Creating custom TreeView/TreeNode

问题:

I need to extend the TreeNode class such that I can add custom properties to each node (seeing as WebForms TreeNode doesn't include the Tag property). So this is my CustomTreeNode:

public class CustomTreeNode : TreeNode
{

    public CustomTreeNode()
    {               
    }

    public CustomTreeNode(int nodeId, string nodeType)
    {
        NodeId = nodeId;
        NodeType = nodeType;
    }

    public string NodeType { get; set; }
    public int NodeId { get; set; }
}

If I create a CustomTreeNode and add it to a TreeView:

CustomTreeNode node = new CustomTreeNode(1, "CustomType");            
treeView.Nodes.Add(node);

I would then get a casting exception doing the following:

CustomTreeNode selectedNode = (CustomTreeNode)TreeView.SelectedNode;

because TreeView returns a TreeNode, not a CustomTreeNode.

I've done some reading, and it looks like I need to extend the TreeView class, and override the CreateNode() method to return CustomTreeNode instead of TreeNode. So I created this:

public class CustomTreeView : TreeView
{
    protected override TreeNode CreateNode()
    {
        return new CustomTreeNode();
    }
}

The problem is however, CreateNode() doesn't take any arguments, so you have to have call the empty constructor for the CustomTreeNode class. So when I created my CustomTreeNode above, when I get it back from the CustomTreeView, the nodeId and nodeType values have been lost because the empty constructor returns a node without any values.

Any help much appreciated.

回答1:

This is what I came up with (experts, any advice welcomed). Instantiate the CustomTreeNodes in your code behind and set the properties via setters. Modify your CustomTreeNode class to persist the values in ViewState. The node returned by your custom tree view's CreateNode will load the ViewState information.

TreeNode class:

[DefaultProperty("Text")]
[ToolboxData("<{0}:CustomTreeNode runat=server></{0}:CustomTreeNode>")]
public class CustomTreeNode : TreeNode
{
    private const int NODE_TYPE = 1;
    private const int NODE_ID = 2;

    public string NodeType { get; set; }
    public int NodeId { get; set; }

    protected override void LoadViewState(Object savedState)
    {
        if (savedState != null)
        {
            object[] myState = (object[])savedState;
            if (myState[0] != null)
                base.LoadViewState(myState[0]);
            if (myState[NODE_TYPE] != null)
                this.NodeType = (string)myState[NODE_TYPE];
            if (myState[NODE_ID] != null)
                this.NodeId = (int)myState[NODE_ID];

        }
    }

    protected override Object SaveViewState()
    {
        object baseState = base.SaveViewState();
        object[] allStates = new object[3];
        allStates[0] = baseState;
        allStates[NODE_TYPE] = this.NodeType;
        allStates[NODE_ID] = this.NodeId;

        return allStates;
    }
}

TreeView class:

[DefaultProperty("Text")]
[ToolboxData("<{0}:CustomTreeView runat=server></{0}:CustomTreeView>")]
public class CustomTreeView : TreeView
{
    protected override TreeNode CreateNode()
    {
        // Tree node will get its members populated with the data from VIEWSTATE
        return new CustomTreeNode();
    }
}

Simple .aspx file (Assuming that your custom control is defined in an assembly "Foo" and a namespace "Bar":

<%@ Register TagPrefix="customControl" Assembly="Foo" Namespace="Bar"  %>

<customControl:CustomTreeView ID="sampleTree" 
    runat="server" onselectednodechanged="sampleTree_SelectedNodeChanged"></customControl:CustomTreeView>
<asp:Label ID="lblSelectedNode" runat="server" ></asp:Label>

CodeBehind:

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            PopulateTree();
        }
    }

    private void PopulateTree()
    {
        sampleTree.Nodes.Clear();
        CustomTreeNode root = new CustomTreeNode();
        root.Value = "root node";

        sampleTree.Nodes.Add(root);

        // Creating some fake nodes (you would of course be using real data)
        for (int i = 0; i < 10; i++)
        {
            CustomTreeNode child = new CustomTreeNode();
            child.NodeId = i;               // Saved in ViewState
            child.NodeType = "Type " + i;   // Saved in ViewState
            child.Value = child.NodeType;
            root.ChildNodes.Add(child);
        }
    }

    protected void sampleTree_SelectedNodeChanged(object sender, EventArgs e)
    {
        CustomTreeView cTreeView = (CustomTreeView) sender;
        lblSelectedNode.Text = ((CustomTreeNode)cTreeView.SelectedNode).NodeType;
    }