Web Client Programming with Perl-Chapter 7: Graphical Examples with Perl/Tk- P2

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Web Client Programming with Perl-Chapter 7: Graphical Examples with Perl/Tk- P2 Chapter 7: Graphical Examples with Perl/Tk- P2The do_search( ) function will take an optional $url argument, to give it analternative place to connect to. Otherwise it expects $word to containsomething. We just hit Return from the entry widget, so $word contains thestring example, and $url is undefined. If we accidentally hit Return beforetyping anything, we dont want to search for a nonstring, so we return fromthe subroutine if thats the case: $INFORMATION = Connect: $url; $text->configure(-cursor=> watch); $mw->idletasks;We give the user some feedback by placing along the bottom of theapplication a Connect... string, and we also change the cursor to a watch.$mw->idletasks just tells the window to do anything it was waiting to do, sothat we can actually see the watch and information string: my $request = new HTTP::Request(GET, $url); my $response = $ua->request($request); if ($response->is_error) { $INFORMATION = ERROR: Could not retrieve$url; } elsif ($response->is_success) { my $html = parse_html($response->content); ## Clear out text item $text->configure(-state => normal); $text->delete(1.0, end); $html->traverse(\&display_html); $text->configure(-state => disabled); $html_text = ; $INFORMATION = Done; } $text->configure(-cursor => top_left_arrow);}Next we try to connect to the $url. If we fail, the program should display asimple error message in the information area. If we succeed, then we want toget the actual document out and parse it. $html will contain the HTML treeobject. We reconfigure the text object to normal so that we can place textin it,[5] delete anything that might have been there previously, and then calltraverse for the HTML object (telling traverse to call display_html for eachitem). After the entire document has been traversed (well see what that doesin a minute), we re-disable the text widget, and declare ourselves done forthat particular word lookup.Our function, display_html, gets called with three arguments: a $nodepointer, a $startflag flag, and the $depth we are into the tree. We only careabout the first two arguments, since they will help us decide what action toperform.sub display_html { my ($node, $startflag, $depth) = @_; my ($tag, $type, $coderef); ## This tag is theHTML tag... if (!ref $node) { $html_text .= $node; } else { if ($startflag) { $tag = $node->starttag;} else { $tag = $node->endtag;}## Gets rid of any extra stuff in the tag, andsaves itif ($tag =~ /^(/) { $tag = $1>; $extra = $2;}if (exists $html_action{$tag}) {$html_text =~ s/\s+/ /g; &{ $html_action{$tag} }($tag,$html_text); $html_text = ;} } 1;}Thats the entire function, but it does quite a bit. The $node could either bean object or a simple text string. For the simple case, when its just text, weappend it to any prior text (remember, we could be ignoring HTML tags,along the way, that had text before them) and save it for future use. If $nodeis an object pointer, then we have to determine what kind it is, and decide ifwe care about the HTML tag its telling us about.HTML tags usually come in pairs, so $startflag tells us when we found thefirst of a pair. We want to know what that tag was, so we call the starttagmethod. Certain tags have other information associated with them (i.e., the tag), and we want to save that for future use in $extra. Remember thatwe are trying to get just the plain simple tag to use in our lookup array.We do a few more things to clean up, and then we can do our lookup. If wecare about this $tag, then we compress all spaces in the current text string(makes the display a little bit nicer) and call the function specified in ourlookup array, passing it $tag and $html_text. We left $extra as a globalbecause most of our functions wont use it.All that work was just to figure out what function to call. We could havedone a big huge if..then..else statement instead of utilizing a lookup hash,but that would have been large and unwieldy, and would also have made itmore difficult to add new tag handling functions. The following are those taghandling functions, and most of them are pretty short:sub end_title { $mw->title(xword: . $_[1]);}When we find the end title tag, we change our window title to reflect it (a lotlike a standard web browser).sub start_heading { &flush_text(@_); $text->insert(end, \n\n);}When we start a heading, we need to delimit it from the prior text (which weinsert into our text widget with the flush_text( ) function) with a few returns.Note that flush_text( ) takes the same arguments as any of our tag handlers.This allows us to specify it explicitly in the lookup hash if we want to:sub end_heading { $text->insert(end, $_[1], $_[0]); $text->insert(end, \n);}At the end of the heading, we insert the heading text and another returncharacter. The third argument to the insert function is our actual HTML tag.(In this case it could be or and so on.) This tells the text widgetto use that tag to format the text. For our headings, we set up that text tag tobe a font-changing tag:sub paragraph { &flush_text(@_); $text->insert(end, \n\n);}A paragraph marker, , just means insert a few returns. We also have toflush out any text prior to it:sub line_break { &flush_text(@_); $text->insert(end, \n);}Similar to , the also just inserts a return:sub draw_line { &flush_text(@_); $text-> ...