Loading Progress and Status Displays
In This Chapter
Displaying a "Please Wait" Page
Replacing the Existing Page in the Browser
Displaying a Progress Bar Graphic
Implementing a Staged Page Load Process
ASP.NET is extremely fast when you're creating and delivering Web pages. However, no matter how fast and efficient your Web server and the software it runs (including your Web applications) are, the delay between the user clicking a button and seeing the results can vary tremendously. On a good ADSL or direct Internet connection, it might be a "wow, that was quick" few seconds. On a dial-up connection, especially when the server is on the other side of the world, it's more likely to be the seemingly interminable "did I remember to pay the phone bill?" response.
One feature that most executable applications offer but that is hard to provide in a Web application is accurate status information and feedback on a long-running process. However, this can be achieved in at least two different ways, depending on the process your application is carrying out and the kind of status or feedback information you want to provide.
One technique is a "smoke and mirrors" approach, in that it makes the user feel comfortable that something is happeningwhile in fact the information the user sees bears no real relationship to the progress of the server-based operation. The other approach, covered toward the end of the chapter, provides accurate status and feedback details but imposes limitations on client device type and the kinds of operation for which it is suitable.
In this chapter you will see what is possible regarding loading progress and status displays. You'll learn how to use and adapt a variety of techniques to suit your own applications and requirements. This chapter starts with a look at the theory of the process and examines the simplest way it can be achieved.
Displaying a "Please Wait" Page
Many ASP.NET developers find that despite their best efforts in producing efficient code that minimizes response times, the vagaries of database response times, the transit time over the Internet, and user input criteria that are not specific enough can result in a lengthy delay before a page appears in the browser. The result is that users often click the submit button several times to try to elicit a response from your server, sometimes causing all kinds of unfortunate side effects.
Chapter 6, "Client-Side Script Integration," looks at some specific solutions for creating a one-click button. However, an alternative approach is to provide a page that loads quickly and that displays a "please wait" message or some suitable graphic feature, while the real target page is being processed and delivered. In ASP 3.0 and other dynamic Web programming environments, it's common to handle this process with separate pages that implement the three execution stages shown in Figure 3.1.
Figure 3.1 The traditional separate-pages approach to providing a "please wait" message.
ASP.NET engenders the single-page postback architecture approach. However, you can build similar features into ASP.NET applications by implementing the three pages as separate sections of a single page. The server control approach to populating elements and attributes on the page also makes it easier to work with elements such as the <meta> element that you use as part of the process. Figure 3.2 shows the ASP.NET approach, as it is adopted in the example described in the following sections.
Passing Values Between Requests
Of course, what's missing from Figures 3.1 and 3.2 is how any values submitted by the user are passed from the "please wait" page to the code that creates the results. In ASP 3.0 and other dynamic Web page technologies, the usual technique is to include a placeholder within the content attribute of the <meta> element that gets replaced by a query string containing the values sent from the <form> section. You can then extract these from the query string in the page or section of code that generates the results. You'll see this discussed in more detail in the section "Displaying the "Please Wait" Message," later in this chapter.
Figure 3.2 The ASP.NET single-page approach to providing a "please wait" message.
A Simple "Please Wait" Example
Figure 3.3 shows the initial display of a simple sample page that displays a "please wait" message while the main processing of the user's request is taking place. The page queries the Customers table in the sample Northwind database that is provided with SQL Server. In the text box on the page, the user enters all or part of the ID of the customer he or she is looking for.
Figure 3.3 The initial page of the simple "please wait" example.
When the user clicks the Go button, the value in the text box is submitted to the server, and the page shown in Figure 3.4 is displayed. No complex processing is required to display this page, and the total size of the content transmitted across the wire is small, so it should appear very quickly. The user knows that his or her request is being handled, and there is no submit button for the user to play with in the meantime.
Obtaining the Sample Files
You can download this example and the other examples for this book from the Sams Web site at http://www.samspublishing.com, or from http://www.daveandal.net/books/6744/. You can also run many of this book's examples online at http://www.daveandal.net/books/6744/.
Figure 3.4 The "please wait" message that is displayed while processing the main page.
After a short delay (about 3 or 4 seconds, in this example), the main page, which contains the results, is returned to the user and replaces the "please wait" message. You can see in Figure 3.5 that the main page contains a list of customers matching the partial ID value that was provided. At the bottom of the page is a New Customer link that takes the user back to the first page.
Figure 3.5 The main page, displaying the results of a search for matching customers.
The HTML and Control Declarations
Listing 3.1 shows the relevant parts of the sample page shown in the preceding section. Notice that although you include a <meta> element in the <head> section of the page, you don't specify any attributes for it. Instead, you give it an ID and specify that it is a server control by including the runat="server" attribute. However, this <meta> element will have no effect on the page or the behavior of the browser until you specify the attributes for it in the server-side code.
Listing 3.1 The HTML and Control Declarations for the Simple "Please Wait" Sample Page
<html> <head> <!----- dynamically filled META REFRESH element -----> <meta id="mtaRefresh" runat="server" /> </head> <body> <!----- form for selecting customer -----> <form id="frmMain" Visible="False" runat="server"> Enter Customer ID: <asp:Textbox id="txtCustomer" runat="server" /> <asp:Button id="btnSubmit" Text="Go" runat="server" /> </form> <!----- "please wait" display -----> <div id="divWait" Visible="False" runat="server"> <center> <p> </p> <p> </p> <b>Searching, please wait...</b><p /> <p> </p> <span id="spnError"></span> <p> </p> </center> </div> <!----- section for displaying results -----> <div id="divResult" Visible="False" runat="server"> <b><asp:Label id="lblResult" runat="server" /></b><p /> <asp:DataGrid id="dgrResult" runat="server" /><p /> <asp:Hyperlink id="lnkNext" Text="New Customer" runat="server" /> </div> </body> </html>
The remainder of the page is made up of the three sections that implement the three pages shown in Figures 3.3 through 3.5. All three pages include a Visible="False" attribute in their container elementeither the <form> element itself for the first one or the containing <div> element for the other two pages. So all three sections will be hidden when the page is loaded, and you can display the appropriate one by simply changing its Visible property to True.
Meta Refresh and Postback Issues
As you can see from the figures and code so far in this chapter, this example uses a <meta> element in the "please wait" page to force the browser to load the main page. This much-used technique is a handy way to redirect the browser to a different page, and it is supported in almost every browser currently in use today.
When you use the server-side Response.Redirect method in an ASP.NET (or ASP 3.0) page, the server sends two HTTP headers to the client to indicate that the browser should load a different page from the one that was requested. The 302 Object Moved header indicates that the requested resource is now at a different location, and the Location new-url header specifies that the resource is located at the URL denoted by new-url.
The <meta> element supports the http-equiv attribute, which is used to simulate the effects of sending specific HTTP headers to the browser. To redirect the browser to a different URL, using a <meta> element, you can use this:
<meta http-equiv="refresh" content="[delay];url=[new-url] />
In this syntax, [delay] is the number of seconds to wait before loading the page specified in [new-url]. All browsers will maintain the current page they are displaying until they receive the first HTTP header sent by the server for the new page. So if the processing required for creating the new page takes a while and the server does not send any response until the processing is complete, the user will continue to see the page containing the <meta> element (the "please wait" message). By default, ASP.NET enables response buffering, so it does not generate any output until the new page is complete and ready to send to the browser.
Replacing the Existing Page in the Browser
Web browsers continue to display the existing page when you click a link in that page or enter a new URL in the address bar, while they locate and start to load the new page. However, as soon as the first items of the page that will be rendered are received (as opposed to the HTTP headers), the existing page is removed from the display, to be replaced by the progressive rendering of the new page.
One important point to note, however, is that if you disable output buffering by setting Response.Buffer = False, or if you force intermediate output to be sent to the response by using Response.Flush, the page currently displayed in the browser will be discarded as soon as the partial output of the new page is received.
You can delay the removal of the existing page in some browsersfor example, Internet Explorer supports page translations, which take advantage of the built-in Visual Filters and Transitions feature (see http://msdn.microsoft.com/workshop/author/filter/filters.asp#Interpage_Transition).
However, the issue here is that unlike when you submit an ASP.NET <form> element, the redirection caused by the <meta> element doesn't perform a postback. This means that viewstate for the page will not be maintained, and the values of any controls on the whole page (including the nonvisible sections) will be lost. So any values that you want to pass to the page the next time it loads (that is, when you display the results of processing the main section of the page) must be passed in the query string of the URL specified in the <meta> element.
Of course, this is what you would have to do in the pre-ASP.NET example shown in Figure 3.1 as well. Code in the page must collect the values from all the controls in the <form> section of the page when it is posted to the server, and it must build up a query string containing these within the <meta> element. You'll see how to do this in the following section.
The Page_Load Event Handler
The Page_Load event handler for the sample page first has to determine the current stage of the three-step process:
Stage 1The user has just posted the <form> element containing his or her input to the server.
Stage 2The "please wait" message is displayed, and the <meta> element has caused the browser to request the page containing the results.
Stage 3The user has clicked the New Customer link to go back to Stage 1.
The following sections describe the code and page content that is used in the example to implement these three stages.
Listing 3.2 shows the first section of the Page_Load event handler for the sample page. The only time a postback will have occurred is at Stage 1 because the other two stages are initiated by a <meta> element or a hyperlink. (Code in a section of the Page_Load event handler makes the <form> element visible when the page first loads, as you'll see shortly.)
Listing 3.2 The First Part of the Page_Load Event Handler
Sub Page_Load() If Page.IsPostback Then ' user submitted page with customer ID ' create URL with query string for customer ID ' next page will not be a postback, so viewstate will be lost Dim sRefreshURL As String = Request.Url.ToString() _ & "?custID=" & txtCustomer.Text ' use META REFRESH to start loading next page mtaRefresh.Attributes.Add("http-equiv", "refresh") mtaRefresh.Attributes.Add("content", "0;url=" & sRefreshURL) ' hide <form> section and show "wait" section frmMain.Visible = False divWait.Visible = True Else ...
The Page.IsPostback property will be True only at Stage 1. At that point, you can extract the value of the text box (and any other control values that you might have in more complex examples) and build up the URL and query string for the <meta> element. You obviously want to reload the same page, so you get the URL from the Url property of the current Request instance. In this example, the only value you need to maintain as the page is reloaded is the value of the text box, and you use the name custID for this as you create the query string.
Then, as shown in Listing 3.2, you add the attributes you need to the <meta> element already declared in the page. You declare the <meta> element as a server control by using the following:
<meta id="mtaRefresh" runat="server" />
ASP.NET will implement this element as an instance of the HtmlGenericControl class because there is no specific control type within the .NET Framework class library for the <meta> element. However, the HtmlGenericControl type has an Attributes collection that you can use to add the attributes you need to it. You add the http-equiv="refresh" attribute and the content attribute, with a value that will cause the browser to immediately reload the page. If you view the source of the page in the browser (by selecting View, Source), you'll see the complete <meta> element:
<meta id="mtaRefresh" http-equiv="refresh" content="0;url= /daveandal/books/6744/loadwait/simplewait.aspx?custID=a"></meta>
The next line of code hides the <form> section of the page. Because this stage is a postback, the viewstate of the controls on the page is maintained, so the form will remain visible if you don't hide it. The final code line makes the section containing the "please wait" message visible.
The HtmlGenericControl Class
The HtmlGenericControl class is described in more detail in Chapter 1, "Web Forms Tips and Tricks," where it is used for another control type that is not part of the .NET Framework class library.
Displaying the Results
Listing 3.3 shows the second section of the Page_Load event handler. This section is executed only if the Page.IsPostback property is False; however, you have to detect whether the page is being loaded by the <meta> element in the "please wait" page (Stage 2) or the hyperlink in the results page (Stage 3).
Listing 3.3 The Second Part of the Page_Load Event Handler
... Else ' get customer ID from query string Dim sCustID As String = Request.QueryString("custID") If sCustID > "" Then ' page is loading from META REFRESH element and ' so currently shows the "please wait" message ' a customer ID was provided so display results divResult.Visible = True ' set URL for "Next Customer" hyperlink lnkNext.NavigateUrl = Request.FilePath ' get data and bind to DataGrid in the page FillDataGrid(sCustID) Else ' either this is the first time the page has been ' loaded, or no customer ID was provided ' display controls to select customer frmMain.Visible = True End If End If End Sub
You've just seen how the code that runs in Stage 1, when the user submits the form, adds the customer ID to the query string as custID=value. (When the user loads the page by clicking the hyperlink in the results page, there will be no query string.) So you test for the presence of a customer ID value and, if there is one, you can make the section of the page that displays the results visible, set the URL of the hyperlink in that section of the page so that it will reload the current page, and then call a separate routine, named FillDataGrid, that calculates the results and fills the ASP.NET DataGrid control in this section of the page.
At the end of Listing 3.3 you can see the code that runs for Stage 3 of the process. In this case, you know that it's not a postback, and there is no customer ID in the query string. So either this is the first time the page has been accessed or the user did not enter a customer ID value in the text box. In either case, you just have to make the <form> section visible, and the user ends up back at Stage 1 of the process.
Viewstate and the Visible Property
Notice that because the page does not maintain viewstate for Stages 2 and 3, you don't need to hide the other sections of the page content. All three carry the Visible="False" attribute, so they will not be displayed unless you specifically change the Visible property to True when the page loads each time.
Populating the DataGrid Control
The only other code in the sample page is responsible for fetching the required data from the database and populating the DataGrid control on the page. The full or partial customer ID, extracted from the query string at Stage 2 of the process, is passed to the FillDataGrid routine, which is shown in full in Listing 3.4.
Listing 3.4 The Final Part of the Page_Load Event Handler
Sub FillDataGrid(sCustID As String) Dim sSelect As String _ = "SELECT CustomerID, CompanyName, City, Country, Phone " _ & "FROM Customers WHERE CustomerID LIKE @CustomerID" Dim sConnect As String _ = ConfigurationSettings.AppSettings("NorthwindSqlClientConnectString") Dim oConnect As New SqlConnection(sConnect) Try ' get DataReader for rows from Northwind Customers table Dim oCommand As New SqlCommand(sSelect, oConnect) oCommand.Parameters.Add("@CustomerID", sCustID & "%") oConnect.Open() dgrResult.DataSource = oCommand.ExecuteReader() dgrResult.DataBind() oConnect.Close() lblResult.Text = "Results of your query for Customer ID '" _ & sCustID & "'" ' force current thread to sleep for 3 seconds ' to simulate complex code execution Thread.Sleep(3000) Catch oErr As Exception oConnect.Close() lblResult.Text = oErr.Message End Try End Sub
The code here is fairly conventional. It creates a parameterized SQL statement and then executes it with a Command instance to return a DataReader instance that points to the result set generated by the database. You use the customer ID passed to the routine as the value of the single Parameter instance you create, and the resulting DataReader instance is bound to the DataGrid control. See the section "Using Parameters with SQL Statements and Stored Procedures" in Chapter 10, "Relational Data-Handling Techniques," for more details on using parameterized SQL statements.
Simulating a Complex or Lengthy Process
The code used to populate the DataGrid control in this example is unlikely to qualify as a complex or lengthy operation. Unless someone pulls the network cable out, it won't take long enough for the user to see the "please wait" message in the demonstration page. So to simulate a long process, you can insert a call to the Sleep method of the static Thread object, specifying that the current thread should wait 3 seconds before continuing:
The only point to watch for here is that you have to import the System.Threading namespace into the page to be able to access the Thread object:
<%@Import Namespace="System.Threading" %>