Terrence Brannon

NAME

HTML::Seamstress::Quickstart - A gentle introduction to HTML::Seamstress

Introduction

This guide is designed to get you started with dynamically generating and modifying ("templating") HTML with HTML::Seamstress.

We will work through several examples, with each one increasing your ability to work with Seamstress effectively.

Sample files

All the files for the samples are in the directory lib/HTML/Seamstress/Quickstart

Pure TreeBuilder

Welcome to the first example. This is our bare-bones example. Let's say we want to dynamically modify the following HTML:

<tt>pod_code 'Quickstart/html/greeting.html'</tt>

Let's not use Seamstress at all in this case. Remember Seamstress just makes using HTML::Tree more convenient when writing software - it is completely optional and totally non-magical. So here's the (admittedly verbose) pure TreeBuilder solution:

<tt>pod_code 'Quickstart/greeting-treebuilder.pl'</tt>

There's a convenience function in HTML::Element::Library which makes it easy to replace all the content of an element. This will make our script shorter. If we simply use Seamstress, its new_from_file() method will bless the HTML tree into a class which inherits from HTML::Element::Library, making it easy for us to shorten our program. So let's rework the example using bare-bones Seamstress.

Base bones Seamstress rework

Since we used HTML::Seamstress instead of HTML::TreeBuilder, our $tree was blessed as an instance of HTML::Seamstress. Since HTML::Seamstress inherits from HTML::TreeBuilder and HTML::Element::Library, we have a $tree which can use the methods of both.

We will take advantage of the replace_content method in HTML::Element::Library to shorten our program:

<tt>pod_code 'Quickstart/greeting-seamstress-pure.pl'</tt>

Now of course, this program is just itching to not repeat itself, so we will clean it up just a tad:

<tt>pod_code 'Quickstart/greeting-seamstress-pure-mapped.pl'</tt>

Abstract the file and our operations on it into a Perl LOOM

Ok sweet, we have a nice tight program. But is this really application-level code? As the user of ultra-scaffolded frameworks such as Class::DBI and Catalyst, I can say no. Our inline code must be much tighter. It must do no more than use, new, and operation() whatever operation() may be in this case.

The key abstraction technique of the uber-modules is a package. Normally a package collects together a set of methods. In our case, it is collecting together an HTML file and the object-oriented operations on it. alert, alert: acronym, alert from this point forward, a Perl class abstracting a file and tree operations on the file will be called a LOOM - (L)ibrary (O)f (O)bject-oriented (M)ethods for HTML files.

On the whole, there are two ways to build a LOOM. The quick and dirty way is to stick a .pm file in the same directory as the html file. This is fine for most purposes and is what I like to use.

However in some cases it is not desirable or possible for the HTML and Perl to be in the same directory. This is the non quick and dirty approach. There are two ways to be squeaky clean as of this writing:

1 Provide an absolute path to the HTML

We will demonstrate this approach

2 Inline the HTML

There is no Seamstress API or utility support for this. But it is an idea and I just wanted to mention it for completeness. You can create a .pm with the HTML along the lines of this.

 package html::hello_world;

 sub new {...}

 sub process {...} 

 __HTML__
 <html>
  <head>
    ...
  </head>
  <body>
    ...
  </body>

 </html>

And then you could use File::Slurp to read it in and submit an appropriate HTML::Seamstress::new_from_content() method to do the proper blessings.

But I don't like this approach. It makes it hard to stay synchronized with the designer as he continually makes updates.

Quick and dirty LOOM building: .pm and .html in same directory

We have an html::Greeting module like this:

<tt>pod_code 'Quickstart/html/Greeting.pm'</tt>

which we make nice tight application-level use of like this:

<tt>pod_code 'Quickstart/callGreeting.pl'</tt>

Cleaning up our Perl class

We are flowing smoothly now with nice tight code in our application. But should we be happy with this module? I see a few drawbacks which require improvement:

  • our file name is given as a relative path name

    Relative paths are fine as long as we are certain to start in the same directory, but we cannot be sure of that when building applications, so we need an absolute path.

  • we had to manually create this package

Let's fix the first problem first.

Make path to HTML file absolute

Again, Seamstress just happens to have a subroutine which guesses the name of the HTML file associated with a Seamstress-style Perl module. It is called html() and here we see it in use to give us the path to our file in absolute fashion:

<tt>pod_code 'Quickstart/html/GreetingAbs.pm'</tt>

and main code body is still the same:

<tt>pod_code 'Quickstart/callGreetingAbs.pl'</tt>

Slow and clean LOOM building: .pm and .html in different directory

Here we need to slip a class in between HTML::Seamstress and our LOOM:

<tt>pod_code 'Quickstart/lib/HTML/Seamstress/Base.pm'</tt>

This class will make it easy to track down the directory of our HTML files.

The LOOM class inherits from it and makes use of it in its constructor:

<tt>pod_code 'Quickstart/lib/html/Greeting.pm'</tt>

And out main body code is just as tight. We have a few statements to make sure to use the right version of html::Greeting, but other than that, no changes:

<tt>pod_code 'Quickstart/slowClean.pl'</tt>

Automated creation of Seamstress-style packages

Instead of manually creating or copying and pasting packages to create Seamstress-style packages, the spkg.pl script in the Seamstress was off use. It is designed to help build the slow-clean LOOMs not the quick-and-dirty ones.... it really should be updated to support either usage mode.

Use subroutines and Params::Validate to "componentize" your operations

The final phase in Seamstress best practices is to break each tree manipulation down into a separate subroutine whose parameters are specified by Params::Validate.

If you do this, then you can control the dynamic HTML generation by parameterizing your subs properly. This advice will make more sense as you do more complex things with Seamstress

Now you're ready for the big time! Have fun!

COPYRIGHT AND LICENSE

Copyright 2002-2006 by Terrence Brannon.

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.