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CHAPTER 6 s OBJECT-ORIENTED APPLICATION DESIGN
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Figure 6-5. Merged child items with assignment information
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Perhaps business logic could be added to properties to throw exceptions if they were called from an inappropriate context. But the obvious complexity of this sort of logic should give you pause. The problem is that one object is trying to handle more than one responsibility. Such a scenario means that the object model is flawed. Going down such a path will lead to complex, hard-to-maintain code.
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Historically, this sort of complex code was referred to as spaghetti code. It turns out that with improper object design, it is very possible to end up with spaghetti code in business objects. The result is terrible, and is exactly what good object design is intended to prevent!
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It should be quite clear at this point that merging the two collections or their child objects into a single set of objects isn t the right answer. They have different responsibilities, and so they should be separate objects. But this leaves one glaring issue: what about the common properties and any common business logic they might require How can two objects use the same data without causing duplication of business logic
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Dealing with Common Behaviors and Information
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When designing relational databases, it is important to normalize the data. There are many aspects to normalization, but one of the most basic and critical is avoiding redundant data. A given data element should exist exactly once in the data model. And that s great for relational modeling. Unfortunately, many people struggle with object design because they try to apply relational thinking to objects. But object design is not the same as relational design. Where the goal with relational design is to avoid duplication of data, the goal of object design is quite different. There s no problem with a data field being used or exposed by different objects. I realize this may be hard to accept. We ve all spent so many years being trained to think relationally that it is often very hard to break away and think in terms of objects. Yet creating a good object model requires changing this mode of thought.
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Object design isn t about normalizing data. It is about normalizing behavior.
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The goal in object design is to ensure that a given behavior exists only once within the object model. Simple examples of behavior include the idea of a string being required, or one value being larger than another. More complex behaviors might be the calculation of a tax or discount amount. Each behavior should exist only once in the object model, though it may be used from many different objects.
CHAPTER 6 s OBJECT-ORIENTED APPLICATION DESIGN
This is why collaboration is so critical to good object design. For example, one object the DiscountCalculator will implement the complex calculation for a discount. Many other objects may need to determine the discount, and so they collaborate with DiscountCalculator to find that value. In this manner, the behavior exists exactly once in the model.
Dealing with Common Information
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 3, the CSLA .NET framework defined 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 6-6 illustrates this relationship.
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