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CHAPTER 2 s FRAMEWORK DESIGN
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Another option is to move the knowledge of the application server into the objects themselves. The UI can just interact with the objects, allowing them to load defaults, retrieve data, or update themselves. In this model, simply using the New keyword creates a new object: Dim cust As New Customer Within the object s constructor, you would then write the code to contact the application server and retrieve default values. It might be something like this: Public Sub New() Dim svr As AppServer = _ CType(Activator.GetObject(GetType(AppServer), _ "http://myserver/myroot/appserver.rem"), AppServer) Dim values() As Object = svr.GetCustomerDefaults ' Copy the values into our local fields End Sub Notice that the above code does not take advantage of the built-in support for passing an object by value across the network. Ideally, the code would look more like this: Public Sub New() Dim svr As AppServer = _ CType(Activator.GetObject(GetType(AppServer), _ "http://myserver/myroot/appserver.rem"), AppServer) Me = svr.CreateCustomer End Sub But it won t work because you can t change the value of Me. While the compiler won t complain, at runtime the value won t be set. This means you re left to retrieve the data in some other manner (Array, Hashtable, DataSet, an XML document, or some other data structure) and then load it into the object s fields. The end result is that you have to write code on both the server and in the business class in order to manually copy the data values. Given that both the UI-in-charge and class-in-charge techniques avoid all this extra coding, let s just abort the discussion of this option and move on.
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Class-in-Charge (Factory Pattern)
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The UI-in-charge approach uses .NET s ability to pass objects by value, but requires the UI developer to know about and interact with the application server. The object-in-charge approach enables a very simple set of UI code, but makes the object code prohibitively complex by making it virtually impossible to pass the objects by value. The class-in-charge option provides a good compromise by providing reasonably simple UI code that s unaware of application servers, while also allowing the use of .NET s ability to pass objects by value, thus reducing the amount of plumbing code needed in each object. Hiding more information from the UI helps create a more abstract and loosely coupled implementation, thus providing better flexibility.
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CHAPTER 2 s FRAMEWORK DESIGN
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s Note
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The class-in-charge approach is a variation on the Factory design pattern, in which a factory method is responsible for creating and managing an object. In many cases, these factory methods are Shared methods that may be placed directly into a business class hence the class-in-charge moniker.1
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In this model, I ll make use of the concept of Shared factory methods on a class. A Shared method can be called directly, without requiring an instance of the class to be created first. For instance, suppose that a Customer class contains the following code: <Serializable()> _ Public Class Customer Public Shared Function NewCustomer() As Customer Dim svr As AppServer = _ CType(Activator.GetObject(GetType(AppServer), _ "http://myserver/myroot/appserver.rem"), AppServer) Return svr.CreateCustomer End Function End Class Then the UI code could use this method without first creating a Customer object, as follows: Dim cust As Customer = Customer.NewCustomer A common example of this tactic within the .NET Framework itself is the Guid class, whereby a Shared method is used to create new Guid values, as follows: Dim myGuid As Guid = Guid.NewGuid This accomplishes the goal of making the UI code reasonably simple; but what about the Shared method and passing objects by value Well, the NewCustomer() method contacts the application server and asks it to create a new Customer object with default values. The object is created on the server and then returned back to the NewCustomer() code, which is running on the client. Now that the object has been passed back to the client by value, the method simply returns it to the UI for use. Likewise, you can create a Shared method in the class in order to load an object with data from the data store, as shown: Public Shared Function GetCustomer(ByVal criteria As String) As Customer Dim svr As AppServer = _ CType(Activator.GetObject(GetType(AppServer), _ "http://myserver/myroot/appserver.rem"), AppServer) Return svr.GetCustomer(criteria) End Function Again, the code contacts the application server, providing it with the criteria necessary to load the object s data and create a fully populated object. That object is then returned by value to the GetCustomer() method running on the client, and then back to the UI code.
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1. Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, Design Patterns: Elements of Reusable Object-Oriented Software (Addison-Wesley, 1995).
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