All ASP.NET controls from
version 2.0 can be associated with a
control adapter. A
control adapter is a class that inherits from ControlAdapter and it has the chance to interact with the control(s) it is targeting so as to change some
of its properties or alter its output. I talked about
control adapters before and they really a cool feature. The ControlAdapter class exposes virtual methods for some well known lifecycle events, OnInit, OnLoad, OnPreRender and OnUnload that closely match their
Control counterparts, but are fired before them. Because the
control adapter has a reference to its target
Control, it can cast it to its concrete class and do something with it before its lifecycle events are actually fired. The adapter is also notified before the
control is rendered (BeginRender), after their children are renderes (RenderChildren) and after itself is rendered (Render): this way the adapter can modify the control’s output.
Control adapters may be specified for any class inheriting from
Control, including abstract classes, web server controls and even pages. You can, for example, specify a
control adapter for the WebControl and UserControl classes, but, curiously, not for
Control itself. When specifying a
control adapter for a page, it must inherit from PageAdapter instead
of ControlAdapter. The adapter for a
control, if specified, can be found on the protected Adapter property, and for a page, on the PageAdapter property. The first use
of control adapters that came to my attention was for changing the output
of standard ASP.NET web controls so that they were more based on CSS and less on HTML tables: it was the CSS Friendly
Control Adapters project, now available at http://code.google.com/p/aspnetcontroladapters/. They are interesting because you specify them in one location and they apply anywhere a
control of the target type is created. Mind you, it applies to controls declared on markup as well as controls created by code with the new operator. So, how do you use
control adapters? The most usual way is through a browser definition file. In it, you specify a set
of control adapters and their target controls, for a given browser. This browser definition file is a XML file with extension .Browser, and can either be global (%WINDIR%\Microsoft.NET\Framework64\vXXXX\Config\Browsers) or local to the web application, in which case, it must be placed inside the App_Browsers folder at the root
of the web site. It looks like this: 1: <browsers>
2: <browser refID="Default">
3: <controlAdapters>
4: <adapter controlType="System.Web.UI.WebControls.TextBox" adapterType="MyNamespace.TextBoxAdapter, MyAssembly" />
5: </controlAdapters>
6: </browser>
7: </browsers>
A browser definition file targets a specific browser, so you can have different definitions for Chrome, IE, Firefox, Opera, as well as for specific
version of each
of those (like IE8, Firefox3). Alternatively, if you set the target to Default, it will apply to all. The reason to pick a specific browser and
version might be, for example, in order to circumvent some limitation present in that specific
version, so that on markup you don’t need to be concerned with that.
Another option is through the the current Browser object
of the request:
1: this.Context.Request.Browser.Adapters.Add(typeof(TextBox).FullName, typeof(TextBoxAdapter).FullName);
This must go very early on the page lifecycle, for example, on the OnPreInit event, or even on Application_Start. You have to specify the full class name for both the target
control and the adapter.
Of course, you have to do this for every request, because it won’t be persisted.
As an example, you may know that the classic TextBox
control renders an HTML input tag if its TextMode is set to SingleLine and a textarea if set to MultiLine. Because the textarea has no notion
of maximum length, unlike the input, something must be done in order to enforce this. Here’s a simple suggestion:
1: public class TextBoxControlAdapter : ControlAdapter
2: {
3: protected TextBox Target
4: {
5: get
6: {
7: return (this.
Control as TextBox);
8: }
9: }
10:
11: protected override void OnLoad(EventArgs e)
12: {
13: if ((this.Target.MaxLength > 0) && (this.Target.TextMode == TextBoxMode.MultiLine))
14: {
15: if (this.Target.Page.ClientScript.IsClientScriptBlockRegistered("TextBox_KeyUp") == false)
16: {
17: if (this.Target.Page.ClientScript.IsClientScriptBlockRegistered(this.Target.Page.GetType(), "TextBox_KeyUp") == false)
18: {
19: String script = String.Concat("function TextBox_KeyUp(sender) { if (sender.value.length > ", this.Target.MaxLength, ") { sender.value = sender.value.substr(0, ", this.Target.MaxLength, "); } }\n");
20:
21: this.Target.Page.ClientScript.RegisterClientScriptBlock(this.Target.Page.GetType(), "TextBox_KeyUp", script, true);
22: }
23:
24: this.Target.Attributes["onkeyup"] = "TextBox_KeyUp(this)";
25: }
26: }
27:
28: base.OnLoad(e);
29: }
30: }
What it does is, for every TextBox
control, if it is set for multi line and has a defined maximum length, it injects some JavaScript that will filter out any content that exceeds this maximum length. This will occur for any TextBox that you may have on your site, or any class that inherits from it. You can use any
of the previous options to register this adapter.
Stay tuned for more ASP.NET Web Forms extensibility tips!
