10: Advanced DSL Customization in C#.NET

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10: Advanced DSL Customization
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Rules Rules are the most general purpose mechanism for propagating and responding to change within the store Rules are triggered by changes in the store such as property value changes, or the creation or deletion of an object or link, shape, or connector In the usual case, the rule executes during the Commit() operation of the outermost transaction in which the triggering change occurred A rule can trigger further rings, which are added to the end of the queue The transaction is completed when the rule ring queue is empty It is of course possible for badly organized rules to cause a transaction never to complete, until the ring queue runs out of space
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TIP Consider other mechanisms before rules
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Because rules can execute in a dif cult-to-control order, a large set of rules can be dif cult to debug So although rules are a powerful and general mechanism, it s good to look rst for other ways of achieving the result you want
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Here is the code of a rule This rule responds to the change of any shape s absolute bounds (that is, if it moves or changes shape)
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[RuleOn(typeof(NodeShape), FireTime = TimeToFireTopLevelCommit)] public sealed class ContainerShapeChangesRule : ChangeRule { public override void ElementPropertyChanged (ElementPropertyChangedEventArgs e) { NodeShape stateShape = eModelElement as NodeShape; if (stateShape == null) return; if (stateShapeStoreTransactionManager CurrentTransactionIsSerializing)) return; if (eDomainPropertyId == NodeShapeAbsoluteBoundsDomainPropertyId) { RectangleD oldBounds = (RectangleD)eOldValue; RectangleD newBounds = stateShapeAbsoluteBoundingBox; HandleAbsoluteBoundsChange(stateShape, oldBounds, newBounds); } }
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Re sponding to Change s
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Notice: The RuleOn attribute marks this class as a rule The rule class can be called anything you like, but must inherit from one of a xed set of abstract rules The rule is de ned on a class (NodeShape, in this case); it does not have to be separately registered for each instance The code of the rule is entirely separate from the code of the class it observes
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The RuleOn Attribute
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[RuleOn(typeof(NodeShape), FireTime = TimeToFireTopLevelCommit)]
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The attribute speci es the observed class, which may be any domain class, relationship, shape, connector, or diagram You cannot specify that a rule observes a particular property of a class, nor that it observes speci c instances You must specify the class to which the property you are interested in belongs In this case, we specify NodeShape so that we can look at the AbsoluteBounds properties of the shapes NodeShape is the common base class of all shapes To catch all connectors, use BinaryLinkShape; all connectors or shapes, ShapeElement; all relationships, ElementLink ModelElement is the superclass of everything You can apply as many rules as you like to the same class Changes in any instance of the class (and its subclasses) will trigger all the applied rules TopLevelCommit is the usual FireTime, but you can also specify LocalCommit, which means that the rule executes at the end of the innermost transaction; or you can specify InLine, which means that the rule executes as soon as possible after the triggering change You can also set a Priority integer The problem with using priorities is that you pretty soon get to devising some big table of all the relative priorities of your rules and start depending on them ring in that order, with one rule depending on another having set something up Pretty soon after that, you re into rule spaghetti, nding it impossible to debug heaps of rules that re in the wrong order So on the whole, it s easier to leave the priority at the default and write your rules not to assume any particular order of execution
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10: Advanced DSL Customization
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You can set InitiallyDisabled = true in the RuleOn attribute, and rules can be turned on and off using the store s RuleManager In 7, we saw an example where the rule was turned off until the model had loaded
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storeRuleManagerEnableRule(typeof(ContainerShapeChangesRule));
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