aspnet05-01.htm

tutorialssss.... for everybody!!!!!!!!!!!!!!!!!!!

<p>Script and control coding for this output is shown below. Notice that the 
buttons have <span class="code">CommandName</span> values indicating the <span class="code">BookType</span> field from the database to be used for selecting 
subsets of records.</p><pre class="divcode"><span class="script">&lt;SCRIPT Runat="Server"&gt;

Sub Show_Books (Src As Object, Args As CommandEventArgs)
  
  Dim SQLString As String
  SQLString = "SELECT BookID, BookTitle, BookPrice FROM Books " &amp; _
              "WHERE BookType = '" &amp; <b>Args.CommandName</b> &amp; "'"
  BookSource.SelectCommand = SQLString

End Sub

&lt;/SCRIPT&gt;
</span>
&lt;form Runat="Server"&gt;

&lt;h3&gt;View Book Types&lt;/h3&gt;

&lt;asp:Button Runat="Server"
  Text="Database"
  OnCommand="Show_Books"
  CommandName=<b>"Database"</b>
  Width="80"/&gt;

&lt;asp:Button Runat="Server"
  Text="Graphics"
  OnCommand="Show_Books"
  CommandName=<b>"Graphics"</b>
  Width="80"/&gt;

&lt;asp:Button Runat="Server"
  Text="Hardware"
  OnCommand="Show_Books"
  CommandName=<b>"Hardware"</b>
  Width="80"/&gt;&lt;br/&gt;

&lt;asp:Button Runat="Server"
  Text="Software"
  OnCommand="Show_Books"
  CommandName=<b>"Software"</b>
  Width="80"/&gt;

&lt;asp:Button Runat="Server"
  Text="Systems"
  OnCommand="Show_Books"
  CommandName=<b>"Systems"</b>
  Width="80"/&gt;

&lt;asp:Button Runat="Server"
  Text="Web"
  OnCommand="Show_Books"
  CommandName=<b>"Web"</b>
  Width="80"/&gt;

&lt;asp:AccessDataSource id="BookSource" Runat="Server"
  DataFile="../Databases/BooksDB.mdb"
  SelectCommand="SELECT BookID, BookTitle, BookPrice FROM Books
                 WHERE BookType = 'Database'"/&gt;

&lt;p&gt;&lt;asp:GridView DataSourceID="BookSource" Runat="Server"/&gt;&lt;/p&gt;

&lt;/form&gt;
</pre>
<div class="listing"><b>Listing 5-9.</b> Code to issue SQL command depending on 
CommandName of clicked button.</div>
<p>The AccessDataSource is originally coded with a <span class="code">SelectCommand</span> to retrieve a subset of records for initial 
GridView display. Subsequently, the GridView displays other subsets of records 
corresponding with button clicks.</p>
<p><span class="code">BookType</span> values are received by subprogram <span class="code">Show_Books</span> through the <span class="code">CommandName</span> 
property of its <span class="code">CommandEventArgs</span> argument (<span class="code">Args.CommandName</span>). These command names, which are also the 
<span class="code">BookType</span> values appearing in the database, are 
concatenated within a <span class="code">SELECT</span> statement to select records 
with these <span class="code">BookType</span> values. For instance, if the first 
button is clicked (<span class="code">CommandName="Database"</span>), the 
following <span class="code">SQLString </span>value is produced.</p><pre class="divcode"><span class="script">SQLString = "SELECT BookID, BookTitle, BookPrice FROM Books " &amp; _
            "WHERE BookType = '" &amp; <b>Args.CommandName</b> &amp; "'"
</span>
-- becomes --

SELECT BookID, BookTitle, BookPrice FROM Books WHERE BookType = 'Database'
</pre>
<div class="listing"><b>Listing 5-10.</b> SQL statement produced by clicking a 
button.</div>
<p>When this <span class="code">SQLString</span> variable is script-assigned to 
the AccessDataSource's <span class="code">SelectCommand</span> property, the data 
source is immediately bound to the GridView to display these selected 
books.</p><a name="POST">
<p class="head2">Posting to a Different Page</p></a>
<p>Normally, a button click posts back to the same page on which the button 
appears. This is the default behavior of the button's <span class="code">OnClick</span> event handler. You can, instead, post to a different 
page and still be able to access controls on the original page. Posting to a 
different page takes place through the button's <span class="code">PostBackUrl</span> property. This property gives the URL of the page 
to which to post.</p>
<p>In the following example, a page contains two controls, a Button and a Label. 
The button includes the <span class="code">PostBackUrl</span> property to post to 
page <span class="code">PostPage.aspx</span> located in the same directory. When 
<span class="code">PostPage.aspx</span> opens, it accesses the button <span class="code">id</span> and label <span class="code">Text</span> from the previous 
(posting) page.</p>
<div class="page">
<h4>Using PostBackUrl</h4>Button: <input id="PostingButton" onclick='javascript:WebForm_DoPostBackWithOptions(new WebForm_PostBackOptions("PostingButton", "", false, "", "PostPage.aspx", false, false))' value="PostPage.aspx" name="PostingButton" type="submit"><br><br>Label:&nbsp;&nbsp;<span id="PostedValue" style="border-style: ridge; padding-right: 10px; padding-left: 10px; padding-top: 3px;">Text 
value to post</span> <br><br></div>
<div class="figure"><b>Figure 5-7.</b> Posting to a different page.</div>
<p>Code for the posting page is shown below with a Button containing the <span class="code">PostBackUrl</span> property and a Label containing a text string for 
posting to <span class="code">PostPage.aspx</span>.</p><pre class="divcode">&lt;h4&gt;Using PostBackUrl&lt;/h4&gt;

Button: &lt;asp:Button id="PostingButton" Text="PostPage.aspx" Runat="Server"
         <b>PostBackUrl="PostPage.aspx"</b>/&gt;&lt;br/&gt;&lt;br/&gt;

Label:&nbsp;&nbsp;&lt;asp:Label id="PostedValue" Text="Text value to post" Runat="Server"
         BorderStyle="Ridge" Style="padding-left:10px; padding-right:10px"/&gt;
</pre>
<div class="listing"><b>Listing 5-11.</b> Code to post to a different page.</div>
<p>The <span class="code">PostBackUrl</span> page, <span class="code">PostPage.aspx</span>, includes a script to <i>locate</i> the Button 
and Label controls on the posting page and to report their <span class="code">id</span> and <span class="code">Text</span> values, respectively.</p><pre class="divcode"><span class="script">&lt;SCRIPT Runat="Server"&gt;

Sub Page_Load
  
  Dim FoundButton As Button
  FoundButton = PreviousPage.FindControl("PostingButton")
  PostButton.Text = FoundButton.id
  
  Dim FoundLabel As Label
  FoundLabel = PreviousPage.FindControl("PostedValue")
  PostValue.Text = FoundLabel.Text

End Sub

&lt;/SCRIPT&gt;
</span>
&lt;form Runat="Server"&gt;

&lt;h3&gt;This is PostPage.aspx&lt;/h3&gt;

&lt;p&gt;The button with the id 
"&lt;asp:Label id="PostButton" Font-Bold="True" Runat="Server"/&gt;"
posted the text value 
"&lt;asp:Label id="PostValue" Font-Bold="True" Runat="Server"/&gt;"
from the previous page.

&lt;p&gt;&lt;a href="PostingPage.aspx"&gt;Return&lt;/a&gt;&lt;/p&gt;

&lt;/form&gt;
</pre>
<div class="listing"><b>Listing 5-12.</b> Code to retrieve and displays values 
from posting page.</div>
<p class="head3">The FindControl() Method</p>
<p>The page to which a posting is made through the <span class="code">PostBackUrl</span> property of a button does <i>not</i> have direct 
access to controls on the posting page. That is, a script on <span class="code">PostPage.aspx</span> cannot directly reference the Button or the 
Label on the posting page. It cannot, as can be done on a page posted back to 
itself, use <span class="code">PostingButton.id</span> or <span class="code">PostedValue.Text</span> to directly retrieve the <span class="code">id</span> and <span class="code">Text</span> properties of these 
controls on the posting page.</p>
<p>The technique that must be used involves first <i>finding</i> the controls of 
interest on the previous page and then referencing properties of these 
<i>found</i> controls. The posting page is identified through the <span class="code"><b>PreviousPage</b></span> property; controls on this page are 
located with its <span class="code"><b>FindControl()</b></span> method. The <span class="code">FindControl()</span> method of <span class="code">PreviousPage</span> 
has the general formats shown in Figure 5-8.</p>
<table class="format" cellpadding="10">
  <tbody>
  <tr>
    <td class="code"><pre><b>Dim</b> <i>FoundControl</i> <b>As</b> <i>ControlType</i>
<i>FoundControl</i> = <b>PreviousPage</b>.<b>FindControl</b>("<i>id</i>")

or

<b>Dim</b> <i>FoundControl</i> <b>As</b> <i>ControlType</i> = <b>PreviousPage</b>.<b>FindControl</b>("<i>id</i>")
</pre></td></tr></tbody></table>
<div class="figure"><b>Figure 5-8.</b> General format to locate a control on a 
posting page.</div>
<p>First, an object of the type to be located is declared. When locating a 
Button, for instance, the object is declared as type <span class="code">Button</span>; when locating a Label, the object is declared as type 
<span class="code">Label</span>. Any ASP.NET control type can be declared to match 
the control type to be located.</p>
<p>Second, this declared object is assigned the control of interest from the 
<span class="code">PreviousPage</span> using the <span class="code">FindControl("<i>id</i>")</span> method to identify the control 
through its <span class="code">id</span>. Once "found" and assigned to the locally 
declared object, properties of a control appearing on the posting page can be 
referenced through this local object. This technique is used in the <span class="code">Page_Load </span>subprogram on the <span class="code">PostPage.aspx</span> page.</p><pre class="divscript">Sub Page_Load
  
  Dim FoundButton As Button
  FoundButton = PreviousPage.FindControl("PostingButton")
  PostButton.Text = FoundButton.id
  
  Dim FoundLabel As Label
  FoundLabel = PreviousPage.FindControl("PostedValue")
  PostValue.Text = FoundLabel.Text

End Sub
</pre>
<div class="listing"><b>Listing 5-13.</b> Script to locate and report control 
properties from posting page.</div>
<p>Object reference <span class="code">FoundButton</span> (any programmer-supplied 
name can be used) is declared as a <span class="code">Button</span> object. It is 
then assigned the Button with <span class="code">id="PostingButton"</span> located 
on the <span class="code">PreviousPage</span> (posting page). Likewise for the 
Label. Object reference <span class="code">FoundLabel </span>is declared as a 
<span class="code">Label</span> object. The Label with <span class="code">id="PostedValue"</span> on the <span class="code">PreviousPage</span> 
is assigned to this script object. Thereafter, properties associated with these 
controls appearing on the posting page are accessible through their surrogate 
references on the current page. In this example, <span class="code">FoundButton.id</span> accesses the <span class="code">id</span> 
property of the Button on the posting page; <span class="code">FoundLabel.Text</span> accesses the <span class="code">Text</span> 
property of the Label on the posting page. These two values are displayed in 
Labels appearing on the posted page.</p>
<p>You need to become familiar with the <span class="code">FindControl()</span> 
method. It has many uses under ASP.NET besides locating controls on pages which 
issue <span class="code">PostBackUrls</span>. It is routinely used to locate 
controls embedded inside other controls on the same page.</p><br></div>
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