Working with Client-Side Script
Summary: While ASP.NET performs most of its processing on the server, some actions are better served by client-side processing. Scott Mitchell shows how your ASP.NET pages and controls can add client-side code. (27 printed pages)
Creating a Base Class as the Foundation for Adding Client-Side Script
Adding Client-Side Script from the Code-Behind Class
Executing Client-Side Code in Response to User Action
Implementing Common Client-Side Functionality
When working with dynamic Web-based scripting technologies, like classic ASP or PHP, developers must have a keen understanding of the logical, temporal, and physical separation between the client and the server. For a user's action to trigger the execution of server-side code, for example, a developer working with classic ASP must explicitly cause the user's browser to make a request back to the Web server. Creating such interactions can easily consume much development time and lead to unreadable code.
While Web Forms in ASP.NET greatly simplify running server-side script when client-side actions are performed, such power, if misused, can lead to unacceptable performance. While Web Forms hide the complexities involved, each time server-side code needs to be executed, the end user's browser must make a request back to the Web server by resubmitting the form. When submitting the form, all form fields—textboxes, drop-down lists, check boxes, and so on—must have their values sent back as well. Additionally, the page's view state is sent back to the Web server. In total, each time the Web page is posted back, potentially several kilobytes of data will need to be sent back to the Web server. Frequent postbacks, then, can quickly lead to an unusable Web application, especially for those users still stuck on dial-up. The need for frequent postbacks can be reduced by pushing functionality to the client.
Note ASP.NET Web Forms emit a hidden form field titled VIEWSTATE, which contains a base-64 encoded representation of the changed state of the Web controls in the Web Form. Depending on the Web controls present, the view state can range anywhere from a few dozen bytes, to tens of kilobytes. To learn more about view state check out my article Understanding ASP.NET View State.
With classic ASP, adding data-driven, custom client-side script was simple, albeit not very readable. To display a popup window in classic ASP that loads a URL based on some ID field, for instance, you would type in the appropriate client-side script, using the
<%=id%>syntax to insert the value of the ID field. ASP.NET allows you to create such data-driven client-side script with some assorted methods in the Page class.
This article examines techniques for adding client-side script to your ASP.NET Web pages. Client-side script is, as its name implies, script code that runs in the visitor's browser. We'll see how to accomplish common client-side tasks, such as displaying alerts, confirm boxes, and popup windows. (One of the main uses of client-side script—form field validation—is a bit of a moot topic with ASP.NET, since the validator Web controls provide client-side form validation out of the box.) The focus of this article will be on the server-side classes, methods, and techniques for injecting client-side script; we will not be examining the actual client-side script in detail, as this information is covered in numerous other articles and sites around the Web.
Creating a Base Class as the Foundation for Adding Client-Side Script
One of the major differences between classic ASP and ASP.NET is the programming model of each technology. ASP pages are atomic, procedural scripts interpreted on each page visit. ASP.NET, however, is a fully object-oriented programming technology. All ASP.NET Web pages are classes with properties, methods, and events. All Web pages are derived, either directly or indirectly, from the Page class in the System.Web.UI namespace; the Page class contains the base functionality of an ASP.NET Web page.
One of the concepts of object-oriented programming is that of inheritance. Inheritance allows you to create a new class that extends the functionality of another class. (If class B inherits class A, it is said to extend class A; class A is said to be the base class.) When using the code-behind model for creating ASP.NET Web pages, you can quite clearly see that the code-behind class inherits the Page class:
Public Class WebForm1 Inherits System.Web.UI.Page ... End Class
By having your code-behind class inherit the Page class, it automatically receives the functionality inherent in the Page class, such as the Request, Response, Session, and ViewState objects, as well as common events, like Init, Load, Render, as so on. As we'll see in this article, if you have a need for some common functionality to be available for all ASP.NET Web pages, one approach is to create a class that derives from the Page class and has additional methods and properties to accomplish these desired enhancements. Then, to have an ASP.NET Web page utilize these enhancements, all we need to do is update the Inherits statement in the page's code-behind class to use the class that extends the Page class.
In this article we'll create a class, called ClientSidePage, that derives from the Page class and provides extra methods to help with performing common client-side tasks. By having a code-behind class inherit ClientSidePage, rather than Page, adding script code will be as simple as calling a method and passing in a few parameters. Specifically, this class will contain methods for:
- Displaying a modal, client-side dialog box.
- Setting the focus to a specific form field on page load.
- Using a modal confirm dialog box to determine if a user wants to postback the form or not.
- Displaying popup windows.
Before we delve into the ClientSidePage class, let's first examine the pertinent methods in the Page class for injecting client-side script into a Web page. Once we've covered these Page methods, we'll jump into extending their functionality with the ClientSidePage class, and see how to tie everything together and use the extended class in an ASP.NET Web page.
Adding Client-Side Script from the Code-Behind Class
All ASP.NET Web pages must be derived directly or indirectly from the Page class in the System.Web.UI namespace. The Page class contains the base set of methods, properties, and events required for a functioning Web page. Among the class's many methods are a few methods designed for injecting client-side script into the rendered HTML. These methods are called from the code-behind class and can therefore be used to emit data-driven client-side script. The pertinent Page class methods for emitting client-side script follow.
The base class is derived from the System.Web.UI.Page class, so you can access the Page class's public methods by calling them directly from your code-behind class.
Note To access the Page class's methods, you can either type in the method name directly, or utilize IntelliSense in Microsoft Visual Studio .NET by entering MyBase. (for Microsoft Visual Basic .NET), this. (for C#), or Page. (for either C# or Visual Basic .NET). If you are using Visual Basic .NET as your programming language of choice, be sure to configure Visual Studio .NET to not hide advanced members, or you won't see these client-side script methods. (To show advanced members, go to Tools | Options | Text Editor | Basic and uncheck Hide advanced members.)
When emitting client-side script through the code-behind class of an ASP.NET Web page, typically the value of the key parameter isn't of paramount importance. Simply choose a descriptive key value. The key parameter is more pertinent when injecting client-side script code through a custom, compiled server control. There may be instances where a compiled control requires that there be a set of client-side functions. Multiple instances of the server control on one page might be able to share these common client-side script functions, so these functions need only be emitted once for the entire page, and not once per control instance. For example, the validation controls utilize client-side code to enhance the user experience. This client-side code must be present if there are any validation controls on the page, but if there are multiple validation controls, all can use this single set of shared functions.
By giving a script block a key, a control developer building a control that utilizes a set of common client-side functions can check to see if the required set of common functions has already been added by another instance of the control on the page. If so, it need not re-add the common script. To check if a script block has been added with the same key, use the IsClientScriptBlockRegistered(key) method, which will return a Boolean value indicating whether or not a script block with the same key has been registered. Realize that you can add a script block without first checking if it's registered. If you attempt to add a script block with a key that's already registered, the added script block will be ignored and the original script block will remain assigned to that key.
Note The IsClientScriptBlockRegistered method is particularly useful in two situations. First, it comes in handy when you're adding similar, but unique script blocks, and you need to insure that each script block is given a unique key. The code we'll examine later on in this article illustrates the utility of the "is registered" method. A second use is when building a control that needs some common script, especially if the script is not trivially generated. By using the IsClientScriptBlockRegistered method, you can ensure that the script common to all instances of the server control on the page is generated only once per page load, rather than once per control instance on the page.
The RegisterClientScriptBlock method is useful for adding client-side script that does not rely on any of the form fields present within the Web Form. A common use of this method is to display a client-side alert box. For example, imagine that you had a Web page with some TextBox Web controls and a Save Button. The TextBox controls might have particular values from a database. Imagine that this page allowed the user to modify these values and commit their changes by clicking the Save button. When clicking Save, the Web page would be posted back, and the Button's Click event would fire. You could create a server-side event handler for this event that updates the database. To let the user know that their changes had been saved, you might want to display an alert box that says, "Your changes have been saved." This could be accomplished by adding the following line of code to the Button's Click event handler:
The above will add the specified script content within the page's <form>, but before the content within the form. When the page is rendered in the user's browser they will see a client-side alert box displayed upon page load, as shown in Figure 1.
The RegisterStartupScript method is quite similar to the RegisterClientScriptBlock method. The main difference is the location where the client-side script is emitted. Recall that with the RegisterClientScriptBlock the script is emitted after the start of the <form> element, but before the form's contents. RegisterStartupScript, on the other hand, adds the specified script at the end of the form, after all form fields. Use RegisterStartupScript to place client-side script that interacts with the rendered HTML elements. (Later we'll look at an example of setting the focus to a form field upon page load; to accomplish this you'll use the RegisterStartupScript method.)
Like RegisterClientScriptBlock, the script blocks added by RegisterStartupScript need a unique key value. Again, this key value is primarily used by custom control developers. Not surprisingly, there is an IsStartupScriptRegistered(key) method as well, which returns a Boolean value indicating if a script block with the specified key has already been registered or not.
Note For more information on using RegisterStartupScript and RegisterClientScriptBlock in creating custom, compiled server controls, read an earlier article of mine: Injecting Client-Side Script from an ASP.NET Server Control.
To add the values 1, 2, and 3 to a client-side Array object named FavoriteNumbers, you'd use the following server-side code:
RegisterArrayDeclaration("FavoriteNumbers", "1") RegisterArrayDeclaration("FavoriteNumbers", "2") RegisterArrayDeclaration("FavoriteNumbers", "3")
This code would emit the following client-side script:
Notice that each array value passed in must be a string; however, the client-side script rendered sets the values of the Array object as the contents of the string. That is, if you wanted to create an Array with the string values "Scott" and "Jisun", you'd use:
RegisterArrayDeclaration("FavoriteFolks", "'Scott'") RegisterArrayDeclaration("FavoriteFolks ", "'Jisun'")
Notice that the second input parameters are strings that contain
'Jisun'—text delimited by a single apostrophe. This would render the following client-side script:
In classic ASP there was often the need to pass around various bits of information from one page to another. A common way of accomplishing this was using hidden form fields. (A hidden form field is a form field that is not displayed, but whose value is sent on the form's submission. The syntax for creating a hidden form field is
<input type="hidden" name="name" value="value" />.) The need for passing information around by custom hidden form fields in ASP.NET is greatly reduced since the state of the controls in the page is automatically persisted. If, however, you find that you need to create a custom hidden form field, you can do so through the RegisterHiddenField method.
The RegisterHiddenField method accepts two input parameters: the name of the hidden field and the value. For example, to create a hidden form field with the name foo and the value bar, use the following code:
This would add a hidden form field within the page's <form> element, like so:
<form name="_ctl0" method="post" action="test.aspx" id="_ctl0"> <input type="hidden" name="foo" value="bar" /> ... </form>
Understanding How Client-Side Elements Are Rendered
The Page class contains two internal methods responsible for rendering client-side script registered in the methods discussed above: OnFormRender and OnFormPostRender. (A method marked internal can only be called by other classes in the same assembly. Therefore, you cannot call the Page's internal methods from the code-behind classes in your ASP.NET Web application.) Both of these methods are called in the HtmlForm class's RenderChildren method. The HtmlForm class, in the System.Web.UI.HtmlControls namespace, represents a Web Form; that is, the server-side form in an ASP.NET Web page—
<form runat="server">...</form>—is loaded as an instance of the HtmlForm class during the page's instantiation stage.
Since the client-side script registered by the Page class's assorted methods is rendered in the OnFormRender and OnFormPostRender methods, and since these methods are only called by the HtmlForm class, the client-side script you programmatically add by these methods is only rendered if the Web page contains a Web Form. That is, any script elements you programmatically add through any of the discussed methods above will only be emitted in the page's final markup if the ASP.NET Web page contains a server-side form (a
A Web Form on an ASP.NET Web page is rendered by first adding the starting <form> element. Following that, the Web Form's RenderChildren method is called, which contains three lines of code:
Page.OnFormRender(...) MyBase.RenderChildren(...) Page.OnFormPostRender(...)
The call to the Page class's OnFormRender method adds the following markup:
- Any hidden form fields added by calls to RegisterHiddenField.
- The base-64 encoded view state in a hidden form field named __VIEWSTATE.
- Any script blocks added by calls to RegisterClientScriptBlock.
The second line of code in the Web Form's RenderChildren method calls the base class's RenderChildren method, which renders the contents within the Web Form. After rendering all of the form's contents, a call is made to the Page class's OnFormPostRender method, which adds the following client-side content:
- Any Array objects added by the RegisterArrayDeclaration method.
- Any script blocks added by calls to RegisterStartupScript.
Finally, after the Web Form's RenderChildren method completes, the closing form tag (</form>) is rendered. Figure 2 illustrates this rendering process graphically.
Note Figure 2 assumes you are somewhat familiar with the ASP.NET page life cycle. If you are interested in learning more about the page life cycle, consider reading Understanding ASP.NET View State, focusing on the section titled, "The ASP.NET Page Life Cycle."
Figure 2. Page rendering in ASP.NET
Examining the Rendering Order of Script Blocks
Upon first glance at the register methods of the Page class, it might seem that the order with which the registered elements are rendered in the Web page correspond to the order with which they were added in the code. That is, imagine that you had the following two lines of code in your ASP.NET Web page's code-behind class:
You wouldn't be too surprised when you found that the page rendered the following client-side script blocks (assuming the value of someDataDriveValue was Sam):
The user visiting this page would see an alert box saying "Hello, Sam!"
Based on this test, you might then assume that it was always the case that the order the script blocks were emitted in the HTML page was the order they were specified in the server-side code. However, this would be an incorrect assumption, and one that could cause your pages to break. For example, imagine that the script blocks added above were emitted in the HTML page in the reverse order. Then you'd have:
This would display an alert box reading "Hello, !", since the variable name has not yet been assigned a value. Clearly, there are times when the order with which script blocks are emitted is very important.
The register methods of the Page class—RegisterClientScriptBlock, RegisterStartupScript, RegisterArrayDeclaration, and RegisterHiddenFields—all write the supplied script content to an internal HybridDictionary. A HybridDictionary is a data structure found in the System.Collections.Specialized namespace, and is designed for storing items in a dictionary where the number of items in the dictionary is not known. For a small collection of items, a ListDictionary is the most efficient data structure, but for larger dictionaries, a Hashtable is more efficient. A HybridDictionary splits the difference—it starts by storing items using a ListDictionary. Once the ListDictionary has its ninth item added, the HybridDictionary switches from using a ListDictionary to using a Hashtable.
While this approach is ideal for performance, it can wreck havoc if you are using several script blocks where the order of the script blocks is important. That's because while a ListDictionary maintains the order with which the elements were added, a Hashtable does not. So, if you add eight or fewer items to any one of the particular register methods, the items will be emitted in the order with which they were added. However, if you add a ninth item, the order that the script is emitted will be seemingly random.
Note The ListDictionary stores its elements using a linked list, while the Hashtable stores its elements in an array, where the contents are ordered by the hashed value of a sting key. A thorough discussion on linked lists and hashtables is far beyond the scope of this article. For more information, including an analysis of their performance, consider reading An Extensive Examination of Data Structures, specifically Part 2 and Part 4.
If you plan on having cases where there may be more than eight client-side elements added using a particular register method, and the order with which the elements appear matters, you might want to take a look at Peter Blum's free RegisterScripts library. RegisterScripts provides greater control over the order with which the client-side elements are emitted, and also provides the option to not have to manually add the <script> tags, which you have to add when including client-side scripts with the RegisterClientScriptBlock or RegisterStartupScript methods.
Executing Client-Side Code in Response to User Action
HTML elements have a variety of client-side events that you can tap into and have client-side code execute when the event fires. The required markup simply goes in the HTML element's tag as an attribute. For example, to display an alert box when a button is clicked you can do:
<input type="button" value="Click me to see an alert box!" onclick="alert('Here it is!');" />
To run client-side code when a client-side event transpires, you can add the appropriate attribute to an HTML element. For a Web control, you can programmatically add a client-side attribute using the Attributes collection. For example, imagine that you had a TextBox Web control that you wanted to be highlighted yellow whenever the rendered textbox gains focus. To accomplish this you'd want the TextBox Web control's rendered HTML to look something like the following:
<input type="text" onfocus="this.style.backgroundColor='yellow';" onblur="this.style.backgroundColor='white';" />
To achieve this markup, we can programmatically set the TextBox Web control's onfocus and onblur client-side attributes by the Attributes collection, like so:
TextBoxControl.Attributes("onfocus") = "this.style.backgroundColor='yellow';" TextBoxControl.Attributes("onblur") = "this.style.backgroundColor='white';"
This technique of tying client-side code with client-side events is commonly used to provide a rich, interactive user experience. Later on in this article we'll see how to employ this technique to display confirm dialog boxes based on a user's actions.
Implementing Common Client-Side Functionality
Now that we've looked at the ASP.NET methods involved in dynamically adding client-side script to a Web page, let's turn our attention to applying this knowledge. The remainder of this article focuses on common client-side tasks, such as displaying alert boxes, confirm boxes, popup windows, and so on. Specifically we'll create a class that contains a set of methods that can be used in an ASP.NET project to quickly and easily provide such functionality.
The Visual Basic .NET code we will be examining throughout the remainder of this article is available in this article's code download.
Displaying an Alert Box
In order to make it as easy as possible for a page developer to display an alert box, let's create a class called ClientSidePage that contains a method called DisplayAlert(message). This class will inherit the Page class. A page developer that wants to utilize these client-side helper methods, then, will need to have their code-behind class inherit this ClientSidePage class rather than the default Page class. The following code shows this ClientSidePage class with its first method, DisplayAlert.
To use this code in your ASP.NET Web application, you will need to add a new class to your ASP.NET application. In Visual Studio .NET, right click on the ASP.NET Web application project name in the Solution Explorer and choose to add a new class. Then, cut and paste the above code into the class. Next, in your ASP.NET Web pages where you want to utilize this code, you'll need to modify the code-behind class so that it inherits from the ClientSidePage class rather than from Page. The following code shows a sample code-behind class derived from ClientSidePage and that uses the DisplayAlert method.
Public Class WebForm1 Inherits ClientSidePage Private Sub Page_Load(ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles MyBase.Load DisplayAlert("Hello, World!") End Sub ... End Class
Note that the ClientSidePage class not only has the DisplayAlert method, which generates a full client-side <script> element, but also has a GetAlertScript method, which returns just the client-side script, sans the <script> tag. This second method can be used in cases where you want to display an alert based on some client-side event. For example, if you want to have an alert displayed anytime a particular text box received focus, you could add the following code to your server-side code-behind class:
TextBoxControlID.Attributes("onfocus") = GetAlertScript(message)
Setting Focus to a Form Field on Page Load
The client-side function CSP_focus accepts a string parameter, the ID of the form field to set to focus, and retrieves the HTML element from the DOM. The retrieved element's focus() function is then called. At the bottom of the Web page, after all of the form fields have been specified, we need to call the CSP_focus method passing in the ID of the form field where we want the focus set.
The following method, GiveFocus(Control), uses the RegisterClientScriptBlock and RegisterStartupScript methods to generate the needed client-side script.
To use the GiveFocus method from an ASP.NET Web page whose code-behind class inherits ClientSidePage, simply call GiveFocus in the Page_Load event handler and pass in the Web control that should have its focus set on page load. For example, to set the focus to the TextBox Web control TextBoxControl, use the following Page_Load event handler:
Private Sub Page_Load(ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles MyBase.Load GiveFocus(TextBoxControl) End Sub
Opening a Popup Window
While popup windows have gotten a bad rap on the Internet as a nefarious tool for advertisers, popup windows are used in many Web applications to a good end. For example, you might want a page that displays a list of database items in a DataGrid, with a link to edit each particular item. Rather than using the DataGrid's in-line editing capabilities, you might want to have a popup window opened when the user opts to edit a DataGrid, where the popup window contains a list of text boxes with the editable fields of the DataGrid. (One reason you might want to do this is because there may be a very large number of editable fields, but you only want to show the most pertinent fields in the DataGrid, thereby eliminating the possibility of using the DataGrid's built-in editing features.)
- The URL to load in the popup window.
- A string name for the popup window.
- The features for the popup window, such as its height and width, whether or not the window is resizable, and so on.
A thorough discussion of the window.open() function is beyond the scope of this article; to learn more refer to the technical documentation.
An example of this code in action can be seen in the code download for this article. There you'll find a sample Web page that has a DataGrid listing the files in the same directory as the ASP.NET Web page. This DataGrid has two columns: a TemplateColumn that displays a hyperlink that, when clicked, opens a popup window showing the contents of the selected file; and the name of the file (see Figure 3).
Figure 3. DataGrid with popup window
The DataGrid's markup utilizes the GetPopupScript method, as shown below:
The ASP.NET Web page ViewFile.aspx opens the file whose name is specified in the querystring, and displays its contents (see Figure 4).
Figure 4. Displaying the contents of Web.config in a popup window
Note Popup windows are best suited for intranet applications only, because a number of Internet users utilize some sort of popup blocking software, such as Google Toolbar. In fact, with the Microsoft Windows XP Service Pack 2, Microsoft Internet Explorer will be configured to block popups by default. However, popups will still work when a user visits a site in the Trusted Sites or Local Intranet zones. For more information on the popup blocking features for Internet Explorer in the Windows XP Service Pack 2, be sure to read Changes to Functionality in Microsoft Windows XP Service Pack 2.
Confirming Before Postback
Typically confirm dialog boxes are used to ensure that the user wants to continue before submitting a form. When an HTML element is clicked to submit a form (such as a submit button), if that HTML element fires a client-side event handler that returns false, the form submission is canceled. Commonly the confirm dialog box is used in a Web page as follows:
<form ...> <input type="submit" value="Click Me to Submit the Form" onclick="return confirm('Are you sure you want to submit this form?');" /> </form>
When the user clicks the "Click Me to Submit the Form" button, they'll see a confirm dialog box that asks them if they are sure they want to submit the form (see Figure 5). If the user clicks OK, confirm() will return true and the form will be submitted. If, however, they click Cancel, confirm() will return false and the form's submission will be canceled.
Imagine you had a DataGrid with a column of buttons labeled "Delete." Upon clicking this button, the form will postback and the selected record will be deleted. In this instance, you might want to double-check that the user really wanted to delete this record. Here would be a great place to use a client-side confirm dialog box. You could prompt the user with a dialog box stating something like: "This will permanently delete the record. Are you sure you want to continue?" If the user clicks OK, the form will postback and the record will be deleted; if they click Cancel, the form will not be posted back, and hence the record will not be deleted.
return confirm(message);. In order to provide such functionality for a DataGrid's ButtonColumn, you'll need to programmatically reference the Button or LinkButton control in either the DataGrid's ItemCreated or ItemDataBound event handlers and set the onclick attribute there. For more information, see http://aspnet.4guysfromrolla.com/articles/090402-1.aspx.
Confirmation with AutoPostBack DropDownLists
While confirm dialog boxes are typically used when a button is clicked, they can also be used when a drop-down list is changed. For example, you might have a Web page that automatically posts back when a particular DropDownList Web control is changed. (The DropDownList Web control has an AutoPostBack property that, if set to True, causes the form to postback whenever the DropDownList's selected item is changed.)
Intuitively you might think adding a confirm dialog box for a DropDownList is identical to adding such a dialog box for a Button Web control. That is, simply set the DropDownList's client-side onchange attribute to something like:
return confirm(...);. using:
DropDownListID.Attributes("onchange") = "return confirm(...);"
<select onchange="return confirm(...);__doPostBack(...);"> ... </select>
Noting this, what we really want to happen is have the __doPostBack function called if confirm returns true, because then the page will be posted back. We can accomplish this by setting the onchange event by the Attributes collection to:
if (confirm(...)), which will render the following markup, which is what we are after:
<select onchange="if (confirm(...)) __doPostBack(...);"> ... </select>
At first glance this will seem to have the desired effect. If a user selects a different item from the drop-down list, a confirm box appears. If the user clicks OK, the form will postback; if the user clicks Cancel, the form's postback is halted. The problem, though, is that the drop-down list retains the item the user selected to initiate the drop-down list's onchange event. For example, imagine the drop-down list loads with item x being selected, and then the user chooses item y. This will trigger the drop-down list's client-side onchange event, which will display the confirm dialog box. Now, imagine that the user hits Cancel—the drop-down list will still be selected on item y. What we want is to have the selection reverted back to item x.
To accomplish this we need to do two things:
- In the drop-down list's client-side onchange event, if the user clicks Cancel, you need to revert the drop-down list back to the "remembered" value.
Step 1 entails creating a global script variable for the drop-down list and a function that runs when the page loads that will record the drop-down list's value. Step 2 requires changing the client-side onchange attribute for the drop-down list to look like:
if (!confirm(...)) resetDDLIndex(); else __doPostBack();
This necessary script can be easily generated by creating a helper method in the ClientSidePage class.
To use this, simply call this method for each AutoPostBack DropDownList on the Web page that you want to display a confirm dialog box for when its selected item changes.
Confirming When Exiting Without Saving
In most every data-driven Web application I've created there's always been some page where users can edit particular bits of information from the database. A very simple example might be a page with a series of TextBox and DropDownList Web controls, with the database data populated within these controls. The user can make any suitable changes and click the Save button to persist their changes to the database.
When I create these pages, I usually end the page with two Button Web controls: a Save button and a Cancel button. The Save button saves any changes back to the database, while the Cancel button exits the page without persisting any changes. While two buttons may seem like a perfect design, sometimes users accidentally click the Cancel button when they meant to click the Save button, thereby losing any changes they made to the data. To prevent this from happening, you can use a confirm box on the Cancel button that only appears if any of the textboxes or drop-down lists on the Web page have been changed. That is, if the user makes any changes to the data and then clicks Cancel, a confirm box will prompt them to see if they are sure they want to exist without saving. (If the user just clicks Cancel without changing any data, no such confirm box is shown.)
This script can be easily generated by moving its generation to the ClientSidePage
class. Specifically, we can create the following three methods:
To create a Web page that exhibits this behavior, we simply need to have its server-side code-behind class derive from ClientSidePage and in the Page_Load event handler have a call to MonitorChanges for each Web control that needs a client-side onchange event and a call to ConfirmOnExit for each Button, LinkButton, and ImageButton that, when clicked, should display a warning if the user has made changes and is exiting the page.
Note Notice that the MonitorChanges method uses the onclick client-side event instead of onchange for the CheckBox, CheckBoxList, and RadioButtonList Web controls. This is because these controls wrap a <span> tag or <table> around the check box or series of check boxes or radio buttons. In my tests with Internet Explorer, I found that the onchange event, when applied to the <span> or <table>, was not picked up when a check box or radio button was clicked, but the onclick event was raised.
Figure 6 shows an example ASP.NET Web page with two TextBox Web controls, a DropDownList Web control, and a CheckBox Web control. As the Page_Load event handler below shows, all of these Web controls are being monitored for changes. The Cancel button, btnCancel, is configured so that if it's clicked after changes have been made, a confirm dialog box will be displayed.
Figure 6. Dialog with confirmation
Public Class ConfirmOnExit Inherits ClientSidePage Private Sub Page_Load(ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles MyBase.Load 'Specify what controls to check for changes MonitorChanges(name) MonitorChanges(age) MonitorChanges(favColor) MonitorChanges(chkSilly) ConfirmOnExit(btnCancel, _ "You have made changes to the data since last saving." & _ " If you continue, you will lose these changes.") End Sub ... End Class
Note The client-side onchange event does not work in older versions of Netscape. Also, Internet Explorer 5.0 has had some reported problems with the onchange event (which were fixed in Internet Explorer 5.01, SP 1).
Furthermore, this approach won't work as desired with DropDownList Web controls with AutoPostBack set to True, as the postback will reset the value of isDirty. There are a couple of workarounds for this problem, such as using a hidden form field that indicates whether or not the postedback form data is dirty to begin with or not. I leave implementing this as an exercise for the reader.
Creating a Client-Side MessageBox Control
In a past project, I had a need for a client-side, modal message box that would cause a postback no matter what button was clicked. In response, I built a custom compiled ASP.NET server control that met these requirements. In addition, the client-side message box uses VBScript's MsgBox function to provide a richer message box experience.
A thorough discussion of this custom server control could warrant an entire article in itself, so rather than focus on the inner workings of the control, let's examine how to use the MessageBox control in an ASP.NET Web page. (The complete source for the control, as well as a sample ASP.NET Web page using the control, is available in this article's download.)
To use the MessageBox control in an ASP.NET Web page, first add the MessageBox control to the Visual Studio .NET Toolbox. This can be accomplished by right-clicking on the Toolbox and choosing to Add/Remove Items from the Toolbox, and then browsing to the MessageBox assembly. To add the client-side message box to a Web page, simply drag it from the Toolbox onto the Designer. Figure 7 shows the MessageBox control in the Visual Studio .NET Designer.
Figure 7. Displaying a modal messagebox
The MessageBox class has a number of properties that you can configure to tweak the appearance of the message box:
.Specifies what buttons are displayed. The options are defined in the ButtonOptions enumeration and can be:
.The ID of the Button Web control that, when clicked, will display the client-side message box. Use this property if you want the message box displayed due to a specific button being clicked.
.The icon displayed in the message box; the options are defined in the IconOptions enumeration. The legal values are:
.The text displayed within the message box.
.The title of the message box.
Once you have added the MessageBox control to an ASP.NET Web page, the next challenge is having it displayed due to some client-side action. The DisplayWhenButtonClicked property allows you to specify the ID of a Button Web control on the page that, when clicked, will cause the message box to be displayed. Alternatively, you can have the message box displayed by calling the client-side function mb_show(id), where ID is the ID of the MessageBox control.
Regardless of what button configuration you choose to display in the message box, when any button is clicked, a postback ensues and the MessageBox control's Click event fires. You can create an event handler for this event by simply double-clicking on the MessageBox in the Designer. The event handler's second input parameter is of type MessageBoxClickedEventArgs, which contains a ButtonClicked property that returns information on what message-box button the user clicked.
The MessageBox control is useful in situations where you want to quickly present the user with a modal dialog box that, regardless of the user's choice, results in a postback. To see the MessageBox control in action, check out the MsgBoxDemo.aspx page in the source code download.
This article began with a look at common uses of client-side script in a Web page, and then turned to examine the methods and techniques for injecting client-side script into an ASP.NET Web page. As we saw, the Page class contains a number of methods designed for programmatically inserting client-side script blocks from the server-side code-behind class. These methods are also commonly used from custom, compiled server controls, as discussed in an earlier article of mine: Injecting Client-Side Script from an ASP.NET Server Control.
In addition to adding script blocks, client-side functionality often must be tied to a client-side event raised by some HTML element. To programmatically specify a Web control's client-side event handler through the server-side code-behind class, use the Attributes collection, which is available as a property to all Web controls.
The second half of this article applied the topics covered in the first half, showing how to implement common client-side functionality in an ASP.NET Web page. We saw how to extend the Page class so that from a code-behind class we could easily display an alert box, set the focus upon page load to a specific Web control, how to display a popup, and how to display a confirm dialog box. We also looked at creating a custom server control that used VBScript to provide a richer client-side message box user experience that caused a postback regardless of the button clicked.
Special Thanks To...
Before submitting my article to my MSDN editor, I have a handful of volunteers help proofread the article and provide feedback on the article's content, grammar, and direction. Primary contributors to the review process for this article include Maxim Karpov, Carlos Santos, Milan Negovan, and Carl Lambrecht. If you are interested in joining the ever-growing list of reviewers, drop me a line at email@example.com.
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About the author
Scott Mitchell, author of five books and founder of 4GuysFromRolla.com, has been working with Microsoft Web technologies for the past five years. Scott works as an independent consultant, trainer, and writer. He can be reached at firstname.lastname@example.org or through his blog, which can be found at http://ScottOnWriting.NET.