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CHAPTER 6 s OBJECT-ORIENTED APPLICATION DESIGN
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At this point, the key requirements for the application have been gathered from the use cases. Based on these use cases, it is possible to create an object-oriented design. There are a variety of techniques used in object-oriented design (you may have heard of CRC cards and decomposition, in addition to others), and in this chapter, I ll use ideas from both decomposition and CRC cards. A form of decomposition will be used to identify the nouns in the use cases, and then narrow down which of these are actual business objects. These objects will be described in terms of their class, responsibility, and collaborators (CRC).
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The first step in the process, then, is to assemble a list of the nouns in the use case write-ups. By using a bit of judgment, you can eliminate a few nouns that are obviously not objects, but still end up with a good-sized list of potential business objects or entities, as shown in Table 6-1. Table 6-1. Potential Entities Discovered in the Initial Design Project manager Project name Administrator Resource Supervisor List of roles List of resources Project Start date List of projects Employee name List of assignments Assignment List of assigned resources Project number End date Employee Employee ID Role Date assigned
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Using your understanding of the business domain (and probably through further discussion with business users and fellow designers), the options can be narrowed. Some of these aren t objects, but rather data elements, or security roles. These include the following: Project manager Administrators Supervisor
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I am assuming there s already an object to deal with a user s role. Such an object will be created by subclassing the Csla.Security.BusinessPrincipalBase class later in the chapter. But these security roles should not be confused with the role a resource (person) plays on a project they re two very different concepts.
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Pulling out these nouns, along with those that are likely to be just data fields (such as project name and employee ID), you can come up with a smaller list of likely business objects, allowing you to start creating a basic class diagram or organizing the classes using CRC cards. Table 6-2 lists the high-level CRC data for each potential object.
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CHAPTER 6 s OBJECT-ORIENTED APPLICATION DESIGN
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Table 6-2. Potential Objects and Their Associated Class Names
Potential Class
Project Resource Employee ProjectList ResourceList ProjectResources ResourceAssignments RoleList Role RoleEditList RoleEdit
Responsibility
Adds and edits a valid project Adds and edits a valid resource Adds and edits a valid employee Gets a read-only list of projects Gets a read-only list of resources Maintains a list of resources assigned to a project Maintains a list of projects to which a resource is assigned Gets a read-only list of roles Provides read-only role data Maintains a list of roles in the system Adds and edits a valid role
Collaborators
ProjectResources ResourceAssignments, Employee None Project Resource Resource, RoleList Project, RoleList Role None RoleEdit None
One key aspect of CRC-based design is that an object s responsibility should be short and to the point. Long, complex responsibility descriptions are an indication that the object model is flawed, and that the complicated object should probably be represented by a set of simpler objects that collaborate to achieve the goal. The diagram should also include relationships between the entities in the diagram. For the most part, these relationships can be inferred from the use case descriptions for instance, we can infer that a list of projects will likely contain Project objects; and that a Project object will likely contain a list of assigned resources, which in turn will likely contain Resource objects. Note that I use the word likely here, rather than will. We re still very much in a fluid design stage here, so nothing is yet certain. We have a list of potential objects, and we re inferring a list of potential relationships. Figure 6-1 is an illustration of how these objects relate to each other. Looking at the CRC list and this diagram, there is some indication that there s more work to do. There are several issues that you should look for and address, including duplicate objects, trivial objects, objects that have overly complex relationships in the diagram, and places that can be optimized for performance.
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