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DYNAMIC TREE NODES
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There are a number of ways tree nodes can be expanded and collapsed. These include the following: From the mouse. The user can double-click on a tree node to toggle between expand and collapse operations. When the ShowPlusMinus property is true, a click on a plus + sign will expand the node while a click on a minus sign will collapse a node. From the keyboard. The user can press the right arrow key to expand the selected node in the tree, and the left arrow key to collapse the selected node. From code. The TreeNode class includes an Expand method to expand the node, a Collapse method to collapse the node, and a Toggle method to switch the node to the opposite of its current state. The TreeView class includes the ExpandAll and CollapseAll methods to expand or collapse all nodes in the tree. In addition, the TreeNode.EnsureVisible method will expand nodes as required to have the node appear within the containing TreeView control. Regardless of how a node is expanded or collapsed, the BeforeExpand, AfterExpand, BeforeCollapse, and AfterCollapse events occur in the TreeView class for each node as it alters its state. The before events receive a TreeViewCancelEventArgs class instance as their event parameter, while the after events receive a TreeViewEventArgs class instance. The TreeViewCancelEventArgs class is summarized in .NET Table 15.4. The TreeViewEventArgs class provides the same two Action and Node properties shown in the table, but inherits from the System.EventArgs class rather than the CancelEventArgs class. The CancelEventArgs class is discussed in chapter 8.
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.NET Table 15.4 TreeViewCancelEventArgs class
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The TreeViewCancelEventArgs class is a CancelEventArgs object that contains event data for events in the TreeView class that occur before an operation takes place. The event handler receiving this class has the opportunity to cancel the operation by setting the inherited Cancel property to true. This class is part of the System.Windows.Forms namespace, and inherits from the System.ComponentModel.CancelEventArgs class. Action Public Properties Node Gets the TreeNode object that is the target of the current operation. Gets the TreeViewAction enumeration member representing the action that caused this event to occur.
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Let s get back to our application and make use of some of these constructs. We would like to insert a set of nodes for the photos in an album whenever the album node is expanded. We can do this by handling the BeforeExpand event for our tree.
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CHA PTE R 15
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TREE VIEWS
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Before we actually do this, a utility method to open a PhotoAlbum using a given album node will turn out to be useful here and later on in the chapter. We will create this method first.
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CREATE AN OPENTREEALBUM METHOD Action 1 In the MainForm.cs code window, create a new OpenTreeAlbum method that accepts a TreeNode object and returns an album. Begin this method by opening the album associated with the node.
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Result
private PhotoAlbum OpenTreeAlbum (TreeNode node) {
string s = node.Tag as string; PhotoAlbum album = OpenAlbum(s);
Use the OpenAlbum method created in chapter 14. 3 Update the image index values for this node.
How-to
if (album == null) { // Unable to open album node.ImageIndex = ErrorIndex; node.SelectedImageIndex = ErrorIndex; } else { // Album opened successfully node.ImageIndex = AlbumIndex; node.SelectedImageIndex = SelectedAlbumIndex; } return album; }
a. If the album cannot be opened, use the error icon for both images. b. Otherwise, use the standard album images.
Return the result of the OpenAlbum call.
With this method in place, we can create a BeforeExpand event handler for our TreeView control. The following table continues the previous steps to create this handler.
HANDLE THE BEFOREEXPAND EVENT Action 5 In the MainForm.cs [Design] window, add a BeforeExpand event handler for the TreeView control. Result
private void treeViewMain_BeforeExpand (object sender, System.Windows. Forms.TreeViewCancelEventArgs e) { TreeNode node = e.Node;
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HANDLE THE BEFOREEXPAND EVENT (continued) Action 6 To implement this handler, see if the expanding node is an album.
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