shown by the cross-hair of the default connectable in the middle of the symbol, role lines to the Child port will not be exactly vertical. You can correct the position of the cross-hairs in the Symbol Editor. Select the cross-hair at the centre of the symbol, and with the shift key down add the cross-hair for the Child connectable. Then choose Align | Align Center to shift the Child connectable along so it is centered over the default connectable’s cross-hair. The Parent connectable can be corrected similarly, but using Align | Align Middle to shift it up or down.