The Watch example distribution already contains an implementation of a visual debugger. To try it out, set the Generation target platform to ‘API’ in the property dialog for the ‘2008Models’ graph, make sure that API server is started and all browser windows are closed, and run the ‘Autobuild’ generator. The test environment will start the web browser again, and you can choose which watch model you want to test. When you activate the watch model and play around with it, the test environment will make a SOAP call to MetaEdit+, opening the diagram for the state machine that is being executed and highlighting the active state in the diagram (as in Figure 5-2). Thus, you can now run watch applications and follow their execution through the models.

How does it work, then? As the general behavior of animation (i.e. highlighting the target state after a state transition) remains the same for all elements, we found it most feasible to implement it as a framework feature. Therefore, API-specific versions of Master and AbstractWatchApplication framework Java components were created. It took one day from one developer to make these extensions. The ultimate benefit of this solution is of course that there were no changes in models and only a minimum amount of changes in code generators (i.e. we had to create new API-specific high-level ‘_create make for API’ and ‘_compile and execute for API’ generators).

The basic implementation idea of our visual debugger is very simple: at the end of each state transition, a SOAP message requesting highlighting of the specified model element is sent to the SOAP server running in MetaEdit+. MetaEdit+ processes the message, highlights the requested element and sends an acknowledging SOAP message back to the client application (this return message may also contain return values from API operations, but in this example only a NULL value is returned). For more information about the actual implementation of the visual debugger, we advise you to study the Java code in the Master and AbstractWatchApplication framework classes.