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Because controls are client components, you can access them with the $find method, passing the ID of the control as an argument.
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The ID of a control can t be set programmatically. It s automatically set by the Sys.UI.Control class to the same ID as the associated element. You can get a reference to the control by passing the ID of the associated DOM element to $find.
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Another way to access a control is through the associated element. Because an element can have one and only one associated control, a property called control which stores the reference to the control is created on the DOM element when the control is initialized. Supposing that you have a DOM element stored in the someElement variable, the following statement accesses the associated control (if it exists, of course) and stores a reference in the controlInstance variable:
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var controlInstance = someElement.control;
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Next, we ll examine two examples of custom controls created with the Microsoft Ajax Library. The first example is relative to an element wrapper: a control called TextBox that wraps a text box element on the client side. The second example illustrates how to use client controls to work on a block of structured markup code.
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The first client control you ll create is an element wrapper: a control that represents a DOM element on the client side. Your mission is to wrap a text box element with a client TextBox control. The reasons for using an element wrapper are varied. In this case, you want to be able to prevent the web form from being submitted when the Enter key is pressed in the text field, as normally happens in a web page. The logic needed to accomplish this task is controlled through a public property called ignoreEnterKey, which is exposed by the control. If you set the property to true, a press of the Enter key in the text field is ignored. If the property is set to false, the form is submitted to the server. Listing 8.9 shows the code for the Samples.TextBox control.
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ASP.NET AJAX client components
Listing 8.9 Code for the Samples.TextBox control
Type.registerNamespace('Samples'); Samples.TextBox = function(element) { Samples.TextBox.initializeBase(this, [element]); this._ignoreEnterKey = false; } Samples.TextBox.prototype = { initialize : function() { Samples.TextBox.callBaseMethod(this, 'initialize');
$addHandlers(this.get_element(), {keypress:this._onKeyPress}, this); }, dispose : function() { $clearHandlers(this.get_element()); Samples.TextBox.callBaseMethod(this, 'dispose'); },
B Control lifecycle
_onKeyPress : function(evt) { if(this._ignoreEnterKey && evt.charCode == 13) { evt.preventDefault(); } }, get_ignoreEnterKey : function() { return this._ignoreEnterKey; }, set_ignoreEnterKey : function(value) { this._ignoreEnterKey = value; }
C Event handler
} Samples.TextBox.registerClass('Samples.TextBox', Sys.UI.Control);
The structure of a control is similar to that of a behavior. As always, you see the overrides of the initialize and dispose methods B, where you attach and detach handlers for the events raised by the associated element. In this example, you re interested in handling the text box s keypress event, which notifies you of any key pressed by the user in the text field. The corresponding event handler C
Controls
_onKeyPress does a check to determine if the ignoreEnterKey property is set to true and if the user pressed the Enter key. If the check is positive, it calls the preventDefault method on the event object to prevent execution of the event s default
action. This, in turn, prevents the form from being submitted to the server. This functionality can be enabled or disabled through the ignoreEnterKey property. To test the control, create a new ASP.NET AJAX page, declare a text box element, and associate it with a new instance of the TextBox control, as shown in listing 8.10.
Listing 8.10 Code for testing the Samples.TextBox control
<input type="text" id="myTextBox" /> <script type="text/javascript"> Sys.Application.add_init(pageInit); function pageInit() { $create(Samples.TextBox, {'ignoreEnterKey':true}, {}, {}, $get('myTextBox')); } </script>
It s no surprise that you instantiate the component with a $create statement during the init stage of the page lifecycle. The $create method sets the value of the ignoreEnterKey property to true. This activates the custom functionality and executes its logic every time the user presses a key in the text field.
The ASP.NET Futures package contains more examples of element wrappers, such as Label, HyperLink, and Button controls. They re defined in the PreviewScript.js file; you ll use them in chapter 11, when we discuss the XML Script declarative language. Appendix A contains instructions for how to install the ASP.NET Futures package.
So far, you ve seen examples of visual components (both behaviors and controls) that target a single DOM element. In many situations, you have to deal with complex UIs that consist of a hierarchy of DOM elements a DOM subtree. For example, the UI of a menu is composed by many different elements labels, hyperlinks, panels and the same is true for complex controls such as the ASP.NET GridView or the TreeView. Is it possible to associate a client control with the complex markup code rendered by a GridView or in general to a portion of structured markup code Are you restricted to developing only simple element wrappers
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