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<HTML><HEAD><TITLE>Recipe 19.12. Writing a Multiscreen CGI Script (Perl Cookbook)</TITLE><METANAME="DC.title"CONTENT="Perl Cookbook"><METANAME="DC.creator"CONTENT="Tom Christiansen & Nathan Torkington"><METANAME="DC.publisher"CONTENT="O'Reilly & Associates, Inc."><METANAME="DC.date"CONTENT="1999-07-02T01:45:44Z"><METANAME="DC.type"CONTENT="Text.Monograph"><METANAME="DC.format"CONTENT="text/html"SCHEME="MIME"><METANAME="DC.source"CONTENT="1-56592-243-3"SCHEME="ISBN"><METANAME="DC.language"CONTENT="en-US"><METANAME="generator"CONTENT="Jade 1.1/O'Reilly DocBook 3.0 to HTML 4.0"><LINKREV="made"HREF="mailto:online-books@oreilly.com"TITLE="Online Books Comments"><LINKREL="up"HREF="ch19_01.htm"TITLE="19. CGI Programming"><LINKREL="prev"HREF="ch19_12.htm"TITLE="19.11. Creating Sticky Widgets"><LINKREL="next"HREF="ch19_14.htm"TITLE="19.13. Saving a Form to a File or Mail Pipe"></HEAD><BODYBGCOLOR="#FFFFFF"><img alt="Book Home" border="0" src="gifs/smbanner.gif" usemap="#banner-map" /><map name="banner-map"><area shape="rect" coords="1,-2,616,66" href="index.htm" alt="Perl Cookbook"><area shape="rect" coords="629,-11,726,25" href="jobjects/fsearch.htm" alt="Search this book" /></map><div class="navbar"><p><TABLEWIDTH="684"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch19_12.htm"TITLE="19.11. Creating Sticky Widgets"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 19.11. Creating Sticky Widgets"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch19_01.htm"TITLE="19. CGI Programming"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch19_14.htm"TITLE="19.13. Saving a Form to a File or Mail Pipe"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 19.13. Saving a Form to a File or Mail Pipe"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch19-36190">19.12. Writing a Multiscreen CGI Script</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch19-pgfId-1328">Problem<ACLASS="indexterm"NAME="ch19-idx-1000005503-0"></A><ACLASS="indexterm"NAME="ch19-idx-1000005503-1"></A></A></H3><PCLASS="para">You want to write a single CGI script that can return several different pages to the browser. For instance, you want a single CGI script for administering a database of products. The script will be called to display the form to add a product, to process the add-product form, to display a list of products to delete, to process the delete-product form, to display a list of product to edit, to display a form of the product's attributes for the user to change, and to process the edit-product form. You can use these multiscreen CGI scripts to form an elementary <ACLASS="indexterm"NAME="ch19-idx-1000007382-0"></A>shopping-cart-type application.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch19-pgfId-1334">Solution</A></H3><PCLASS="para">Use a hidden field to encode the current screen.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch19-pgfId-1340">Discussion</A></H3><PCLASS="para">It is easy to generate sticky hidden fields with the CGI module. The <CODECLASS="literal">hidden</CODE><ACLASS="indexterm"NAME="ch19-idx-1000005509-0"></A> function returns HTML for a hidden widget and will use the widget's current value if you only give <CODECLASS="literal">hidden</CODE> the widget name:</P><PRECLASS="programlisting">use CGI qw(:standard);print hidden("bacon");</PRE><PCLASS="para">To determine which page ("display product list", "display all items in shopping cart", "confirm order") to display, use another hidden field. We'll call this one <CODECLASS="literal">.State</CODE> so it won't conflict with any field we might have called <CODECLASS="literal">State</CODE> (for instance, in credit card billing information). To let the user move from page to page, use submit buttons that set <CODECLASS="literal">.State</CODE> to the name of the page to go to. For instance, to make a button to take the user to the "Checkout" page, use:</P><PRECLASS="programlisting"> print submit(-NAME => ".State", -VALUE => "Checkout");</PRE><PCLASS="para">We wrap this in a function to make it easier to type:</P><PRECLASS="programlisting">sub to_page { return submit( -NAME => ".State", -VALUE => shift ) }</PRE><PCLASS="para">To decide what code to display, check the <CODECLASS="literal">.State</CODE> parameter:</P><PRECLASS="programlisting">$page = param(".State") || "Default";</PRE><PCLASS="para">Put the code to generate each page in separate subroutines. You could decide which subroutine to call with a long <CODECLASS="literal">if</CODE> ... <CODECLASS="literal">elsif</CODE> ... <CODECLASS="literal">elsif</CODE>:</P><PRECLASS="programlisting">if ($page eq "Default") { front_page();} elsif ($page eq "Checkout") { checkout();} else { no_such_page(); # when we get a .State that doesn't exist}</PRE><PCLASS="para">This is tedious and clumsy. Instead use a hash that maps a page name to a subroutine. This is another strategy for implementing a C-style <CODECLASS="literal">switch</CODE><ACLASS="indexterm"NAME="ch19-idx-1000005595-0"></A> statement in Perl.</P><PRECLASS="programlisting">%States = ( 'Default' => \&front_page, 'Shirt' => \&shirt, 'Sweater' => \&sweater, 'Checkout' => \&checkout, 'Card' => \&credit_card, 'Order' => \&order, 'Cancel' => \&front_page,);if ($States{$page}) { $States{$page}->(); # call the correct subroutine } else { no_such_page();}</PRE><PCLASS="para">Each page will have some persistent widgets. For instance, the page that lets the user order t-shirts will want the number of t-shirts to persist even when the user continues and orders shoes as well. We do this by calling the page-generating subroutines with a parameter that lets them know whether they're the active page. If they're not the active page, they should only send back hidden fields for any persistent data:</P><PRECLASS="programlisting">while (($state, $sub) = each %States) { $sub->( $page eq $state );}</PRE><PCLASS="para">The <CODECLASS="literal">eq</CODE> comparison returns true if the page is the current page, and false if it isn't. The page-generating subroutine then looks like this:</P><PRECLASS="programlisting">sub t_shirt { my $active = shift; unless ($active) { print hidden("size"), hidden("color"); return; } print p("You want to buy a t-shirt?"); print p("Size: ", popup_menu('size', [ qw(XL L M S XS) ])); print p("Color:", popup_menu('color', [ qw(Black White) ])); print p( to_page("Shoes"), to_page("Checkout") );}</PRE><PCLASS="para">Because the subroutines all generate HTML, we have to print the HTTP header and start the HTML document and form before we call the subroutines. This lets us print a standard header and footer for all the pages, if we want. Here, we assume we have subroutines <CODECLASS="literal">standard_header</CODE> and <CODECLASS="literal">standard_footer</CODE> for printing the headers and footers:</P><PRECLASS="programlisting">print header("Program Title"), start_html();print standard_header(), begin_form();while (($state, $sub) = each %States) { $sub->( $page eq $state );}print standard_footer(), end_form(), end_html();</PRE><PCLASS="para">Don't make the mistake of encoding prices in the forms. Calculate prices based on the values of the hidden widgets, and sanity-check the information where you can. For example, compare against known products, to make sure they're not trying to order a burgundy XXXXXXL t-shirt.</P><PCLASS="para">Using hidden data is more robust than using cookies, because you can't rely on the browser supporting or accepting cookies. A full explanation is in <ACLASS="xref"HREF="ch19_11.htm"TITLE="Managing Cookies">Recipe 19.10</A>.</P><PCLASS="para">We show a simple shopping cart application as the program <EMCLASS="emphasis">chemiserie</EM> at the end of this chapter.<ACLASS="indexterm"NAME="ch19-idx-1000005505-0"></A><ACLASS="indexterm"NAME="ch19-idx-1000005505-1"></A></P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch19-pgfId-1468">See Also</A></H3><PCLASS="para">The documentation for the standard CGI module.</P></DIV></DIV><DIVCLASS="htmlnav"><P></P><HRALIGN="LEFT"WIDTH="684"TITLE="footer"><TABLEWIDTH="684"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch19_12.htm"TITLE="19.11. Creating Sticky Widgets"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 19.11. 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