Figure 7-5: A basic error dialog from the properties window in Visual C#.NET

Creator Quick Response Code in Visual C#.NET Figure 7-5: A basic error dialog from the properties window
Figure 7-5: A basic error dialog from the properties window
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In itself, the message is not very informative, but clicking the Details button reveals the true error message, shown in Figure 7-6
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Figure 7-6: An expanded error dialog from the properties window
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While this isn t a superb user experience, it does share a comon look and feel with other validation errors for properties, so we ll accept that and move on A similar experience is presented if you use the toolbox inheritance connector to create a looped inheritance The cursor indicates that the choice is valid, but when the target is selected, the dialog shown in Figure 7-7 is presented
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Figure 7-7: Drag-drop error dialog
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At least this time the error message is presented directly; however, in the case of a drag-drop tool, it seems much more jarring The typical experience
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for such tools is that they present a No entry sign for invalid choices when they are simply hovered over, before the user has even made a selection This allows a much more exploratory style of user interaction You ll see this type of interaction automatically supplied by the generated code for multiplicity constraints on relationships, so let s see how to improve the experience for our cyclical rule in this case A nested partial class has to be added to the designer:
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public partial class GeneralizationConnectAction : ConnectAction { private partial class GeneralizationConnectionType : GeneralizationConnectionTypeBase { /// <summary> /// Helper method to skip from compartment /// shapes up to their parents /// </summary> /// <param name="shape"></param> /// <returns></returns> private static ShapeElement RemovePassThroughShapes (ShapeElement shape) { if (shape is Compartment) { return shapeParentShape; } SwimlaneShape swimlane = shape as SwimlaneShape; if (swimlane != null && swimlaneForwardDragDropToParent) { return shapeParentShape; } return shape; } /// <summary> /// Only allow connections that don't /// create a cycle in the inheritance chain /// </summary> /// <param name="sourceShapeElement"></param> /// <param name="targetShapeElement"></param> /// <param name="connectionWarning"></param> /// <returns></returns> public override bool CanCreateConnection (ShapeElement sourceShapeElement, ShapeElement targetShapeElement, ref string connectionWarning) { ShapeElement sourceShape =
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RemovePassThroughShapes(sourceShapeElement); ShapeElement targetShape = RemovePassThroughShapes(targetShapeElement); if (sourceShape != null && targetShape != null) { ModelClass sourceClass = sourceShapeSubject as ModelClass; ModelClass targetClass = targetShapeSubject as ModelClass; if (!CyclicInheritanceAddRule TestValidInheritance(sourceClass, targetClass, ref connectionWarning)) { return false; } } // Fall through to the base test if // we haven't detected a cycle return baseCanCreateConnection(sourceShapeElement, targetShapeElement, ref connectionWarning); } } }
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This code overrides the CanCreateConnection() method in the double-derived GeneralizationConnectionType nested class within the GeneralizationConnectAction class This method is called repeatedly as the mouse is moved over the design surface when the inheritance connection tool is selected The ConnectAction-derived type effectively forms the handler for the modal interaction and the method allows or disallows potential connections for any pair of shapes This code deals in the world of Shapes rather than raw ModelElements, but once the toplevel shapes have been identified and their underlying model elements retrieved, it can make use of the same TestValidInheritance() method that the previously described rule uses, ensuring that the logic isn t duplicated If the test fails, then the method simply returns false to indicate that the No entry message should be shown over the element currently under the mouse cursor Additionally, if the reference parameter connectionWarning is set, then a tooltip will be shown that makes it very clear why this particular connection would not be acceptable
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The nal case to examine is loading a model that already contains an instance of cyclical inheritance Perhaps this model came from an earlier version of the tool that didn t have validation rules, or perhaps the XML was supplied from some other tool in a tool chain or was hand-edited Whatever its source, the tool needs to decide on a strategy for handling such les As the code stands, when an attempt is made to load an erroneous le, the rule will re in the load transaction and the load will then fail, causing an error message similar to the one shown in Figure 7-8
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