Thursday, July 27, 2006

 

Panel Dynamics

Browser renderings of data and controls can produce dynamic displays that traditional client-side applications could only hope for. Particularly useful is a scrollable collection of controls, including ones such as buttons that produce actions. In a sterling example of cuteness and opacity, Microsoft's ASP.NET 2.0 documentation hints that creating such a display might be possible but won't tell you how to do it. Neither will the usually helpful Dino Esposito, although his book "Programming ASP.NET 2.0 Core Reference" does have a sentence explaining that dynamic controls must be regenerated each time a page loads (see page 113).

Microsoft recommends its PlaceHolder as a container for dynamic controls, but that is a dead object, offering no mobility. Much more useful is Panel, which can provide both horizontal and vertical scrolling. Action controls inside a Panel operate normally as long as they are visible, no matter where they may be moved on the display.

Each action control has one or more collections of event handlers. Button has Click, CheckBox has CheckChanged, and so on. Server-side event handler functions are connected to the corresponding events by adding a delegate in C#. To get the events from dynamic controls, the code to create the controls and specify their event handler functions can be placed in the Page_Load function and must execute every time. For example, using a statically declared Panel:

In the .aspx markup --
    < asp:Panel id="ofPanelView" runat="server" >

In the .aspx.cs Page_Load function --
    ControlCollection oControls = ofPanelView.Controls;
    oControls.Clear();
    Button oSpecialButton1 = new Button();
    oSpecialButton1.CommandName = "1";
    oSpecialButton1.Click += new System.EventHandler(SpecialButton_Click);
    oControls.Add(oSpecialButton1);
    Button oSpecialButton2 = new Button();
    oSpecialButton2.CommandName = "2";
    oSpecialButton2.Click += new System.EventHandler(SpecialButton_Click);
    oControls.Add(oSpecialButton2);

Elsewhere in the .aspx.cs code --
    SpecialButton_Click(object sender, EventArgs e)
    {   string sCommandName = ((Button)sender).CommandName;
        . . .

If you will be changing a pattern of dynamic controls for a page, then you must also assign the .ID property of each control and do so in a consistent way, so that on postback values and events will be paired with corresponding server-side objects. If a pattern of dynamic controls does not change, default ID values will do the job.


[Note: because of display limitations, characters "< " and " >" here are shown with a space after or before them.]

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