The Page_Load Event

Intro3.aspx illustrates how you can dynamically initialise the contents of a Web Page using the Page_Load event. This example also introduces another type of Web Control, the DropDownList object.

<%@Register TagPrefix="tutorial" Namespace="Tutorial" Assembly="tutorial" %>
<script language="Dyalog" runat="server">

∇Page_Load 
:Access Public
list.Items.Add ⊂'Apples'
list.Items.Add ⊂'Oranges'
list.Items.Add ⊂'Bananas'
∇

∇Select (obj ev)
:Access Public
:Signature Select Object obj, EventArgs ev
out.Text←'You selected ',list.SelectedItem.Text
∇
</script>

<html>
<head>
<title>Initialising the contents of the Page using the Page_Load method</title>
<link rel="stylesheet" type="text/css" href="apl.css">
</head>

<body>
<h1>intro3: The Page_Load method</h1>
<form runat="server">
<asp:DropDownList id="list" runat="server"/>
<p>
<asp:Label id="out" runat="server" />
</p>
<asp:Button id="btn" 
    Text="Submit"
    runat="server"
    OnClick="Select"/>
</form>
<tutorial:index runat="server"/>
</body>
</html>

When an ASP.NET web page is loaded, it generates a Page_Load event. You can use this event to perform initialisation simply by defining a public function called Page_Load in your APLScript. This function will automatically be called every time the page is loaded. The Page_Load function should be niladic.

Note that, if the page employs the technique illustrated in Intro1.aspx, whereby the page is continually POSTed back to itself by user interaction, your Page_Load function will be run every time the page is loaded and you may not wish to repeat the initialisation every time. Fortunately, you can distinguish between the initial load, and a subsequent load caused by the post back, using the IsPostBack property. This property is inherited from the System.Web.UI.Page class, which is the base class for any .aspx page.

The Page_Load function in this example checks the value of IsPostBack. If 0 (the page is being loaded for the first time) it initialises the contents of the list object, adding 3 items "Apples", "Oranges" and "Bananas". The explanation for the statement:

      list.Items.Add ⊂'...'

is that the DropDownList WebControl has an Items property that is a collection of ListItem objects. The collection implements an Add function that takes a String Argument that can be used to add an item to the list.

Notice that the name of the object list is defined by the id="list" attribute of the DropDownList control that is defined in the page layout section of the page.

In this example, the page is processed by a POST back caused by pressing the Submit button. As it stands, changing the selection in the list object does not cause the text in the out object to be changed; you have to press the Submit button first.

However, you can make this happen automatically by adding the following attributes to the list object (see intro4.aspx):

AutoPostback="true"

OnSelectedIndexChanged="Select"/>

AutoPostback causes the object to generate HTML that will provoke a post back whenever the selection is changed. When it does so, the OnSelectedIndexChanged event will be generated in the server-side script which in turn will call Select, which in turn will cause the text in the out object to change.

Note that this technique, which can be used with most of the ASP.NET controls including CheckBox, RadioButton and TextBox controls, relies on a round trip to the server every time the value of the control changes. It will not perform well except on a fast connection to a lightly loaded server.