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Dealing with Common Information
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So the real question isn t whether the Assigned and Role properties can be put into a common object that s relational thinking. Instead, the question is whether those properties have common behaviors (business rules or logic) that can be put into a common object. As it turns out, the Role property must be validated to ensure any new value is a real role. Since the Role property can be set in both ProjectResource and ResourceAssignment, that behavior could be duplicated. A better answer is to normalize that behavior, putting it into a central object. Let s call this new object Assignment, since it will be responsible for centralizing the code common to assignments of projects to resources, and resources to projects. Then both ProjectResource and ResourceAssignment can collaborate with Assignment to ensure that the Role property is validated. This means that Assignment will contain the rule method that implements the role-validation behavior. In 6, you ll see how the CSLA .NET framework defines the RuleHandler delegate to support exactly this type of scenario. Given a ValidRole() rule method in Assignment, both ProjectResource and ResourceAssignment merely have to associate that rule method with their Role properties to share the common behavior. Figure 3-8 illustrates this relationship.
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C HA PTER 3 OBJEC T-ORIE NTED APPLIC ATION DES IGN
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Figure 3-8. ProjectResource and ResourceAssignment collaborating with Assignment In 17, I discuss the code to do exactly this.
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Dealing with Common Behaviors
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The responsibility of the Assignment object from Figure 3-8 is to manage the association between a project and a resource. This means that the Assignment object s behavior could include the idea of associating a project with a resource. This is a broader behavior than that provided by ProjectResources, which assigns a resource to a project, or by ResourceAssignments, which assigns a project to a resource. In fact, the behavior of Assignment is more general and encompasses the needs of both other objects. Of course, the ProjectResource and ResourceAssignment classes handle the real work of dealing with a resource assigned to a project, or a project associated with a resource. The collection classes really just add and remove these child objects, leaving it to the child objects to handle the details. The end result is that ProjectResource, to fulfill its behavior, can ask Assignment to do the actual work, as shown in Figure 3-9. The same is true of ResourceAssignment. The implication is that Assignment could have a method such as AddAssignment() that accepts a project s Id property and a resource s Id property, along with the role the resource will play on the project.
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Tip Object models should be simple and intuitive, even when underlying behaviors are complex. By centralizing common behaviors using objects internal to the business layer, a simpler and more tailored public interface can be exposed to the UI developer.
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CH APT ER 3 O BJECT -O RIEN TED A PPLI CA TI O N D ESI GN
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Similarly, ProjectResource and ResourceAssignment have behaviors that involve removing a resource from a project or removing a project from a resource. Assignment, then, will have a more general behavior to remove an association between a project and a resource. Figure 3-9 shows the full extent of Assignment, including all the methods that implement behaviors common to both ProjectResource and ResourceAssignment.
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Figure 3-9. Objects collaborating with Assignment At this point, all the common behaviors from ProjectResource and ResourceAssignment have been normalized into a single location in the object model.
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Optimizing for Performance
Part of object design includes reviewing things to ensure that the model won t lead to poor performance. This isn t really a single step in the process, but rather something you should do on a continual basis during the whole process. However, once you think the object model is complete, you should always pause to review it for performance issues. One primary performance issue with many object models deals with the use of relational thinking when designing the objects. Normalizing data within the object model is perhaps the most common flaw causing performance issues. Due to the design of ProjectResource, ResourceAssignment, and Assignment, the object model has already eliminated this issue by normalizing behavior instead of data. This helps avoid loading entire business objects just to display a couple of common data elements. There is, however, another performance issue in the model. The ProjectList and ResourceList collection objects, as modeled, retrieve collections of Project and Resource business objects so that
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