Factory Methods in VB.NET

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Factory Methods
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The Factory Methods region contains two factory methods and a Private constructor, much like the Project class.
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CHAPTER 8 s BUSINESS OBJECT IMPLEMENTATION
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Factory Methods
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The two factory methods are declared as Friend scope since they are not for use by the UI code. Rather, they are intended for use by the Project object that contains the collection: Friend Shared Function NewProjectResources() As ProjectResources Return New ProjectResources End Function Friend Shared Function GetProjectResources( _ ByVal dr As SafeDataReader) As ProjectResources Return New ProjectResources(dr) End Function In both cases, the factory methods simply use the New keyword to create and return a new instance of the collection object. The NewProjectResources() method returns an empty, new collection. This method is called by Project when a new Project object is created. GetProjectResources() is used to load the collection with child objects based on data from the database. It is called from DataPortal_Fetch() in the Project class when a Project object is in the process of being loaded from the database. This method accepts a data reader as a parameter, and that data reader is provided to the constructor, which is responsible for loading the collection with data. That parameterized constructor is found in the Data Access region.
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The default constructor, called from NewProjectResources(), is located in the Factory Methods region, just like it is in the template from 7: Private Sub New() MarkAsChild() End Sub The fact that MarkAsChild() is called here is very important. Remember that the ProjectResources collection is contained within a Project object and is a child of that Project. Due to this, the collection object must be marked as a child object as it is created. The BusinessListBase code relies on this information to make sure that the object behaves properly as a child of another object. The GetProjectResources() factory method also calls a constructor, passing it a data reader object: Private Sub New(ByVal dr As SafeDataReader) MarkAsChild() Fetch(dr) End Sub This method also calls MarkAsChild(), and then calls a Fetch() method, which will actually load the object s data from the data reader.
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Data Access
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The Data Access region in a child collection object is quite different from that of any root object like Project. Remember that the data portal never directly interacts with child objects, leaving it instead to the root object to initiate all data access in its children. In this case, that means that the
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CHAPTER 8 s BUSINESS OBJECT IMPLEMENTATION
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Project object is responsible for initiating all data access activity in its child ProjectResources collection. Recall that in the DataPortal_XYZ methods of Project, calls were made to the GetProjectResources() factory method and to an Update() method on the collection.
Loading Data
In the DataPortal_Fetch() method of Project, a call is made to the GetProjectResources() factory method in ProjectResources. That factory method calls a parameterized constructor, passing a data reader that contains the collection of data for the child objects to be loaded into the collection. That constructor then calls the following Fetch() method to load the object with data: Private Sub Fetch(ByVal dr As SafeDataReader) Me.RaiseListChangedEvents = False While dr.Read() Me.Add(ProjectResource.GetResource(dr)) End While Me.RaiseListChangedEvents = True End Sub This method loops through all the items in the data reader, using each row of data to create a new ProjectResource child object. I ll discuss the GetResource() factory method later in the chapter, but you can see that it accepts the data reader object as a parameter so the new child object can populate itself with data from the current row. As discussed in 7, the RaiseListChangedEvents property is set to False and then True to suppress the ListChanged events that would otherwise be raised as each item is added.
Updating Data
The DataPortal_Insert() and DataPortal_Update() methods of Project call the collection s Update() method. This method is Friend in scope, as it is intended only for use by the parent Project object. The Update() method is responsible for deleting, inserting, and updating all the child objects in the collection into the database. More precisely, it is responsible for asking each child object to do the appropriate operation. This means looping through both the list of child objects marked for deletion and the list of active objects that may require insert or update operations: Friend Sub Update(ByVal project As Project) Me.RaiseListChangedEvents = False ' update (thus deleting) any deleted child objects For Each obj As ProjectResource In DeletedList obj.DeleteSelf(project) Next ' now that they are deleted, remove them from memory too DeletedList.Clear() ' add/update any current child objects For Each obj As ProjectResource In Me If obj.IsNew Then obj.Insert(project) Else obj.Update(project) End If Next Me.RaiseListChangedEvents = True End Sub
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