XML Reference Guide

Mar 14, 2003

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At this point, we have a working, static version of a hierarchical tree structure. The text is displayed based on whether a particular nodte is open or closed, and the appropriate "plus" or "minus" icon is displayed based on whether the node is open or closed. What we don't have is a way to make sure that the page is displayed with the appropriate node open to start with. Several ways exist to accomplish this. We could, if we choose, add a handler to the event that signifies that the page is loaded, and create JavaScript that would read the URL, detect the appropriate node, and display it in the open state.

In this case, however, we are going to use this opportunity to look at the way in which we can use .Net to display hierarchical data on a page. In a later entry, we'll do this with XML, but before we get to that point, let's look at the basic process.

.Net was designed from the beginning to make it easier to create dynamic web sites. One way in which it accomplishes this task is to create "controls" to which you can bind a set of data. For example, you can easily create an HTML table of data from a database by "binding" the data to a DataGrid control. The control then inserts all of the appropriate HTML, and even gives you the opportunity to specify styles are the table was a whole and of alternate rows.

But we need even more control than that. After all, we are not creating a table. If we follow the HTML at the end of our last entry, in order to match the existing structure of the InformIT.com site, we will need to use lists instead. Fortunately, .Net provides a control that gives us complete power over what it outputs. It's called a Repeater.

Let's look at what we're actually going to do. We have a set of data, or rather two sets of data, that represent topics in the InformIT XML and Web Services Guide. One set represents the main topics, or "C heads", and the other represents the subtopics, or "D heads" that appear under them in the hierarchy. We can represent them in a database such as the one in figure 1:

Guide Entry table

Notice that we have a pair of columns, id and parent_id, that relate the rows. For example, entry 12 is the parent for entries 14, 15, and 16. So we know that those items need to be displayed as children of entry 12. We can use this data relation in displaying the page, even if it doesn't formally exist in the database.

To display these items on the page, we'll start with a basic Repeater:

<%@ Page Language="VB" %>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
            "http://www.w3.org/TR/REC-html40/loose.dtd">
<html>
<head>
   <base href="http://www.informit.com" />
</head>
<body>
   <asp:Repeater id="CHeadRepeater" runat="server">

      <HeaderTemplate>
        <h1>Other page content, such as the top of the page</h1>
      </HeaderTemplate>

      <ItemTemplate>
          
         <p><a href="/guides/content.asp?g=xml&seqNum=<%# DataBinder.Eval(Container.DataItem, "id") %>">

            <%# DataBinder.Eval(Container.DataItem, "guideEntryTitle") %>
         </a></p>

      </ItemTemplate>

      <FooterTemplate>
        (The bottom of the page.)

      </FooterTemplate>
   </asp:Repeater>

</body>
</html>

Notice that the Repeater control uses a prefix of asp:. You're probably tempted to think, "oh, I recognize that, that's an XML namespace alias." Well, not really, but it does serve much of the same purpose; it tells the server that this is not a simple HTML element to display, and but rather an element to process. The runat attribute tells the server it's the one that should process it, and not the client.

As you might suspect, the HeaderTemplate and FooterTemplate elements are processed before the first item, and after the last item, respectively. The ItemTemplate element gets processed for each item the Repeater encounters, and that's where it gets interesting.

When the server encounters content in the <%# %> notation, it knows that it is supposed to process the contents based on the current row. For example, in this case we are executing the DataBinder.Eval function to get the id and guideEntryTitle attributes of the object representing the current item. We can add any function we want into one of these evaluation blocks, as we'll see shortly.

Now let's populate the Repeater:

<%@ Page Language="VB" %>
<%@ Import Namespace="System.Data" %>
<%@ Import Namespace="System.Data.Odbc" %>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
            "http://www.w3.org/TR/REC-html40/loose.dtd">

<script language="VB" runat="server">

    Sub Page_Load(Src As Object, E As EventArgs)
        
        Dim MyConnection As OdbcConnection
        Dim MyCommand As OdbcCommand
        Dim ConnectionString As String
        Dim ds As DataSet 

        ConnectionString = _
           "Driver={Microsoft Access Driver (*.mdb)};" & _
           "Dbq="&Server.MapPath("guideentries.mdb")&";"
        
        myConnection = new Odbc.OdbcConnection(ConnectionString)

        Dim CHeads As OdbcDataAdapter = _
                 New OdbcDataAdapter( _
                     "select * from GuideEntries where parent_id=0 order by listOrder", _
                     myConnection )
        ds = New DataSet()
        CHeads.Fill(ds, "cheads")

        CHeadRepeater.DataSource = ds.Tables("cheads")
        CHeadRepeater.DataBind()

        myConnection.Close()
    End Sub
</script>

<html>
<head>
   <base href="http://www.informit.com" />

</head>
<body>
   <asp:Repeater id="CHeadRepeater" runat="server">
      <HeaderTemplate>
...

First, note that the server executes the Page_Load subroutine on, as you might have guessed, loading the page. To that end, we can use this opportunity to create a set of data and bind it to the Repeater. In my case, I'm running my ASP.net pages in a hosted environment in which I can only access an Access database, and only directly, so my first step is to create an ODBC connection. (If you are using, say, SQL Server, you'll load the System.Data.SqlClient classes instead.) Once we've got the connection, we create a DataAdapter that uses the connection to select all of the main topics, and use it to create a table in the DataSet. (We can tell they are main topics because they have a parent_ID of 0.)

It's important to note that we didn't just "populate" the DataSet, as we might have done with a RecordSet. One of the advantages of using a DataSet is that it can hold more than one table of data. In this case, we've created one arbitrarily called cheads. In a few moments, will create a second one and related to the first.

In the meantime, we are using this table as the data source for the Repeater, which we can identify from the id attribute we gave it in the body of the page. Once we assign it a data source, we bind the data to it, which gives us a page like figure 2:

a simple repeater

Now we can go ahead and create the data relation so we can display the subtopics:

...
        Dim CHeads As OdbcDataAdapter = _
                 New OdbcDataAdapter( _
                     "select * from GuideEntries where parent_id=0 order by listOrder", _
                     myConnection )
        ds = New DataSet()
        CHeads.Fill(ds, "cheads")

        Dim DHeads As OdbcDataAdapter = _
                 New OdbcDataAdapter( _
                      "select * from GuideEntries where parent_id > 0 order by listOrder", _
                      myConnection )
        DHeads.Fill(ds, "dheads")

        ds.Relations.Add("topicRelation", _
                             ds.Tables("cheads").Columns("id"), _
                             ds.Tables("dheads").Columns("parent_id"))

        CHeadRepeater.DataSource = ds.Tables("cheads")
        CHeadRepeater.DataBind()

        myConnection.Close()
    End Sub
</script> 
...

Here we have a second data adapter that adds a second table of data, called "dheads", to the DataSet. We can then create the parent-child relationship, topicRelation, by relating to the parent_id column in the child table to the id column in the parent table.

It's important to note that this case, in which both sides of the relation represent the same table in the database, is actually a rarity. In most cases, you will use this capability to draw a relations between, say, a customer table and an order table, or even different types of data.

Now that we've got the relation, we can actually nest another Repeater within the first:

...
   <asp:Repeater id="CHeadRepeater" runat="server">
      <HeaderTemplate>
        <h1>Other page content, such as the top of the page</h1>

      </HeaderTemplate>

      <ItemTemplate>
          
         <p><a href="/guides/content.asp?g=xml&seqNum=<%# DataBinder.Eval(Container.DataItem, "id") %>">
            <%# DataBinder.Eval(Container.DataItem, "guideEntryTitle") %>

         </a></p>
         <asp:Repeater id="DHeadRepeater" runat="server" 
              datasource='<%# Container.DataItem.Row.GetChildRows("topicRelation") %>'>

              <HeaderTemplate>
                 <div style="border: 1px dashed black">
              </HeaderTemplate>

              <ItemTemplate>
                  <%# Container.DataItem("guideEntryTitle") %><br />
              </itemtemplate>

              <FooterTemplate>
                  </div>

              </FooterTemplate>

          </asp:Repeater>
      </ItemTemplate>

      <FooterTemplate>
        (The bottom of the page.)
      </FooterTemplate>

   </asp:Repeater>
...

Notice that the new Repeater is within the ItemTemplate of the first one. That means it will be executed once for each row of the original set of data. But what will it use for data? Well, notice the datasource attribute. Remember, when the server encounters that type of evaluation block, it knows it has to calculate the contents based on the current row. Because we have created a data relation, the server can pull all of the child rows for the current row. The results look something like figure 3:

embedding a Child

Now that we have the basic Repeaters working, we can insert the actual HTML we want to output:

<html>
<head>
   <base href="http://www.informit.com" />

   <script type="text/javascript">

      function toggle(plusminus){
         if (plusminus.src=="http://www.informit.com/display/common/images/icons/plus.gif"){
            plusminus.src="/display/common/images/icons/minus.gif";
         } else {
            plusminus.src="/display/common/images/icons/plus.gif";
         }

         base = plusminus.parentNode;

         allChildren = base.getElementsByTagName("ul")[0].getElementsByTagName("li");
         for (i = 0; i < allChildren.length; i++){
            el = allChildren[i];
            if (el.style.display!="block"){
               el.style.display="block";
            } else {
               el.style.display="none";
            }
         }
      }

   </script>
   <style type="text/css">
      ul { display: block;
           list-style-type: none; }
   </style>

</head>
<body>
   <asp:Repeater id="CHeadRepeater" runat="server">
      <HeaderTemplate>
        <h2>Other page content, such as the top of the page</h2>
        <ul>

      </HeaderTemplate>

      <ItemTemplate>
         <li class="chead">
           <img src="/display/common/images/icons/plus.gif" alt="Expand" onclick="toggle(this)" />
           <span class="cheadT">
              <a href="/guides/content.asp?g=xml&seqNum=<%# DataBinder.Eval(Container.DataItem, "id") %>">

               <%# DataBinder.Eval(Container.DataItem, "guideEntryTitle") %>
              </a>
           </span>
           <ul style="display: block;">
          
             <asp:Repeater id="DHeadRepeater" runat="server" 
                datasource='<%# Container.DataItem.Row.GetChildRows("topicRelation") %>'>

                <ItemTemplate>
                   <li class="dhead" style="display: block">
                      <img src="/display/common/images/icons/plus.gif" alt="Expand" onclick="toggle(this)" />
                      <a href="/guides/content.asp?g=xml&seqNum=<%# Container.DataItem("id") %>">
                        <%# Container.DataItem("guideEntryTitle") %>

                      </a>
                   </li>
                </itemtemplate>

             </asp:Repeater>

           </ul>

         </li>

      </ItemTemplate>

      <FooterTemplate>
        </ul>
        (The bottom of the page.)
      </FooterTemplate>

   </asp:Repeater>

</body>
</html>

Now the page is virtually identical to the one we created earlier. (There is a slight change to the toggle script based on the fact that we will be manipulating the li elements directly.) As before, the JavaScript gets executed by the browser, not the server. But it is the server that creates the HTML, including the links.

The result looks like figure 4:

adding the correct HTML

At this point, we are essentially right back where we started. We have a page that displays our information, and opens and closes the appropriate node when requested. But we still need to have the server display nodes in their proper state to start with. We'll start by determining which main topic should be open:

<%@ Page Language="VB" %>
<%@ Import Namespace="System.Data" %>
<%@ Import Namespace="System.Data.Odbc" %>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
            "http://www.w3.org/TR/REC-html40/loose.dtd">

<script language="VB" runat="server">

    Dim openId as String
    Dim ds As DataSet 

    function getParentId(childId as String)

       Dim targetRows As Array
       targetRows = ds.Tables("dheads").select("id="&childId)
       if (targetRows.Length = 0)
          return 0
       else 
          Dim thisRow As DataRow = targetRows.GetValue(0)
          return thisRow("parent_id")
       end if

    end function

    Sub Page_Load(Src As Object, E As EventArgs)
        
        Dim MyConnection As OdbcConnection
        Dim MyCommand As OdbcCommand
        Dim ConnectionString As String

        ConnectionString = _
           "Driver={Microsoft Access Driver (*.mdb)};" & _
           "Dbq="&Server.MapPath("guideentries.mdb")&";"
        
        myConnection = new Odbc.OdbcConnection(ConnectionString)

        Dim CHeads As OdbcDataAdapter = _
                 New OdbcDataAdapter( _
                     "select * from GuideEntries where parent_id=0 order by listOrder", _
                     myConnection )
        ds = New DataSet()
        CHeads.Fill(ds, "cheads")

        Dim DHeads As OdbcDataAdapter = _
                 New OdbcDataAdapter( _
                      "select * from GuideEntries where parent_id > 0 order by listOrder", _
                      myConnection )
        DHeads.Fill(ds, "dheads")

        ds.Relations.Add("topicRelation", _
                             ds.Tables("cheads").Columns("id"), _
                             ds.Tables("dheads").Columns("parent_id"))

        Dim selectedId As String = Request.QueryString("seqNum")&" "
        if (selectedId = " ")
           selectedId = 0
           openId = 0
        else 
           Dim parentId As String = getParentId(selectedId)
           if (parentId = 0)
              openId = trim(selectedId)
           else
              openId = parentId
           end if
        end if

        CHeadRepeater.DataSource = ds.Tables("cheads")
        CHeadRepeater.DataBind()

        myConnection.Close()
    End Sub
</script> 
...

A couple of things to notice here. First, we have declared two global variables, which will be needed by other functions. One, the DataSet, we've moved from inside the Page_Load subroutine so that we can use it in the getParentId function, which accepts an id value, and uses it to to locate a specific row in the dheads table. If the record exists in that table, it returns the value of the parent_id column. If not, it is assumed to be a main topic, which has a parent_id of 0.

We can then use that function to determine which main node should be open. If the URL for the request contains a main topic ID, then of course that is the topic that should be open. If the URL contains the id of a sub topic, we'll want to use its parent. The actual openId variable is global so we can use it to set the style for each subtopic:

...
          return thisRow("parent_id")
       end if

    end function

    function itemStyle(thisId as String)

       Dim style As String

       if (thisId = openId or getParentId(thisId) = openId) 
          style = "style='display: block'"
       else 
          style = "style='display: none'"
       end if
       return  style

    end function

    Sub Page_Load(Src As Object, E As EventArgs)
        
...         
             <asp:Repeater id="DHeadRepeater" runat="server" 
                datasource='<%# Container.DataItem.Row.GetChildRows("topicRelation") %>'>

                <ItemTemplate>
                   <li class="dhead" <%# itemStyle(Container.DataItem("id")) %>>

                      <img src="/display/common/images/icons/plus.gif" alt="Expand" onclick="toggle(this)" />
                      <a href="/guides/content.asp?g=xml&seqNum=<%# Container.DataItem("id") %>">
                        <%# Container.DataItem("guideEntryTitle") %>
                      </a>
                   </li>

                </itemtemplate>
...

The itemStyle function simply compares the current id (or rather, its parent id) with the openId and provides the appropriate style, which we add to the element using the evaluation block.

If you now display the page without specifying a node to open, it should open with all nodes closed:

all nodes closed

If you then display the page with a selected node, as in:

http://farmernick.somee.com/guide4.aspx?seqNum=12

one node open

you can see that node 12, "DOM and Java", is open. But what about we want one of the subtopics? For example, topic 29 is a sub topic of "DOM Level 3". So the URL:

http://farmernick.somee.com/guide4.aspx?seqNum=29

gives us a page like:

sub node open

because the server first determines the parent node and opens that one. Now all that's left is to make sure that the open node gets the appropriate icon:

...
          return thisRow("parent_id")
       end if

    end function

    function getPlusMinus(thisId As String)
       if (thisId = openId)
         return "minus.gif"
       else
         return "plus.gif"
       end if
    end function

    function itemStyle(thisId as String)

       Dim style As String

...
      <ItemTemplate>
         <li class="chead">
           <img src="/display/common/images/icons/<%# getPlusMinus(Container.DataItem("id"))%>" alt="Expand" onclick="toggle(this)" />
           <span class="cheadT">

...

Subtopics will never be displayed as open to start with, so they always get the plus sign, but in this case the open main topic does get the minus sign.

So now we have a working tree navigational structure, which displays on the page with the appropriate node open, and enables users to open and close other nodes at will.

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