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CHAPTER 2 s FRAMEWORK DESIGN
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BusinessBase provides a great deal of functionality to the business objects, whether root or child. 3 will cover the implementation of BusinessBase itself, and s 7 and 8 will show how to create business objects using BusinessBase.
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The BusinessListBase class is the base from which all editable collections of business objects will be created. Given an Invoice object with a collection of LineItem objects, BusinessListBase will be the base for creating that collection: <Serializable()> _ Public Class LineItems Inherits BusinessListBase(Of LineItems, LineItem) End Class When creating a subclass, the business developer must provide the specific types of their new business collection, and the child objects the collection contains, as type parameters to BusinessListBase(Of T, C). This allows the generic type to expose strongly typed methods corresponding to the specific business collection type and the type of the child objects. The result is that the business collection automatically has a strongly typed indexer, along with strongly typed Add() and Remove() methods. The process is the same as if the object had inherited from System.ComponentModel.BindingList(Of T), except that this collection will include all the functionality required to support n-level undo, object persistence, and the other business object features.
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BusinessListBase inherits from System.ComponentModel.BindingList(Of T), so it starts with all the core functionality of a data-bindable .NET collection.
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The BusinessListBase class also defines the data access methods and the MarkAsChild() method discussed in the previous BusinessBase section. This allows retrieval of a collection of objects directly (rather than a single object at a time), if that s what is required by the application design.
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Most applications consist not only of interactive forms or pages (which require editable objects and collections), but also of non-interactive processes. In a 1- or 2-tier physical model, these processes run on the client workstation or web server, of course. But in a 3-tier model, they should run on the application server to have optimal access to the database server or other back-end resources. Common examples of non-interactive processes include tasks as simple as checking to see if a specific customer or product exists, and as complex as performing all the back-end processing required to ship an order or post an invoice. The CommandBase class provides a clear starting point for implementing these types of behaviors. A command object is created on the client and initialized with the data it needs to do its work on the server. It is then executed on the server through the data portal. Unlike other objects, however, command objects implement a special execute method: DataPortal_Execute()
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CHAPTER 2 s FRAMEWORK DESIGN
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The optional pre-, post-, and exception data portal methods can also be implemented if desired. But the DataPortal_Execute() method is the important one, since that is where the business developer writes the code to implement the non-interactive back-end processing. I ll make use of CommandBase in 8 when implementing the sample application objects.
ReadOnlyBase
Sometimes, applications don t want to expose an editable object. Many applications have objects that are read-only or display-only. Read-only objects need to support object persistence only for retrieving data, not for updating data. Also, they don t need to support any of the n-level undo or other editing-type behaviors, because they re created with read-only properties. For editable objects, there s BusinessBase, which has a property that can be set to indicate whether it s a parent or child object. The same base supports both types of objects, allowing dynamic switching between parent and child at runtime. Making an object read-only or read-write is a bigger decision, because it impacts the interface of the object. A read-only object should only include read-only properties as part of its interface, and that isn t something you can toggle on or off at runtime. By implementing a specific base class for read-only objects, they can be more specialized, and have less overhead. The ReadOnlyBase class is used to create read-only objects, as follows: <Serializable()> _ Public Class StaticContent Inherits ReadOnlyBase(Of StaticContent) End Class Classes shouldn t implement any read-write properties. Were they to do so, it would be entirely up to the code in the object to handle any undo, persistence, or other features for dealing with the changed data. If an object has editable properties, it should subclass from BusinessBase.
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