c# data matrix render VIEWSTATE AND SCRIPTING in Visual C#.NET

Encoder Data Matrix in Visual C#.NET VIEWSTATE AND SCRIPTING

CHAPTER 4 VIEWSTATE AND SCRIPTING
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this.ClientScript.RegisterClientScriptInclude ("Interdepends", "Interdepends.js"); The first parameter is a name, or a key really, that prevents the same include from being added to a single page multiple times. The second is the URL of the script file. This simplified call automatically generates the script include element for us. The last change you ll make is to use the RegisterClientScriptBlock method of the script manager to inject the dynamically generated array declarations onto the page, instead of using the PlaceHolder. You ll replace this line of code: plScript.Controls.Add(new LiteralControl(sb.ToString())); with this one: this.ClientScript.RegisterClientScriptBlock (this.GetType(), "InterArrays", sb.ToString()); This simple change prevents the same block of script from being loaded more than once. If you broke these lists out into their own control (UserControl or custom control), you d actually want more than one of these sets of array declarations, as you would need a unique set for each instance of the control on the page. This would require you to incorporate the ClientID attribute of the control into the array names, in order to avoid naming conflicts on the client. The client script manager is a nice improvement over the disjointed state of affairs for scripting in 1.x. While existing code will continue to work, the new features and methods of the script manager will be nice to leverage as you move forward into 2.0 development.
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Focus
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Another common problem has been resolved in 2.0 using client-side script. This is the problem of control focus. It s very common to need to programmatically determine the focus of a control. When using custom validation on the server, it s nice to set the focus to the first control that s found to be in an invalid state. This way the user is ready to correct the problem as soon as the postback completes and the browser refreshes. Doing this in 1.x was possible, but it required awkward, and sometimes arbitrary, code generation. In 2.0, the Control type exposes the Focus method. This can be called on any control on the page. The control automatically generates the appropriate client-side script to ensure the focus is placed there when the page is displayed in the browser.
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Out-of-Band Callbacks
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One of the biggest complaints about ASP .NET is the postback infrastructure, which is ironic, because it s also one of things people like the most. But there are some situations where a postback is just overkill. The previous example of interdependent drop-down lists is a good one. You need to affect the second list programmatically, but it just doesn t seem to warrant an entire round trip back to the server. In that example, you do the processing on the client instead, but you do so by shipping all values for all choices from list one. This increases your
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CHAPTER 4 VIEWSTATE AND SCRIPTING
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page size with the array script you generate, and chances are you won t use most of that data each time the page is sent to a user. For some operations, shipping data for all possible user selections is just not an option. A TreeView control is a good example. There may be hundreds or thousands of nodes in a large TreeView. You definitely don t want to ship all of the data for all of those nodes to the client when the client first requests the page. This would cause a huge up-front performance hit, and negatively impact the usability of the application. A postback for each click on the node is annoying too. The page renders quickly but its performance thereafter is kludgey and slow (and probably just plain unusable). So it would be nice, in many circumstances, if another option existed. Can you go back to the server and get just a little more data without having to rerender and resend all of the markup for the entire page This facility has been built into ASP .NET 2.0, and it s the last scripting feature we ll examine. The feature is called out-of-band callbacks, because it executes asynchronously on a worker thread of the web browser. This is good, because the user interface stays responsive while the browser makes a request to the server in the background. The callback is kicked off by an event on the client. A user action can cause it button clicks, change events, mouse events, whatever. The client-side script to launch the call to the server is automatically generated by a call to a method of the ClientScript object. Values from the web page can be gathered and passed back to the server as a string. Two values are passed, an argument and a context (more values could easily be passed using a delimited string). The argument can be used from the server-side processing, but the context cannot. It is actually passed to the callback method to establish, client-side, how the event originated. Another critical parameter that you specify is the name of the JavaScript function that should be called when the work is done and a response is sent back. You can also, optionally, specify the name of a client-side function to call if an error occurs. The server-side processing can do any work, but can only return a string. Again, if you desire a more complex structure, you can build a delimited string on the server and split it when it gets to the client. You can also have the server return HTML, which can be displayed on the client from the callback method using DHTML. Let s modify the interdependent drop-down list example to display the pool hall s address when the user makes a selection out of the second list. This is a good example of not wanting to make a full round trip back to the server, but also not wanting to totally bloat your page size by returning all of these addresses proactively when your user is only seeking a single address. The process of establishing a callback is fairly complex compared to the ease with which you can accomplish most tasks in ASP .NET. The steps are as follows: 1. You must create a page that not only inherits from the Page base class, but also implements the ICallBackEventHandler interface. 2. This interface exposes one method, RaiseCallbackEvent. This is the method that will fire when the callback occurs. It s a lot like any other server-side event trap, except the browser makes the request on a background thread instead of doing a full page refresh. The other big difference is that instead of generating an HTML document (as a normal postback would), this method simply returns a string, which is received by script code on the client.
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