使用AJAX和ASP.NET来制作DropDownList

Introduction<?xml namespace="" ns="urn:schemas-microsoft-com:office:office" prefix="o" ?>

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In the first section of this article, we shall introduce <?xml namespace="" ns="urn:schemas-microsoft-com:office:smarttags" prefix="st1" ?>AJAX and discuss its advantages and disadvantages. Next, we look at the XMLHTTPRequest object, its properties and functions, and how to create an instance of this object. Then, we will illustrate the use of AJAX functionality by creating a sample application. Finally, we will talk about how the sample application works. The sample application is an ASP.NET web application, written in C#, and is available for download.

What is AJAX ?

AJAXstands for Asynchronous JavaScript and XML. AJAX is not a new technology; it is a new way of combining existing technologies. What we do in AJAX is create an asynchronous request to the web server using client-side JavaScript and an XmlHttpRequest object, and map a function to be executed when the response is received. An asynchronous request means that the browser does not need to wait for the response after sending the request to the server. What we gain using AJAX is a responsive user interface, and we get this by sending a request to the web server for a small amount of information, as many times as we want, without sending the complete information on the form. In AJAX , the request is for data, not for a GUI element, so an AJAX request can be handled by an ASP.NET page without any HTML content, a custom HTTP module, or a web service. The core component in AJAX is the XMLHTTPRequest object. As many web developers are not familiar with this object, we will discuss it in detail in the next section. Following are the advantages and disadvantages of AJAX .

Advantages:

1. The use of AJAX the increases the richness and responsiveness of the web page interface.

2. It reduces the network traffic and CPU usage on web server. This is because there will be no post back to the server that will render a complete HTML page. For example, if you are displaying a lot of data in a web page, the HTML page size may be about 100 kilobytes. This big string has to be created on the server and then sent back to the client.

Disadvantages:

1. The use of AJAX requires users to have JavaScript enabled on their browser. Because of this, an AJAX website should provide a non-AJAX alternative for users without JavaScript enabled.

2. AJAXbreaks the normal browsers' Back button behavior. When a page is updated dynamically, returning to the previous state may not be possible, since browsers typically record only complete page requests in their history lists.

3. As not all browsers are complying with W3C standards, AJAX applications have to be tested rigorously to deal with the quirks of different browsers.

The XMLHTTPRequest Object

The XMLHTTPRequest object is used to send a request to, and receive a response from, the web server. Using this object, the web page can make a request to the web server asynchronously, and receive a response asynchronously. Even though this object is not a W3C standard, most of the current browsers (IE5.0, Mozilla 1.0, Netscape 7.0, Safari 1.2, and Opera 8.0) have implemented this object. Hereafter I will also refer an XMLHTTPRequest object as an XMLHTTP object.

Properties and Functions of XMLHTTPRequest object used in the Sample Application

Table 1 – Properties of XMLHTTPRequest Object

Table 2 – Methods of XMLHTTPRequest Object

Creating an Instance of an XMLHTTPRequest Object

While any browser-supported scripting language can be used to access this object, the method by which an instance of an XMLHTTPRequest object is created varies from browser to browser. Internet Explorer (IE) implements this object using ActiveX technology, so creating an instance of this object requires the use of “New ActiveXObject(ProgId)”, where ProgId is either Msxml2.XMLHTTP or Microsoft.XMLHTTP, depending on the version of MSXML installed. Mozilla, Safari, and Netscape implement this as a native object, so creating an instance requires the use of “New XMLHttpRequest()”. The following JavaScript function (Listing 1) creates an instance of an XMLHTTP object independent of browser type. The code presented for this article has been tested with IE 6.0, Netscape 8.0, and Mozilla Firefox 1.0.

Listing 1 – Creating an XMLHTTP Object

function CreateXmlHttp()

{

    //Creating object of XMLHTTP in IE

    try

    {

        XmlHttp = new ActiveXObject("Msxml2.XMLHTTP");

    }

    catch(e)

    {

        try

        {

            XmlHttp = new ActiveXObject("Microsoft.XMLHTTP");

        }

        catch()

        {

            XmlHttp = null;

        }

    }

    //Creating object of XMLHTTP in Mozilla and Safari

    if(!XmlHttp && typeof XMLHttpRequest != "undefined")

    {

        XmlHttp = new XMLHttpRequest();

    }

}

The Sample Application

The sample application uses a pair of DropDownList controls. A change in the selected value of the first drop-down list should populate the second drop-down list with a new collection of items. This paired behavior is found in many applications, such as a Country-State pair and a Category-Product pair. We have chosen to discuss the Country-State DropDownList pair. In traditional web applications, a change in the value of the Country DropDownList results in the web page posting back to server in order to populate the States DropDownList. In our example, we will populate the States DropDownList by retrieving state data from the server using AJAX , without submitting the entire form to the server.

In this section, first we will discuss the CountriesAndStates.xml and CountriesAndStates.xsl files, and then the CountryStateXml class which is used to get country and state data from the XML file. Next, we will discuss the start page of our project, which is AjaxClient.aspx. We will then discuss our JavaScript files, AjaxVariables.js and AjaxScript.js. You will find core AJAX code in the AjaxScript.js file. Finally we will talk about the AjaxServer.aspx page, which handles the AJAX requests.

CountriesAndStates XML and XSL Files

The CountriesAndStates.xml file stores the names of all countries and their states. The format of this XML file is shown below (Listing 2). Each country is represented as a single country node, and all of its states are represented as child nodes. At present this XML file has only three countries and some of the states of these countries. You can edit this XML to add other countries and states.

Listing 2 – CountriesAndStates.xml File

<country name="USA">

    <state>Alabama</state>

    <state>Alaska</state>

    <state>Arizona</state>

    <state>Arkansas</state>

</country>

The CountriesAndStates.xsl file has the transformation logic for converting one form of XML into another form of XML. This transformation takes a country name as an input parameter, and returns its corresponding states in an XML string.

CountryStateXml class

The CountryStateXml class is used to get country or state data in different formats depending on the method called. This class has one constructor and three public methods.  The constructor loads the CountriesAndStates.xml file into an XPathDocument. The public method GetCountryList returns a list of countries as an ArrayList. The second method GetStateList returns a list of states for a given country as an ArrayList. The third method GetStatesXMLString returns an XML document containing all the states for a given country name, along with the country name.

AjaxClient Page

The start page of our sample application is AjaxClient.aspx and Figure 1 shows it viewed in a browser. This page has two DropDownList server controls. The HTML markup of this page is given in Listing 3. The onchange event of the Country DropDownList is tied to the JavaScript function CountryListOnChange and is shown in bold text in the following markup. We will discuss the CountryListOnChange function later in this section.

Figure 1 – AjaxClient.aspx Page in a Browser

<?xml namespace="" ns="urn:schemas-microsoft-com:vml" prefix="v" ?>

Listing 3 – HTML of AjaxClient.aspx page

<form id="Form1" method="post" runat="server">

    <asp:DropDownList id="countryList" onchange="return CountryListOnChange()"

        style="Z-INDEX: 101; LEFT: 20px; POSITION: absolute; TOP: 28px"

        runat="server" Width="174px" Height="28px"></asp:DropDownList>

    <asp:DropDownList id="stateList"

        style="Z-INDEX: 102; LEFT: 218px; POSITION: absolute; TOP: 28px"

        runat="server" Width="174px"></asp:DropDownList>

</form>

Listing 4 shows the code for the Page_Load method of this web page. When the page loads for the first time, this method populates the Country and State DropDownList controls using the GetCountryList and GetStateList methods of CountryStateXml class.

Listing 4 – Page_Load Method of AjaxClient.aspx Page

private void Page_Load(object sender, System.EventArgs e)

{

    CountryStateXml countryStateXml = new CountryStateXml();

    ArrayList countries = countryStateXml.GetCountryList();

    for(int index = 0; index < countries.Count; index++)

    {

        countryList.Items.Add(countries[index].ToString());

    }

    ArrayList states = countryStateXml.GetStateList(countries[0].ToString()) ;

    for(int index = 0; index < states.Count; index++)

    {

        stateList.Items.Add(states[index].ToString());

    }

}

JavaScript Code

The AjaxScript.js file includes the required AJAX script. When the selected value of the country drop-down list changes, the CountryListOnChange function is called. Now we will talk about how the asynchronous request to the server in the CountryListOnChange function works. See Listing 5.

1. Form the request URL with the selected country in the query string.

2. The CreateXmlHttp function (this function is explained in the earlier section, Creating an Instance of an XMLHTTPRequest Object) creates an instance of an XMLHTTP request object. If the browser does not support the XMLHTTP object, the XmlHttp variable will be set to null and the AJAX code will not be executed. If the browser supports the XMLHTTP object, an asynchronous request is submitted to the server using the XMLHTTP object.

3. The onreadystatechange property of the XMLHTTP object is used to set the response handler for the asynchronous request. This response handler gets called for every change of readyState property of the XMLHTTP object. In our code we set the HandleResponse function as the response handler.

4. The open method is called to initialize the request. The first parameter of the open method takes a method definition (GET, POST, etc.), in our case it’s GET. The second parameter takes the request URL. The third parameter takes a Boolean value indicating whether the request submitted is synchronous or asynchronous. In our sample it’s asynchronous, so the third parameter is set to true. Other parameters are optional; we do not use them in our application.

5. The send method submits the request to the server. The send method's behavior is determined by the third parameter of the open method. In our case it is true, so the send function behaves as an asynchronous function, which means that the browser calls the send function and does not wait for the response to come back from the server.

Listing 5 – CountryListOnChange Function

function CountryListOnChange()

{

    var countryList = document.getElementById("countryList");

    var selectedCountry = countryList.options[countryList.selectedIndex].value;

    var requestUrl = AjaxServerPageName + "?SelectedCountry=" 

                     + encodeURIComponent(selectedCountry);

    CreateXmlHttp();

    if(XmlHttp)

    {

        XmlHttp.onreadystatechange = HandleResponse;

        XmlHttp.open("GET", requestUrl,  true);

        XmlHttp.send(null);          

    }

}

When the response comes back from the server, the HandleResponse function (Listing 6) gets called. In fact, the HandleResponse function is called for every change of the readyState property of the XMLHTTP object. The readyState property can take a value from 0 to 4 (0=UNINITIALIZED, 1=LOADING, 2=LOADED, 3=INTERACTIVE, 4=COMPLETED). A value of 4 (i.e., COMPLETED) means that receiving the response from the server is completed and the response data is available through properties of the XMLHTTP object. For the readyState property to reach the value of 4, this property changes the value four times (i.e., from LOADING to COMPLETED), so the HandleResponse function gets called four times for each response. To make sure that the data is available in the XMLHTTP object, the XmlHttp.readyState property is compared to the value of 4 in the HandleResponse function (Listing 6).

If the readyState of the XMLHTTP instance has a value of 4, we then need to check its status property to make sure that a valid response was received. The status property holds the HTTP status code returned from the server. A value of 200 means that data received in the request is OK. Some of the other popular HTTP status codes, with which most developers are familiar, are 404 (page not found), 403 (access forbidden), and 500 (internal server error). We check that the XmlHttp.status property is 200 to make sure that the response received has a status of OK. The responseXML property of the XMLHTTP object returns an XML document. This property is useful only when the response from the server is in XML format. In our sample, the response is a list of states in XML format. If both conditions are satisfied (readyState is 4 and status is 200), we call the function ClearAndSetStateListItems with the XML document as an input parameter.

Listing 6 – HandleResponse Function

function HandleResponse()

{

    if(XmlHttp.readyState == 4)

    {

        if(XmlHttp.status == 200)

        {

            ClearAndSetStateListItems(XmlHttp.responseXML.documentElement);

        }

        else

        {

            alert("There was a problem retrieving data from the server." );

        }

    }

}

The function ClearAndSetStateListItems clears the items in the State DropDownList and populates this drop-down list with the new states list by traversing through the XML document supplied as input.

AjaxServer Page

The AjaxServer.aspx file will handle the request sent by the client-side JavaScript using the XMLHTTPRequest object. This page does not have any GUI elements. Listing 7 shows the Page_Load event handler of the AjaxServer.aspx page. This method gets the selected country name from the query string, retrieves the state names as an XML document, and then writes the XML document into the response object.

Listing 7 – Page_Load Method of the AjaxServer.aspx Page

private void Page_Load(object sender, System.EventArgs e)

{

    if(!IsPostBack )

    {

        string selectedCountry = Request["SelectedCountry"];

        if(selectedCountry.Length > 0)

        {

            Response.Clear();

            CountryStateXml countryStateXml = new CountryStateXml();

            string statesString = countryStateXml.GetStatesXMLString(selectedCountry);

            Response.Clear();

            Response.ContentType ="text/xml";

            Response.Write(statesString);

            Response.End();

        }

        else

        {

            //clears the response written into the buffer and end the response.

            Response.Clear();

            Response.End();

        }

    }

    else

    {

        //clears the response written into the buffer and end the response.

        Response.Clear();

        Response.End();

    }

}

Running Sample Code

After downloading the sample application as a Zip file, extract its contents to your wwwroot directory. Create a virtual directory in IIS with the name AjaxSample, and map it to the extracted AjaxSample folder. Using Visual Studio, open the AjaxSample.sln file and press F5 to run the application.

Conclusion

In this article we have shown that AJAX is a technology that employs JavaScript to initiate an asynchronous request to the server, and to handle an XML response from the server. We then created a sample ASP.NET application that uses AJAX to dynamically re-populate a drop-down list without requiring the form to post back to the server. In this way, we have illustrated that AJAX allows an ASP.NET web page to provide for a more responsive user interface.

References

1. AJAX: A New Approach to Web Applications

2. AJAX: From Wikipedia, the free encyclopedia

3. XMLHTTP : From Wikipedia, the free encyclopedia

Thanks

I would like to provide my sincere thanks to my friend, Sanjay Patnaik, for helping me all through this article.