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HTML::Element::traverse - discussion of HTML::Element's traverse method
# $element->traverse is unnecessary and obscure. # Don't use it in new code.
You can use the "traverse" method:
{
my $counter = 'x0000';
$start_node->traverse(
[ # Callbacks;
# pre-order callback:
sub {
my $x = $_[0];
$x->attr('id', $counter++) unless defined $x->attr('id');
return HTML::Element::OK; # keep traversing
},
# post-order callback:
undef
],
1, # don't call the callbacks for text nodes
);
}
or you can just be simple and clear (and not have to understand the calling format for "traverse") by writing a sub that traverses the tree by just calling itself:
{
my $counter = 'x0000';
sub give_id {
my $x = $_[0];
$x->attr('id', $counter++) unless defined $x->attr('id');
foreach my $c ($x->content_list) {
give_id($c) if ref $c; # ignore text nodes
}
};
give_id($start_node);
}
See, isn't that nice and clear?
But, if you really need to know:
These all mean to traverse the element and all of its children. That is, this method starts at node $h, ``pre-order visits'' $h, traverses its children, and then will ``post-order visit'' $h. ``Visiting'' means that the callback routine is called, with these arguments:
$_[0] : the node (element or text segment),
$_[1] : a startflag, and
$_[2] : the depth
If the $ignore_text parameter is given and true, then the pre-order call will not be happen for text content.
The startflag is 1 when we enter a node (i.e., in pre-order calls) and 0 when we leave the node (in post-order calls).
Note, however, that post-order calls don't happen for nodes that are text segments or are elements that are prototypically empty (like ``br'', ``hr'', etc.).
If we visit text nodes (i.e., unless $ignore_text is given and true), then when text nodes are visited, we will also pass two extra arguments to the callback:
$_[3] : the element that's the parent
of this text node
$_[4] : the index of this text node
in its parent's content list
Note that you can specify that the pre-order routine can be a different routine from the post-order one:
$h->traverse( [\&pre_callback,\&post_callback], ...);
You can also specify that no post-order calls are to be made, by providing a false value as the post-order routine:
$h->traverse([ \&pre_callback,0 ], ...);
And similarly for suppressing pre-order callbacks:
$h->traverse([ 0,\&post_callback ], ...);
Note that these two syntaxes specify the same operation:
$h->traverse([\&foo,\&foo], ...);
$h->traverse( \&foo , ...);
The return values from calls to your pre- or post-order routines are significant, and are used to control recursion into the tree.
These are the values you can return, listed in descending order of my estimation of their usefulness:
Note that "HTML::Element::OK" et al are constants. So if you're running under "use strict" (as I hope you are), and you say: "return HTML::Element::PRUEN" the compiler will flag this as an error (an unallowable bareword, specifically), whereas if you spell PRUNE correctly, the compiler will not complain.
Almost every task to do with extracting information from a tree can be expressed in terms of traverse operations (usually in only one pass, and usually paying attention to only pre-order, or to only post-order), or operations based on traversing. (In fact, many of the other methods in this class are basically calls to traverse() with particular arguments.)
The source code for HTML::Element and HTML::TreeBuilder contain several examples of the use of the ``traverse'' method to gather information about the content of trees and subtrees.
(Note: you should not change the structure of a tree while you are traversing it.)
[End of documentation for the "traverse()" method]
{
my $counter = 'x0000';
my $give_id;
$give_id = sub {
my $x = $_[0];
$x->attr('id', $counter++) unless defined $x->attr('id');
foreach my $c ($x->content_list) {
$give_id->($c) if ref $c; # ignore text nodes
}
};
$give_id->($start_node);
undef $give_id;
}
It's a bit nutty, and it's still more concise than a call to the "traverse" method!
It is left as an exercise to the reader to figure out how to do the same thing without using a $give_id symbol at all.
It is also left as an exercise to the reader to figure out why I undefine $give_id, above; and why I could achieved the same effect with any of:
$give_id = 'I like pie!';
# or...
$give_id = [];
# or even;
$give_id = sub { print "Mmmm pie!\n" };
But not:
$give_id = sub { print "I'm $give_id and I like pie!\n" };
# nor...
$give_id = \$give_id;
# nor...
$give_id = { 'pie' => \$give_id, 'mode' => 'a la' };
{
my @to_do = ($tree); # start-node
while(@to_do) {
my $this = shift @to_do;
# "Visit" the node:
$this->attr('id', $counter++)
unless defined $this->attr('id');
unshift @to_do, grep ref $_, $this->content_list;
# Put children on the stack -- they'll be visited next
}
}
This can under certain circumstances be more efficient than just a normal recursive routine, but at the cost of being rather obscure. It gains efficiency by avoiding the overhead of function-calling, but since there are several method dispatches however you do it (to "attr" and "content_list"), the overhead for a simple function call is insignificant.
my $died_on; # if you need to know where...
sub thing {
... visits $_[0]...
... maybe set $died_on to $_[0] and die "ABORT_TRAV" ...
... else call thing($child) for each child...
...any post-order visiting $_[0]...
}
eval { thing($node) };
if($@) {
if($@ =~ m<^ABORT_TRAV>) {
...it died (aborted) on $died_on...
} else {
die $@; # some REAL error happened
}
}
or you can just do it with flags:
my($abort_flag, $died_on);
sub thing {
... visits $_[0]...
... maybe set $abort_flag = 1; $died_on = $_[0]; return;
foreach my $c ($_[0]->content_list) {
thing($c);
return if $abort_flag;
}
...any post-order visiting $_[0]...
return;
}
$abort_flag = $died_on = undef; thing($node); ...if defined $abort_flag, it died on $died_on
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