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If, however, we wish to display related data from a DataSet, we will need to make use of more exible controls or combinations of controls The TreeView control is ideal, since we can use it to show up a master-detail structure (for example, Publishers and each Title published by them) The code in Listing 129 lls up the TreeView control tvPublishers
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Private Sub FillTree(ByVal d As DataSet) Dim pRow, tRow As DataRow Dim Publisher, Title As String Dim PubID As Integer Dim n As TreeNode tvPublishersBeginUpdate() tvPublishersNodesClear() Try For Each pRow In dTables("Publishers")Rows Publisher = pRowItem("Name")ToString() PubID = CType(pRowItem("PubID"), Integer) n = New TreeNode(Publisher) nTag = PubID For Each tRow _ In pRowGetChildRows _ (dRelations ("PubsTitles")) nNodesAdd(tRowItem("Title")ToString()) Next tvPublishersNodesAdd(n) Next Finally tvPublishersEndUpdate() End Try End Sub Listing 129: Displaying master-detail information from a DataSet
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The outer loop in the listing (For Each pRow In dTables("Publishers") Rows) is similar to any loop you would use to iterate through the rows of any DataTable We use this to pick up Publisher information ("Name" and "PubID") and insert it into TreeNode objects instead of ListViewItem objects A TreeNode is a line in a TreeView control that has its own collection of TreeNodes (each of which in turn can have its own collection of TreeNodes and so on) Note the way data has been added to the TreeNode for each publisher:
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Publisher = pRowItem("Name")ToString() PubID = CType(pRowItem("PubID"), Integer) n = New TreeNode(Publisher) nTag = PubID
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Having created a new TreeNode, using the text we want it to display as a parameter to its constructor, we can use the Tag property to store another, related object We can use this property to store the primary key of the publisher row ("PubID"), which will be useful if we need to add another title to this publisher s node
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Each Publisher row in the Publishers table (pRow) has associated with it a number of rows from the Titles table This is as a result of adding the relation to the DataSet that indicated how the two tables were linked To populate each Publisher s node in the TreeView with sub-nodes (one for each of the Titles published), the inner loop makes use of the publisher DataRow s GetChildRows() property This returns a collection of rows related to the publisher, and we can iterate through these in a second, inner loop:
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For Each tRow In pRowGetChildRows(dRelations("PubsTitles")) nNodesAdd(tRowItem("Title")ToString()) Next
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The variable n is the TreeNode that holds the publisher information, so we add to its own Nodes collection a node that contains the title published Finally, this node is inserted into the TreeView control (tvPublishersNodesAdd(n)) before looping back to deal with the next publisher and its titles Note that all of the code that adds information to the TreeView control is bracketed between calls to the TreeView s BeginUpdate() and EndUpdate() methods This suppresses screen updating while the data is being added, since otherwise, the face of the TreeView would be redrawn at each addition of a node, which would be grossly inef cient Note also the use of a TryFinally block to make sure that whatever happens, the TreeView control is eventually updated on the screen The result of all of this code is a TreeView control that contains a list of publishers, each of which has a list of Titles (see Figure 129) Note that the form shown in the gure contains a group of controls for adding a new Title to the DataSet The operation to insert a new record or row into a DataSet is described next
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The TreeView of Publishers and Titles built by Listing 129
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The TreeView built in Listing 129 allows us to display an entire DataSet, but has no direct connection to the actual tables and rows in the DataSet To gain more direct access to the data in a DataSet, we can bind the information from the tables to individual controls For example, by binding the Name column of the Publishers table in the DataSet to the Text property of a text box, txtPublisher, we can use the statement:
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txtPublisherDataBindingsAdd( _ New Binding("Text", ds, "PublishersName"))
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The Binding object added to the txtPublisher control s DataBindings collection is constructed using the name of the host control s property that is to display the data ("Text"), the data source to be used (the DataSet ds in this case) and the data member to be displayed (using a string "TableNameColumnName") We can bind several controls to various columns in the DataSet s tables to provide a view of whole or partial records (rows) in the set As a result, information in a row/column will be displayed in the control and if the control is editable (eg a TextBox), changes made will be re ected in the DataSet
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