Here's a lot of detailed questions from Kimberly Marcantonio, an active participant in the DSL Tools Newsgroup. I thought it would be more useful to the community to publicise the answers on my blog. Indeed, expect me to do this more, when the answers are likely to be of interest to those following the DSL Tools. The questions also touch on issues concerning the direction we're taking with this technology. I've tried to be open in my responses, without making firm commitments. I hope soon to be more precise about our plans for new features and their roll out.
In what follows, Kimberly's text is in blue, and my responses are in red italic...
I am currently trying to model the Corba Component Model using this DSL package and have run into the following problems/questions:
- Why is it that you can not currently move the classes (drag and drop) around the canvas? This leads to very spread out models that take up a lot of space and do not print well. Is there a better way to print these models to make sure that they fit onto one page?
We took the decision to automate as much of the layout as possible. You can use 'Bring Definition Here' and 'Create Root' context menu items when a node in the diagram is selected, to control where definitions of classes appear in the diagram. Our experience is that this gives a reasonable amount of control of diagram layout, without losing the significant advantages of autolayout. I'd be interested to know if anyone has tried using these, and whether this helps with the printing issue? We would like to add facilities for being able to create partial diagrams, perhaps showing relationships structures as the true graphs which they are, but have to balance this against the myriad of other features we need to build (e.g. see comments about constraints and serialization below).
- Can you offer further explanation into when to use an embedded relationship, and a Reference relationship? I understand the use of Inheritance, but often do not know when to use the other two. Also I feel as if there should just be a regular connection, for sometimes I feel these two types of connections are not fitting. Is containment the same as embedding?
Embedding and reference are used to drive behaviours of the underlying tools, or, to be more accurate will be used to drive the behaviour of the underlying tools. At the moment they drive the deletion behaviour of a designer - the default behaviour is taken from the diagram, deletion is propagated across embeddings relationships but not reference relationships, though this can be overridden using the delete behaviour designer in the DMD. This information will also be used to drive the XML serialization format (the approach to serialization is only an interim measure at the moment) and the default layout of the explorer. There are other aspects of behaviour where this kind of information is useful, though I won't go into that here. Also see answers below.
- Also could I have more information as to what the XML root is used for? I sometimes feel as if my diagrams have no root, or multiple roots, yet this is not supported.
The current serialization solution is only an interim measure. XML root is used to indicate which element is used at the top of the tree when a model is serialized into a file, and teh kind of element a diagram must map to. Our actual approach to serialization should be richer and mor domain specific than this, and the constraint requiring a diagram to map to the XML root is likely to be relaxed.
- Is there anyway to enforce constraints in this modeling language, such as OCL (Object-Constraint Language)?
Not yet, but constraint validation is in our plans. We'll probably just use .Net languages to write the bodies of constraints initially, as you that brings with it the intellisense and debugging support, but all the plumbing into the designer, reported of errors etc. will be handled for you.
- Is it possible to have more than one .dmd file in a project? If so do you have one .dd file for all of these .dmd files, or many .dd files, one for each .dmd files?
Yes you can have more than one dmd file per project, and indeed you can generate the code for each one you have (currently you'll need to make copies of the three .mdfomt files in the ObjectModel directory, giving them names that match the name of your .dmd file and editing the <%@ modelFile ... %> line). However, at the moment a .dd file can only refer to one .dmd file. Our plans include the ability to define many designers per .dmd, and have one designer able to view models from multiple dmd's. Exactly how we'll do this (there are a number of design options) is yet to be worked out. Basically, we're in the business of being able to create multiple designers which can provide different perspectives on the same model, as well as being able to break down the definition of a domain model into component parts. At least that's the plan.
- If you can have multiple .dmd files can you reference classes on other models?
Yes, though the mechanism is a bit clunky at the moment. To create a reference to a class in another domain model, create a class with the same name and namespace, and then set its IsLoaded property to be false. The code generated from the domain model will put in the appropriate references, though, thinking about it, I don't think the designer will quite do the right thing (it needs to reference both models and ensure that their definitions are loaded into the store on startup).
- Can you show Bi-directional connections?
All relationships are bidirectional. It's just that we have chosen to overlay the definition of relationships with a tree navigator - the diagram reflects one way of walking a graph of objects of classes defined in the model connected by links whcih are instances of relationships in the model, as a tree. This tree is used as the default for behaviours in the designer which require this, such as serialization to XML, viewing a model in the explorer and deletion behavior. At present, the DMD allows you to override this default for deletion behaviour in the Delete Behavior Designer. In future versions, we hope to provide a means of defining similar overrides to drive XML serialization and the explorer behaviour. Also see answer to (2).
- Can you cut across the tree hierarchy?
If I understand the question correctly, yes. You can define relationships between any two classes, including different nodes in the tree. When you do so, a 'use node' will be created for the target class, as a child of the source class, wherever the definition of the source class appears. You can do this for both embedding and reference relationships. Also see answer to (2).
- Can two classes share the same value property?
No. We follow standard OO practice in this regard. So the only way to achieve this result is to have a common superclass which defines the value property.
- Why is it not possible to cut and paste? This would make it easier to create similar classes
This is just a feature we have not implemented yet.
- If B is a child of A, and C is a child of B, does C have to be a child of A?
I assume you're not talking about inheritance here, but embedding relationships, and that you are asking whether the definition of C must appear beneath the definition of B which appears beneath the deifnition of A. It is possible to have the defintiion of C appear as a root on the diagram, or anywhere C is referenced, as the target of an embedding or reference relationship, or as a child in an inheritance relationship. Select the node where you want the definition to appear, and choose the 'Bring Definition Here' option, or choose 'Create Root' if you want the definition to appear at the root of the diagram. Details are in the DMD walkthrough. Also see answer to (2).