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The Render method is a virtual (overridable) member of the System.Web.UI.Control class. This is a class that all server controls inherit from.
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For our custom weather web part control, we ll use some logic to randomly produce a weather result for a variable number of days, and then create a weather image for each day of weather. In reality you would likely have some back-end process such as a web service that would return actual weather results. Add the code shown in listing 2.3 to your class and build the project to see that everything compiles.
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WEB PARTS: THE BUILDING BLOCKS OF PORTALS
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Listing 2.3 The Render method for the weather web part displays a series of random weather images.
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private enum WeatherType { Sunny = 0, Rainy = 1, Cloudy = 2, Unknown = int.MaxValue }
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protected override void Render(HtmlTextWriter writer) { Random rand = new Random();
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for (int i = 0; i < this.NumberOfDays; i++) {
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int weatherValue = rand.Next(1000) % 3; WeatherType todaysWeather = (WeatherType) weatherValue;
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Choose one of 4 images Image img = new Image() ; based on result of random number img.ImageUrl = string.Format("~/images/{0}.gif", todaysWeather.ToString()) ; img.AlternateText = "Today's weather";
writer.AddStyleAttribute(HtmlTextWriterStyle.TextAlign, "center"); writer.RenderBeginTag(HtmlTextWriterTag.Div); img.RenderControl(writer); writer.WriteBreak(); Render weather image writer.Write(todaysWeather.ToString()); writer.RenderEndTag(); // end Div } }
In the Render method we have constructed a simple loop that will run for as many days as we have weather to display and, within each loop, a weather picture is produced and rendered.
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The HtmlTextWriter that we ve used to render our weather web part is a customized text writer that simplifies the task of writing HTML and is also capable of rendering specific output based on the device that is targeted. For example, when the page is visited by an older browser, the HtmlTextWriter will automatically emit down-level markup.
Adding custom controls That completes the creation of the custom web part; all that remains is to create a web page to contain and display it and from now on we ll refer to web pages that contain web parts as web part pages, to distinguish them from ordinary web pages. UNDERSTANDING THE WEBPART CLASS 41
Add a new web page to your project named CustomWeather.aspx and, as with all web part pages, add a WebPartManager to it. You must also declare the server control to the page by using a Register directive at the head of the page as shown here:
<%@ Register TagPrefix="wp" Namespace="WebPartTests" %>
When you have registered your controls to the page you can then reference your custom web part within a ZoneTemplate making sure to use the same tag prefix as declared in the register directive:
<asp:WebPartZone ID="WebPartZone1" runat="server"> <ZoneTemplate> <wp:CustomWeatherPart ID="CustomWeatherPart1" runat="server" Title="Weather Forecast" /> </ZoneTemplate> </asp:WebPartZone>
As we saw in figure 2.6, when this page is displayed in a browser, it will render the weather web part with the default four days worth of weather visible. To change the number of days that are displayed you can just add the NumberOfDays property in the markup of the server control or set the value in code. The following snippet shows the property being set within the markup to display an entire week s worth of weather.
<wp:CustomWeatherPart ID="CustomWeatherPart1" runat="server" NumberOfDays="7" Title="Weather Forecast" />
Creating web parts with user controls We ve seen labels and calendars that magically morph into GenericWebParts and custom server controls that derive from the WebPart class being used to create web parts; but we haven t as yet seen a web part created using a UserControl. Being able to use user controls as web parts allows developers to create user interfaces employing exactly the same techniques they applied when creating web pages. This includes having the ability to drag-and-drop controls from the Toolbox onto the design surface. For this reason, user controls may also be easier to understand for someone who is relatively new to ASP.NET and who would benefit from the better design time experience that they would get when creating user controls in Visual Studio 2005. One advantage includes being able to drag controls directly from the Visual Studio 2005 Toolbox onto the surface of user controls as opposed to having to work solely in code. The weather web part that we built had a very simple user interface and therefore the rendering code and logic were not overly complex; but as the amount of presentation code that is required for a control increases, custom server controls can become quite difficult to create and maintain because you tend to end up writing many lines of code to create the user interface layout.
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