how to generate barcode in c#.net with example System.Web.UI.ICallbackEventHandler in Visual C#

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System.Web.UI.ICallbackEventHandler
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Create a constant that will store the path to an Ajax-style refresh GIF:
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const string GEARS_GIF = "/_layouts/images/kpiprogressbar.GIF";
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Create a property and variable that will allow the user to set the refresh interval of the web part, as shown in listing 9.4.
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The Refresh Rate property for the Ajax-style refresh interval
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Listing 9.4
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private int _refresh = 5; [WebBrowsable(true) , Personalizable(PersonalizationScope.User) ] public int Refresh { get { return _refresh; } set { _refresh = value; } }
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Creating a custom BDC Web Part
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Change your CreateChildControls method so that it resembles the one in listing 9.5. The CreateChildControls method will only be called briefly and will then be overwritten by a call that renders straight after the load event. The purpose that it serves here is to simply display the Ajax-style progress bar.
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New CreateChildControls method, which renders an image
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Listing 9.5
protected override void CreateChildControls() { try { Image img = new Image(); img.ImageUrl = GEARS_GIF; this.Controls.Add(img); } catch (Exception ex) { errormessage = ex.Message; } }
We re now going to use the OnLoad event to place some JavaScript on the page. You can see how the ClientScriptManager is used to register the script. The script generates a unique ID for the web part so that more than one of them can be placed on the page. It also gets the refresh interval from the web part s variable. The script is then loaded into the web part s innerHTML. Listing 9.6 is the web part s OnLoad event, which has the job of generating the JavaScript functions that in turn get registered into the web part page.
The Ajax-style OnLoad event, which fires the JavaScript
Listing 9.6
protected override void OnLoad(EventArgs e) { System.Web.UI.ClientScriptManager csm = Page.ClientScript; StringBuilder js = new StringBuilder();
any
//General JavaScript functions that can be added by Web Part string SCRIPT_NAME = "GeneralClientCallbackScript"; if (!csm.IsClientScriptBlockRegistered(SCRIPT_NAME)) { js.Append("function CallServer(arg, context)") .Append("{ \n") .Append(" try \n") .Append(" { \n")
Adding the functional code
.Append(" var element = document.getElementById(context); \n") .Append(" if(element) \n") .Append(" { \n") .Append(" if(arg != 'Initial') \n") .Append(" { \n") //.Append(" element.innerHTML = 'Loading...'; \n") .Append(" } \n") .Append(csm.GetCallbackEventReference( this, "arg", "HandleCallbackResult", "context", true) ) .Append(" } \n") .Append(" } \n") .Append(" catch(e){window.status = 'ERROR:' + e.Message} \n") .Append("} \n"); js.Append("function HandleCallbackResult(arg, context) \n") .Append("{ \n") .Append(" try \n") .Append(" { \n") .Append(" var element = \n") .Append(" document.getElementById(context); \n") .Append(" if(element) \n") .Append(" { \n") .Append(" element.innerHTML = arg; \n") .Append(" } \n") .Append(" } \n") .Append(" catch(e){window.status = 'ERROR:' + e.Message}
\n") .Append("} \n"); csm.RegisterClientScriptBlock( this.GetType(), SCRIPT_NAME, js.ToString(), true); } //Clear the string builder js.Remove(0, js.Length);
1000);
string refreshSeconds = string.Format("{0:d}", this.Refresh * //BootWebPart is needed because functions that run when //Body Onload is fired cannot have parameters
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//Invoke CallServer with a call to innerHTML js.Append("function BootWebPart" + .Append("{ \n") .Append(" CallServer('Initial', "'); \n") .Append(" setInterval( GetTime, .Append("} \n");
populate this Web Part's this.ID + "() \n") 'ctl00_m_' + '" + this.ID + " + 1000 + "); \n")
js.Append("function GetTime() \n") .Append("{ \n") .Append(" CallServer('DisplayDataGrid', 'divTime'); \n") .Append("} \n"); //Run BootWebPart when the Page has fully loaded js.Append("_spBodyOnLoadFunctionNames.push('BootWebPart" + this.ID + "'); \n"); //Unique JavaScript name for this Web Part script SCRIPT_NAME = this.ID + "Script"; csm.RegisterClientScriptBlock( this.GetType(), SCRIPT_NAME, js.ToString(), true); }
The ICallbackEventHandler members keep track of the eventArgument variable, which can be set to Initial or DisplayDataGrid. The Initial call populates the web part initially and fires the CreateChildControls method. Then the DisplayDataGrid eventArgument is passed, creating the DataGrid. This is where you place the code that was in the CreateChildControls method. The callback method in listing 9.7 is fired upon the set interval and displays the data grid periodically without a page refresh.
The callback methods
Listing 9.7
#region ICallbackEventHandler Members //Holds the case of the switch statement requested private string _eventArgument = ""; //Initially called by the client-side JavaScript public void RaiseCallbackEvent(string eventArgument) { //Save aside the function requested _eventArgument = eventArgument; } public string GetCallbackResult() {
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