ch06_04.htm

用perl编写CGI的好书。本书从解释CGI和底层HTTP协议如何工作开始

variables. They can also contain Perl control structures. Instead of
using Embperl's meta-commands as control structures, we could
have used Perl's <tt class="function">foreach</tt> loop instead of
Embperl's to create the table in our previous example:</p>


<blockquote><pre class="code">[# Output a row for each environment variable #]
  [* foreach $varname ( sort keys %ENV ) { *]
    &lt;TR&gt;
      &lt;TD&gt;&lt;B&gt;[+ $varname +]&lt;/B&gt;&lt;/TD&gt;
      &lt;TD&gt;[+ $ENV{$varname} +]&lt;/TD&gt;
    &lt;/TR&gt;
  [* } *]</pre></blockquote>


<p>The difference is brackets versus meta-command blocks. Note that code
within <tt class="literal">[*</tt> and <tt class="literal">*]</tt> must end with
a semicolon or a curly bracket, and these blocks are evaluated even
inside Embperl comment blocks (see below). These bracket pairs were
introduced in Embperl 1.2.</p></dd>


<dt><b><tt class="literal">[#</tt>  ... <tt class="literal">#]</tt></b></dt>
<dd><p>These brackets are used for
<a name="INDEX-1433" />comments. Embperl ignores
and strips anything between these bracket pairs so the contents do
not end up in the output sent to the client. These can also be used
to remove large sections of HTML or code during testing, but
unfortunately this does not work for code within <tt class="literal">[* ...
*]</tt>, since these blocks are evaluated first. These bracket
pairs were introduced in Embperl 1.2.</p></dd>

</dl>


<p>Because blocks begin with <tt class="literal">[</tt> in Embperl, you must
use <tt class="literal">[[</tt> if you need to output the [ character in
your HTML. There is no need to escape <tt class="literal">]</tt> or
<a name="INDEX-1434" /> <a name="INDEX-1,435" />other characters. Also, Embperl ties STDOUT
to its output stream so you can use <tt class="function">print</tt> within
Embperl blocks and it will behave correctly.</p>
</div>



<a name="ch06-73315" /><div class="sect3">
<h3 class="sect3">6.4.2.2. Variable scope</h3>


<p>Each block of code within a set of bracket pairs is evaluated as a
separate block within Perl. This means that each one has a separate
<a name="INDEX-1436" />
<a name="INDEX-1437" />variable
scope. If you declare a <a name="INDEX-1438" />lexical variable (a variable declared
with <tt class="function">my</tt>) in one block, it will not be visible in
another block. In other words, this will not work:</p>


<blockquote><pre class="code">[- my $time = localtime -]
&lt;P&gt;The time is: [+ $time +].&lt;/P&gt;</pre></blockquote>


<p>The result is roughly analogous to the following in Perl:</p>


<blockquote><pre class="code">&amp;{sub { my $time = localtime }};
print "&lt;P&gt;The time is: " . &amp;{sub { $time }} . ".&lt;/P&gt;";</pre></blockquote>


<p>Similarly, <a name="INDEX-1439" />
<a name="INDEX-1440" />pragmas that depend on scope such
as <tt class="literal">use strict</tt> will only affect the current block
of code. To enable the strict pragma globally, you must use the
<tt class="literal">var</tt> meta-command (see <a href="ch06_04.htm#ch06-63926">Table 6-6</a>).</p>


<p>The <tt class="literal">[* ... *]</tt> blocks are a little different. They
all share a common scope so local variables (variables declared with
<tt class="function">local</tt>) can be shared between them. However,
lexical variables still can not. This does not mean that you should
entirely abandon declaring your variables with
<tt class="function">my</tt> in Embperl.</p>


<p>Lexical variables are still useful as <a name="INDEX-1441" />temporary variables that you only need
within a particular block. Using lexical variables for temporary
variables is more efficient than using <a name="INDEX-1442" /> <a name="INDEX-1,443" />global
variables because they are reclaimed by Perl as soon as the
surrounding block ends. Otherwise, they persist until the end of the
HTTP request. Under CGI, of course, all global variables are cleaned
up at the end of the request because <tt class="command">perl</tt> exits.
However, even when running under <em class="emphasis">mod_perl</em>, by
default Embperl undefines all global variables created within the
scope of your pages at the end of each HTTP request.</p>
</div>



<a name="ch06-63352" /><div class="sect3">
<h3 class="sect3">6.4.2.3. Meta-commands</h3>


<p>
<a name="INDEX-1444" />
<a name="INDEX-1445" />
<a name="INDEX-1446" />Embperl offers several meta-commands for
creating control structures plus other miscellaneous functions shown
in <a href="ch06_04.htm#ch06-63926">Table 6-6</a>. The parentheses shown with some of
the control structures are optional in Embperl, but including them
can make these commands clearer and look more like Perl's
corresponding control structures.</p>



<a name="ch06-63926" /><h4 class="objtitle">Table 6-6. Embperl's Meta-commands </h4><table border="1">



<tr>
<th>
<p>Meta-command</p></th>
<th>
<p>Description</p></th>
</tr>




<tr>
<td>
<blockquote><pre class="code">[$ foreach $loop_var ( list ) $]</pre></blockquote></td>
<td>
<p>Similar to Perl's <tt class="function">foreach</tt> control
structure, except <em class="emphasis">$loop_var</em> is required.</p></td>
</tr>

<tr>
<td>
<blockquote><pre class="code">[$ endforeach $]</pre></blockquote></td>
<td>
<p>Indicates the end of a <em class="emphasis">foreach</em> loop.</p></td>
</tr>

<tr>
<td>
<blockquote><pre class="code">[$ while ( expr ) $]</pre></blockquote></td>
<td>
<p>Similar to Perl's <tt class="function">while</tt> control structure.</p></td>
</tr>

<tr>
<td>
<blockquote><pre class="code">[$ endwhile $]</pre></blockquote></td>
<td>
<p>Indicates the end of a <em class="emphasis">while</em> loop.</p></td>
</tr>

<tr>
<td>
<blockquote><pre class="code">[$ do $]</pre></blockquote></td>
<td>
<p>Indicates the beginning of an <em class="emphasis">until</em> loop.</p></td>
</tr>

<tr>
<td>
<blockquote><pre class="code">[$ until ( expr ) $]</pre></blockquote></td>
<td>
<p>Similar to Perl's <tt class="function">until</tt> control structure.</p></td>
</tr>

<tr>
<td>
<blockquote><pre class="code">[$ if ( expr ) $]</pre></blockquote></td>
<td>
<p>Similar to Perl's <tt class="function">if</tt> control structure.</p></td>
</tr>

<tr>
<td>
<blockquote><pre class="code">[$ elsif ( expr ) $]</pre></blockquote></td>
<td>
<p>Similar to Perl's <tt class="function">elsif</tt> control structure.</p></td>
</tr>

<tr>
<td>
<blockquote><pre class="code">[$ else $]</pre></blockquote></td>
<td>
<p>Similar to Perl's <tt class="function">else</tt> control structure.</p></td>
</tr>

<tr>
<td>
<blockquote><pre class="code">[$ endif $]</pre></blockquote></td>
<td>
<p>Indicates the end of an <em class="emphasis">if</em> conditional.</p></td>
</tr>

<tr>
<td>
<blockquote><pre class="code">[$ sub subname $]</pre></blockquote></td>
<td>
<p>This allows you to treat a section containing both HTML and Embperl
blocks as a subroutine that can be called as a normal Perl subroutine
or via Embperl's <tt class="function">Execute</tt> function.</p></td>
</tr>

<tr>
<td>
<blockquote><pre class="code">[$ endsub $]</pre></blockquote></td>
<td>
<p>Indicates the end of a <em class="emphasis">sub</em> body.</p></td>
</tr>

<tr>
<td>
<blockquote><pre class="code">[$ var $var1 @var2 %var3 ... $]</pre></blockquote></td>
<td>
<p>This command is equivalent to the following in a Perl script:</p>


<blockquote><pre class="code">use strict;
use vars qw( $var1 @var2 %var3 ... );</pre></blockquote>


<p>Your pages will be more efficient if you use this, especially when
running with <em class="emphasis">mod_perl</em>. Remember, however, that
if you do, you must declare every variable here that is shared
between Embperl blocks because of the scope restriction (see <a href="ch06_04.htm#ch06-73315">Section 6.4.2.2, "Variable scope"</a>, earlier).</p></td>
</tr>

<tr>
<td>
<blockquote><pre class="code">[$ hidden [ %input %used ] $]</pre></blockquote></td>
<td>
<p>This generates hidden fields for all elements in the first hash that
are not in the second hash. Both hashes are optional, and one
typically uses Embperl's default, which are
<tt class="literal">%fdat</tt> and <tt class="literal">%idat</tt>.
<tt class="literal">%fdat</tt> contains the name and values of the fields
the user submitted, and <tt class="literal">%idat</tt> contains the names
and values of the fields that have been used as elements in the
current form (see <a href="ch06_04.htm#ch06-15817">Section 6.4.4, "Global Variables"</a>, later).</p></td>
</tr>


</table>
</div>
</div>




<a name="ch06-16-fm2xml" /><div class="sect2">
<h3 class="sect2">6.4.3. HTML Logic</h3>


<p>
<a name="INDEX-1447" />
<a name="INDEX-1448" />Embperl monitors and responds to HTML as it
is output. You can have it construct
<a name="INDEX-1449" />tables and prefill
form elements for you automatically.</p>


<a name="ch06-17-fm2xml" /><div class="sect3">
<h3 class="sect3">6.4.3.1. Tables</h3>


<p>If you use the <tt class="literal">$row</tt>, <tt class="literal">$col</tt>, or
<tt class="literal">$cnt</tt> variables in code within a table, Embperl
will loop over the contents of the table, dynamically build the table
for you, and set these variables to the current row index, the
current column index, and the number of cells output, respectively,
with each iteration. Embperl interprets the variables as follows:</p>


<ul><li><p>If <tt class="literal">$row</tt> is present, everything between
&lt;TABLE&gt; and &lt;/TABLE&gt; is repeated until the expression
containing <tt class="literal">$row</tt> is undefined.
<a name="INDEX-1450" />Rows consisting
entirely of &lt;TH&gt; ... &lt;/TH&gt; cells are considered headers
and are not repeated.</p></li><li><p>If <tt class="literal">$col</tt> is present, everything between &lt;TR&gt;
and &lt;/TR&gt; is repeated until the expression containing
<tt class="literal">$col</tt> is undefined.</p></li><li><p><tt class="literal">$cnt</tt> is used in the same manner for either rows or
columns if it is present and <tt class="literal">$row</tt> or
<tt class="literal">$col</tt> are not.</p></li></ul>
<p>Let's look at an example. Because <tt class="literal">$row</tt> and
<tt class="literal">$col</tt> are set to the index of the current row and
column, they are typically used as
<a name="INDEX-1451" />
<a name="INDEX-1452" />array indices when
building tables, as shown here:</p>


<blockquote><pre class="code">[- @sports = ( [ "Windsurfing", "Summer",   "Water"    ],
               [ "Skiing",      "Winter",   "Mountain" ],
               [ "Biking",      "All Year", "Hills"    ],
               [ "Camping",     "All Year", "Desert"   ] ); -]
&lt;TABLE&gt;
  &lt;TR&gt;
    &lt;TH&gt;Sport&lt;/TH&gt;
    &lt;TH&gt;Season&lt;/TH&gt;
    &lt;TH&gt;Terrain&lt;/TH&gt;
  &lt;/TR&gt;
  &lt;TR&gt;
    &lt;TD&gt;[+ $sports[$row][$col] +]&lt;/TD&gt;
  &lt;/TR&gt;
&lt;/TABLE&gt;</pre></blockquote>


<p>The previous code will create the following table:</p>


<blockquote><pre class="code">&lt;TABLE&gt;
  &lt;TR&gt;
    &lt;TH&gt;Sport&lt;/TH&gt;
    &lt;TH&gt;Season&lt;/TH&gt;
    &lt;TH&gt;Terrain&lt;/TH&gt;
  &lt;/TR&gt;
  &lt;TR&gt;
    &lt;TD&gt;Windsurfing&lt;/TD&gt;
    &lt;TD&gt;Summer&lt;/TD&gt;
    &lt;TD&gt;Water&lt;/TD&gt;
  &lt;/TR&gt;
  &lt;TR&gt;
    &lt;TD&gt;Skiing&lt;/TD&gt;
    &lt;TD&gt;Winter&lt;/TD&gt;
    &lt;TD&gt;Mountain&lt;/TD&gt;
  &lt;/TR&gt;
  &lt;TR&gt;
    &lt;TD&gt;Biking&lt;/TD&gt;
    &lt;TD&gt;All Year&lt;/TD&gt;
    &lt;TD&gt;Hills&lt;/TD&gt;
  &lt;/TR&gt;
  &lt;TR&gt;
    &lt;TD&gt;Camping&lt;/TD&gt;
    &lt;TD&gt;All Year&lt;/TD&gt;
    &lt;TD&gt;Desert&lt;/TD&gt;
  &lt;/TR&gt;
&lt;/TABLE&gt;</pre></blockquote>
</div>



<a name="ch06-18-fm2xml" /><div class="sect3">
<h3 class="sect3">6.4.3.2. List elements</h3>


<p>If you use <tt class="literal">$row</tt> within a
<a name="INDEX-1453" />
<a name="INDEX-1454" />list or select menu, Embperl will
repeat each element until <tt class="literal">$row</tt> is undefined, just
as it does with tables. For select menus, Embperl will also
automatically check options that match name and value pairs in
<tt class="literal">%fdat</tt> and add names and values to
<tt class="literal">%idat</tt> (see below).</p>
</div>



<a name="ch06-19-fm2xml" /><div class="sect3">
<h3 class="sect3">6.4.3.3. Form input elements</h3>


<p>Outputting <a name="INDEX-1455" />
<a name="INDEX-1456" /> <a name="INDEX-1,457" />input and
<a name="INDEX-1458" />