2: Creating and Using DSLs in Visual C#

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2: Creating and Using DSLs
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insoluble problem to generate automatically the application that meets a general requirements statement, it is quite easy to generate applications in a restricted domain that meet the requirements expressed by a language speci c to that domain
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Developing the DSL: From Sketches to Domain Model After experimenting with many sketches of state diagrams and taking into consideration both the variability in the code they want to generate, and the concepts that customers want to express the developers come up with a de nition of the language The core of this de nition is the domain model, which states the kinds of things that are dealt with by instances of the language (individual state diagrams in this case) Using the DSL Tools domain modeling tool, they create the diagram in Figure 2-3 (we ll see how in the next major section)
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Figure 2-3: Initial DSL definition for the Issue State language
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A domain model is the central part of a DSL s de nition It de nes, among other things, domain classes (the round-cornered boxes) and domain relationships (with square corners) The model de nes the classes of elements there may be in instances of the language and the relationships there may be among them Reading this diagram from the top left downwards, it states that: An IssueStateModel consists of any number of StateElements; the relationship between the two is called IssueStateModelHasStates
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Proce ss: Incremental Development of DSLs
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The solid line representing the two arms or roles of this relationship show that this is an embedding relationship In a running designer,3 all the elements are linked in a tree of embedding relationships From the point of view of an IssueStateModel, its collection of State Elements is accessible through its property States From the point of view of a StateElement, its parent IssueStateModel is accessible through its property called Model Each StateElement has a domain property called Name, whose type is String There are two kinds of StateElement: IssueState and StartElement (The IssueStates are represented by boxes in the instance example in Figure 2-2, and the black dot is a StartElement) Any StateElement may be related to any number of other StateElements through a relationship called Transition Each StateElement has properties Successors and Predecessors, which provide access to the related StateElements (Transitions represent the arrowed connectors between states on the diagram) The dotted line representing this relationship shows it to be a reference relationship, crosslinking nodes of the embedding tree IssueStateModel is called the root of this DSL, and as is usual each instance of the root is represented by a whole diagram when the DSL is presented in a designer It is also the root of the XML tree when instances of the language are saved to le Domain models are the subject of 3
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Domain Model and Presentation Are Separate The Domain Model the classes and relationships part of the DSL de nition de nes just the concepts dealt with by the DSL It does not de ne how to present the material as a diagram (In fact, as we observed in 1, it can often be useful to create a DSL without a diagrammatic presentation) To do this, the team de nes a set of Shapes and Connectors boxes and lines, if you like In the Issue State DSL, the correspondence between shapes and domain classes
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A designer is a graphical or other non-textual editor hosted in Visual Studio
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2: Creating and Using DSLs
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is straightforward: each IssueState is represented by an IssueStateShape, and each Transition is represented by a TransitionConnector There is a variety of basic shape and connector types available, and there are several ways of displaying information within a shape, as lines of text, lists of values, or variable icons Color, line thickness, shading, and other characteristics can be varied After de ning a set of shapes, the DSL author must de ne a set of shape maps These de ne which shape or connector displays each domain class or relationship as well as what determines the text or other variable features of the shapes In a running designer, while a DSL user is editing a DSL instance (as in, for example, Figure 2-2), it is of course the shapes and connectors that appear on the screen However, for the most part the domain properties that are displayed in the properties window when one of those shapes is selected are those of the underlying model element, and the editing or other operations that are performed are de ned on the model elements The shapes and connectors on the diagram are kept up to date by a process known as view xup, which is managed by the DSL Tools presentation framework The clear separation between presentation and underlying model means that an author of a DSL de nition can, within reason, change the way in which the domain model is presented without having to change the model itself Another aspect of this separation appears when the DSL user saves an instance to le Two les are actually generated, one containing just the domain class and relationship instances and their domain properties, and the other with layout information for the diagrammatic presentation If the latter le is thrown away, the diagram will be recreated automatically The layout will be a mess, but the shape contents and the connections between them will be correct The particular value of this separation is that tools can easily be written that accept the model, uncluttered by the incidental layout information Presentation is the subject of 4
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Refining the DSL How did the team come up with the DSL de nition The essential process is to look at the sketch instances and identify the different types of things you are drawing not forgetting a domain class for the whole model
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