NAME

CGI::Application - Framework for building reusable web-applications

SYNOPSIS

  # In "WebApp.pm"...
  package WebApp;
  use base 'CGI::Application';
  sub setup {
        my $self = shift;
        $self->start_mode('mode1');
        $self->mode_param('rm');
        $self->run_modes(
                'mode1' => 'do_stuff',
                'mode2' => 'do_more_stuff',
                'mode3' => 'do_something_else'
        );
  }
  sub do_stuff { ... }
  sub do_more_stuff { ... }
  sub do_something_else { ... }
  1;


  ### In "webapp.cgi"...
  use WebApp;
  my $webapp = WebApp->new();
  $webapp->run();

USAGE EXAMPLE

CGI::Application is intended to make it easier to create sophisticated, reusable web-based applications. This module implements a methodology which, if followed, will make your web software easier to design, easier to document, easier to write, and easier to evolve.

CGI::Application builds on standard, non-proprietary technologies and techniques, such as the Common Gateway Interface and Lincoln D. Stein's excellent CGI.pm module. CGI::Application judiciously avoids employing technologies and techniques which would bind a developer to any one set of tools, operating system or web server.

Following is an example of the typical usage of CGI::Application.

Imagine you have to write an application to search through a database of widgets. Your application has three screens:

   1. Search form
   2. List of results
   3. Detail of a single record

To write this application using CGI::Application you will create two files:

   1. WidgetView.pm -- Your "Application Module"
   2. widgetview.cgi -- Your "Instance Script"

The Application Module contains all the code specific to your application functionality, and it exists outside of your web server's document root, somewhere in the Perl library search path.

The Instance Script is what is actually called by your web server. It is a very small, simple file which simply creates an instance of your application and calls an inherited method, run(). Following is the entirety of "widgetview.cgi":

   #!/usr/bin/perl -w
   use WidgetView;
   my $webapp = WidgetView->new();
   $webapp->run();

As you can see, widgetview.cgi simply "uses" your Application module (which implements a Perl package called "WidgetView"). Your Application Module, "WidgetView.pm", is somewhat more lengthy:

   package WidgetView;
   use base 'CGI::Application';
   use strict;

   # Needed for our database connection
   use DBI;

   sub setup {
        my $self = shift;
        $self->start_mode('mode1');
        $self->run_modes(
                'mode1' => 'showform',
                'mode2' => 'showlist',
                'mode3' => 'showdetail'
        );

        # Connect to DBI database
        $self->param('mydbh' => DBI->connect());
   }

   sub teardown {
        my $self = shift;

        # Disconnect when we're done
        $self->param('mydbh')->disconnect();
   }

   sub showform {
        my $self = shift;

        # Get CGI query object
        my $q = $self->query();

        my $output = '';
        $output .= $q->start_html(-title => 'Widget Search Form');
        $output .= $q->start_form();
        $output .= $q->textfield(-name => 'widgetcode');
        $output .= $q->hidden(-name => 'rm', -value => 'mode2');
        $output .= $q->submit();
        $output .= $q->end_form();
        $output .= $q->end_html();

        return $output;
   }

   sub showlist {
        my $self = shift;

        # Get our database connection
        my $dbh = $self->param('mydbh');

        # Get CGI query object
        my $q = $self->query();
        my $widgetcode = $q->param("widgetcode");

        my $output = '';
        $output .= $q->start_html(-title => 'List of Matching Widgets');

        ## Do a bunch of stuff to select "widgets" from a DBI-connected
        ## database which match the user-supplied value of "widgetcode"
        ## which has been supplied from the previous HTML form via a 
        ## CGI.pm query object.
        ##
        ## Each row will contain a link to a "Widget Detail" which 
        ## provides an anchor tag, as follows:
        ##
        ##   "widgetview.cgi?rm=mode3&widgetid=XXX"
        ##
        ##  ...Where "XXX" is a unique value referencing the ID of
        ## the particular "widget" upon which the user has clicked.

        $output .= $q->end_html();

        return $output;
   }

   sub showdetail {
        my $self = shift;

        # Get our database connection
        my $dbh = $self->param('mydbh');

        # Get CGI query object
        my $q = $self->query();
        my $widgetid = $q->param("widgetid");

        my $output = '';
        $output .= $q->start_html(-title => 'Widget Detail');

        ## Do a bunch of things to select all the properties of  
        ## the particular "widget" upon which the user has
        ## clicked.  The key id value of this widget is provided 
        ## via the "widgetid" property, accessed via the CGI.pm
        ## query object.

        $output .= $q->end_html();

        return $output;
   }

   1;  # Perl requires this at the end of all modules

CGI::Application takes care of implementing the new() and the run() methods. Notice that at no point do you call print() to send any output to STDOUT. Instead, all output is returned as a scalar.

CGI::Application's most significant contribution is in managing the application state. Notice that all which is needed to push the application forward is to set the value of a HTML form parameter 'rm' to the value of the "run mode" you wish to handle the form submission. This is the key to CGI::Application.

ABSTRACT

The guiding philosophy behind CGI::Application is that a web-based application can be organized into a specific set of "Run Modes." Each Run Mode is roughly analogous to a single screen (a form, some output, etc.). All the Run Modes are managed by a single "Application Module" which is a Perl module. In your web server's document space there is an "Instance Script" which is called by the web server as a CGI (or an Apache::Registry script if you're using Apache + mod_perl).

This methodology is an inversion of the "Embedded" philosophy (ASP, JSP, EmbPerl, Mason, etc.) in which there are "pages" for each state of the application, and the page drives functionality. In CGI::Application, form follows function -- the Application Module drives pages, and the code for a single application is in one place; not spread out over multiple "pages". If you feel that Embedded architectures are confusing, unorganized, difficult to design and difficult to manage, CGI::Application is the methodology for you!

Apache is NOT a requirement for CGI::Application. Web applications based on CGI::Application will run equally well on NT/IIS or any other CGI-compatible environment. CGI::Application-based applications are, however, ripe for use on Apache/mod_perl servers, as they naturally encourage Good Programming Practices. As always, use strict!

DESCRIPTION

CGI::Application is an Object-Oriented Perl module which implements an Abstract Class. It is not intended that this package be instantiated directly. Instead, it is intended that your Application Module will be implemented as a Sub-Class of CGI::Application.

To inherit from CGI::Application, the following code should go at the beginning of your Application Module, after your package declaration:

    use base 'CGI::Application';

Notation and Conventions

For the purpose of this document, we will refer to the following conventions:

  WebApp.pm   The Perl module which implements your Application Module class.
  WebApp      Your Application Module class; a sub-class of CGI::Application.
  webapp.cgi  The Instance Script which implements your Application Module.
  $webapp     An instance (object) of your Application Module class.
  $self       Same as $webapp, used in instance methods to pass around the 
              current object. (Standard Perl Object-Oriented technique)

Instance Script Methods

By inheriting from CGI::Application you have access to a number of built-in methods. The following are those which are expected to be called from your Instance Script.

new()

The new() method is the constructor for a CGI::Application. It returns a blessed reference to your Application Module package (class). Optionally, new() may take a set of parameters as key => value pairs:

    my $webapp = App->new(
                TMPL_PATH => 'App/',
                PARAMS => {
                        'custom_thing_1' => 'some val',
                        'another_custom_thing' => [qw/123 456/]
                }
    );

This method may take some specific parameters:

TMPL_PATH - This optional parameter adds value to the load_tmpl() method (specified below). This sets a path using HTML::Template's path option when you call load_tmpl() to get your HTML::Template object. This run-time parameter allows you to further encapsulate instantiating templates, providing potential for more re-usability. It can be either a scalar or an array reference of multiple paths.

PARAMS - This parameter, if used, allows you to set a number of custom parameters at run-time. By passing in different values in different instance scripts which use the same application module you can achieve a higher level of re-usability. For instance, imagine an application module, "Mailform.pm". The application takes the contents of a HTML form and emails it to a specified recipient. You could have multiple instance scripts throughout your site which all use this "Mailform.pm" module, but which set different recipients or different forms.

QUERY - This optional parameter allows you to specify an already-created CGI.pm query object. Under normal use, CGI::Application will instantiate its own CGI.pm query object. Under certain conditions, it might be useful to be able to use one which has already been created.

run()

The run() method is called upon your Application Module object, from your Instance Script. When called, it executes the functionality in your Application Module.

    my $webapp = WebApp->new();
    $webapp->run();

This method first determines the application state by looking at the value of the CGI parameter specified by mode_param() (defaults to 'rm' for "Run Mode"), which is expected to contain the name of the mode of operation. If not specified, the state defaults to the value of start_mode().

Once the mode has been determined, run() looks at the dispatch table stored in run_modes() and finds the function pointer which is keyed from the mode name. If found, the function is called and the data returned is print()'ed to STDOUT and to the browser. If the specified mode is not found in the run_modes() table, run() will croak().

If the runmode dies for whatever reason, run() will see if you have set a value for error_mode(). If you have, run() will call that method as a run mode.

Sub-classing and Override Methods

CGI::Application implements some methods which are expected to be overridden by implementing them in your sub-class module. These methods are as follows:

setup()

This method is called by the inherited new() constructor method. The setup() method should be used to define the following property/methods:

    mode_param() - set the name of the run mode CGI param.
    start_mode() - text scalar containing the default run mode.
    error_mode() - text scalar containing the error mode.
    run_modes() - hash table containing mode => function mappings.
    tmpl_path() - text scalar or array refefence containing path(s) to template files.

Your setup() method may call any of the instance methods of your application. This function is a good place to define properties specific to your application via the $webapp->param() method.

Your setup() method might be implemented something like this:

        sub setup {
                my $self = shift;
                $self->tmpl_path('/path/to/my/templates/');
                $self->start_mode('putform');
                $self->error_mode('my_error_rm');
                $self->run_modes({
                        'putform'  => 'my_putform_func',
                        'postdata' => 'my_data_func'
                });
                $self->param('myprop1');
                $self->param('myprop2', 'prop2value');
                $self->param('myprop3', ['p3v1', 'p3v2', 'p3v3']);
        }

teardown()

If implemented, this method is called automatically after your application runs. It can be used to clean up after your operations. A typical use of the teardown() function is to disconnect a database connection which was established in the setup() function. You could also use the teardown() method to store state information about the application to the server.

cgiapp_init()

If implemented, this method is called automatically right before the setup() method is called. This method provides an optional initialization hook, which improves the object-oriented characteristics of CGI::Application. The cgiapp_init() method receives, as its parameters, all the arguments which were sent to the new() method.

An example of the benefits provided by utilizing this hook is creating a custom "application super-class" from which which all your CGI applications would inherit, instead of CGI::Application.

Consider the following:

  # In MySuperclass.pm:
  package MySuperclass;
  use base 'CGI::Application';
  sub cgiapp_init {
        my $self = shift;
        # Perform some project-specific init behavior
        # such as to load settings from a database or file.
  }


  # In MyApplication.pm:
  package MyApplication;
  use base 'MySuperclass';
  sub setup { ... }
  sub teardown { ... }
  # The rest of your CGI::Application-based follows...

By using CGI::Application and the cgiapp_init() method as illustrated, a suite of applications could be designed to share certain characteristics. This has the potential for much cleaner code built on object-oriented inheritance.

cgiapp_prerun()

If implemented, this method is called automatically right before the selected run mode method is called. This method provides an optional pre-runmode hook, which permits functionality to be added at the point right before the run mode method is called. To further leverage this hook, the value of the run mode is passed into cgiapp_prerun().

Another benefit provided by utilizing this hook is creating a custom "application super-class" from which all your CGI applications would inherit, instead of CGI::Application.

Consider the following:

  # In MySuperclass.pm:
  package MySuperclass;
  use base 'CGI::Application';
  sub cgiapp_prerun {
        my $self = shift;
        # Perform some project-specific init behavior
        # such as to implement run mode specific
        # authorization functions.
  }


  # In MyApplication.pm:
  package MyApplication;
  use base 'MySuperclass';
  sub setup { ... }
  sub teardown { ... }
  # The rest of your CGI::Application-based follows...

By using CGI::Application and the cgiapp_prerun() method as illustrated, a suite of applications could be designed to share certain characteristics. This has the potential for much cleaner code built on object-oriented inheritance.

It is also possible, within your cgiapp_prerun() method, to change the run mode of your application. This can be done via the prerun_mode() method, which is discussed elsewhere in this POD.

cgiapp_postrun()

If implemented, this hook will be called after the run mode method has returned its output, but before HTTP headers are generated. This will give you an opportunity to modify the body and headers before they are returned to the web browser.

A typical use for this hook is pipelining the output of a CGI-Application through a series of "filter" processors. For example:

  * You want to enclose the output of all your CGI-Applications in 
    an HTML table in a larger page.

  * Your run modes return structured data (such as XML), which you
    want to transform using a standard mechanism (such as XSLT).

  * You want to post-process CGI-App output through another system,
    such as HTML::Mason.

  * You want to modify HTTP headers in a particular way across all
    run modes, based on particular criteria.

The cgiapp_postrun() hook receives a reference to the output from your run mode method, in addition to the CGI-App object. A typical cgiapp_postrun() method might be implemented as follows:

  sub cgiapp_postrun {
    my $self = shift;
    my $output_ref = shift;

    # Enclose output HTML table
    my $new_output = "<table border=1>";
    $new_output .= "<tr><td> Hello, World! </td></tr>";
    $new_output .= "<tr><td>". $$output_ref ."</td></tr>";
    $new_output .= "</table>";

    # Replace old output with new output
    $$output_ref = $new_output;
  }

Obviously, with access to the CGI-App object you have full access to use all the methods normally available in a run mode. You could, for example, use load_tmpl() to replace the static HTML in this example with HTML::Template. You could change the HTTP headers (via header_type() and header_props() methods) to set up a redirect. You could also use the objects properties to apply changes only under certain circumstance, such as a in only certain run modes, and when a param() is a particular value.

cgiapp_get_query()

This method is called when CGI::Application retrieves the CGI query object. The cgiapp_get_query() method loads CGI.pm via "require" and returns a CGI.pm query object. The implementation is as follows:

  sub cgiapp_get_query {  
        my $self = shift;

        # Include CGI.pm and related modules
        require CGI;

        # Get the query object
        my $q = CGI->new();

        return $q;
  }

You may override this method if you wish to use a different query interface instead of CGI.pm. Note, however, that your query interface must be compatible with CGI.pm, or you must wrap your chosen query interface in a "wrapper" class to achieve compatibility.

Application Module Methods

The following methods are inherited from CGI::Application, and are available to be called by your application within your Application Module. These functions are listed in alphabetical order.

delete()

    $webapp->delete('my_param');

The delete() method is used to delete a parameter that was previously stored inside of your application either by using the PARAMS hash that was passed in your call to new() or by a call to the param() method. This is similar to the delete() method of CGI.pm. It is useful if your application makes decisions based on the existence of certain params that may have been removed in previous sections of your app or simply to clean-up your param()s.

dump()

    print STDERR $webapp->dump();

The dump() method is a debugging function which will return a chunk of text which contains all the environment and CGI form data of the request, formatted nicely for human readability. Useful for outputting to STDERR.

dump_html()

    my $output = $webapp->dump_html();

The dump_html() method is a debugging function which will return a chunk of text which contains all the environment and CGI form data of the request, formatted nicely for human readability via a web browser. Useful for outputting to a browser.

error_mode()

    $webapp->error_mode('my_error_rm');

The error_mode contains the name of a run mode to call in the event that the planned run mode call fails eval. No error_mode is defined by default. The death of your error_mode() run mode is not trapped, so you can also use it to die in your own special way.

get_current_runmode()

    $webapp->get_current_runmode();

The get_current_runmode() method will return a text scalar containing the name of the run mode which is currently being executed. If the run mode has not yet been determined, such as during setup(), this method will return undef.

header_add()

    # add or replace the 'type' header
    $webapp->header_add( -type => 'image/png' );

    - or -

    # add an additional cookie
    $webapp->header_add(-cookie=>[$extra_cookie]);

The header_add() method is used to add one or more headers to the outgoing response headers. The parameters will eventuallly be passed on to the CGI.pm header() method, so refer to the CGI docs for exact usage details.

Unlike calling header_props(), header_add() will preserve any existing headers. If a scalar value is passed to header_add() it will replace the existing value for that key.

If an array reference is passed as a value to header_add(), values in that array ref will be appended to any existing values values for that key. This is primarily useful for setting an additional cookie after one has already been set.

header_props()

    $webapp->header_props(-type=>'image/gif',-expires=>'+3d');

The header_props() method expects a hash of CGI.pm-compatible HTTP header properties. These properties will be passed directly to CGI.pm's header() or redirect() methods. Refer to CGI for exact usage details.

Calling header_props will clobber any existing headers that have previously set.

To add additional headers later without clobbering the old ones, see header_add().

IMPORTANT NOTE REGARDING HTTP HEADERS

It is through the header_props() and header_add() method that you may modify the outgoing HTTP headers. This is necessary when you want to set a cookie, set the mime type to something other than "text/html", or perform a redirect. The header_props() method works in conjunction with the header_type() method. The value contained in header_type() determines if we use CGI::header() or CGI::redirect(). The content of header_props() is passed as an argument to whichever CGI.pm function is called.

Understanding this relationship is important if you wish to manipulate the HTTP header properly.

header_type([<'header' || 'redirect' || 'none'>])

    $webapp->header_type('redirect');

The header_type() method expects to be passed either 'header', 'redirect', or 'none'. This method specifies the type of HTTP headers which should be sent back to the browser. If not specified, defaults is 'header'. See the header section of CGI for details.

To perform a redirect using CGI::Application (and CGI.pm), you would do the following:

  sub some_redirect_mode {
    my $self = shift;
    my $new_url = "http://site/path/doc.html";
    $self->header_type('redirect');
    $self->header_props(-url=>$new_url);
    return "Redirecting to $new_url";
  }

If you wish to suppress HTTP headers entirely (as might be the case if you're working in a slightly more exotic environment), you can set header_type() to "none". This will completely hide headers.

load_tmpl()

    my $tmpl_obj = $webapp->load_tmpl('some.tmpl');

This method takes the name of a template file and returns an HTML::Template object. The HTML::Template->new_file() constructor is used for create the object. Refer to HTML::Template for specific usage of HTML::Template.

If tmpl_path() has been specified, load_tmpl() will set the HTML::Template path option to the path(s) provided. This further assists in encapsulating template usage.

The load_tmpl() method will pass any extra parameters sent to it directly to HTML::Template->new_file(). This will allow the HTML::Template object to be further customized:

    my $tmpl_obj = $webapp->load_tmpl('some_other.tmpl', 
         die_on_bad_params => 0,
         cache => 1
    );

If your application requires more specialized behavior than this, you are encouraged to override load_tmpl() by implementing your own load_tmpl() in your CGI::Application sub-class application module.

mode_param()

 # Name the CGI form parameter that contains the run mode name.
 # This is the the default behavior, and is often sufficient.
 $webapp->mode_param('rm');

 # Set the run mode name directly from a code ref 
 $webapp->mode_param(\&some_method);
 
 # Alternate interface, which allows you to set the run 
 # mode name directly from $ENV{PATH_INFO}.
 $webapp->mode_param( 
        path_info=> 1, 
        param =>'rm'
 );

This accessor/mutator method is generally called in the setup() method. It is used to help determine the run mode to call. There are three options for calling it.

 $webapp->mode_param('rm');

Here, a CGI form parameter is named that will contain the name of the run mode to use. This is the default behavior, with 'rm' being the parameter named used.

 $webapp->mode_param(\&some_method);

Here a code reference is provided. It will return the name of the run mode to use directly. Example:

 sub some_method {
   my $self = shift;
   return 'run_mode_x';
 }

This would allow you to programmatically set the run mode based on arbitrary logic.

 $webapp->mode_param( 
        path_info=> 1, 
        param =>'rm'
 );

This syntax allows you to easily set the run mode from $ENV{PATH_INFO}. It will try to set the run mode from the first part of $ENV{PATH_INFO} (before the first "/"). To specify that you would rather get the run name from the 2nd part of $ENV{PATH_INFO}:

 $webapp->mode_param( path_info=> 2 );

This also demonstrates that you don't need to pass in the param hash key. It will still default to rm.

If no run mode is found in $ENV{PATH_INFO}, it will fall back to looking in the value of a the CGI form field defined with 'param', as described above. This allows you to use the convenient $ENV{PATH_INFO} trick most of the time, but also supports the edge cases, such as when you don't know what the run mode will be ahead of time and want to define it with JavaScript.

More about $ENV{PATH_INFO}.

Using $ENV{PATH_INFO} to name your run mode creates a clean seperation between the form variables you submit and how you determine the processing run mode. It also creates URLs that are more search engine friendly. Let's look at an example form submission using this syntax:

        <form action="/cgi-bin/instance.cgi/edit_form" method=post>
                <input type="hidden" name="breed_id" value="4">

Here the run mode would be set to "edit_form". Here's another example with a query string:

        /cgi-bin/instance.cgi/edit_form?breed_id=2

This demostrates that you can use $ENV{PATH_INFO} and a query string together without problems. $ENV{PATH_INFO} is defined as part of the CGI specification should be supported by any web server that supports CGI scripts.

param()

    $webapp->param('pname', $somevalue);

The param() method provides a facility through which you may set application instance properties which are accessible throughout your application.

The param() method may be used in two basic ways. First, you may use it to get or set the value of a parameter:

    $webapp->param('scalar_param', '123');
    my $scalar_param_values = $webapp->param('some_param');

Second, when called in the context of an array, with no parameter name specified, param() returns an array containing all the parameters which currently exist:

    my @all_params = $webapp->param();

The param() method also allows you to set a bunch of parameters at once by passing in a hash (or hashref):

    $webapp->param(
        'key1' => 'val1',
        'key2' => 'val2',
        'key3' => 'val3',
    );

The param() method enables a very valuable system for customizing your applications on a per-instance basis. One Application Module might be instantiated by different Instance Scripts. Each Instance Script might set different values for a set of parameters. This allows similar applications to share a common code-base, but behave differently. For example, imagine a mail form application with a single Application Module, but multiple Instance Scripts. Each Instance Script might specify a different recipient. Another example would be a web bulletin boards system. There could be multiple boards, each with a different topic and set of administrators.

The new() method provides a shortcut for specifying a number of run-time parameters at once. Internally, CGI::Application calls the param() method to set these properties. The param() method is a powerful tool for greatly increasing your application's re-usability.

prerun_mode()

    $webapp->prerun_mode('new_run_mode');

The prerun_mode() method is an accessor/mutator which can be used within your cgiapp_prerun() method to change the run mode which is about to be executed. For example, consider:

  # In WebApp.pm:
  package WebApp;
  use base 'CGI::Application';
  sub cgiapp_prerun {
        my $self = shift;

        # Get the web user name, if any
        my $q = $self->query();
        my $user = $q->remote_user();

        # Redirect to login, if necessary
        unless ($user) {
                $self->prerun_mode('login');
        }
  }

In this example, the web user will be forced into the "login" run mode unless they have already logged in. The prerun_mode() method permits a scalar text string to be set which overrides whatever the run mode would otherwise be.

The use of prerun_mode() within cgiapp_prerun() differs from setting mode_param() to use a call-back via subroutine reference. It differs because cgiapp_prerun() allows you to selectively set the run mode based on some logic in your cgiapp_prerun() method. The call-back facility of mode_param() forces you to entirely replace CGI::Application's mechanism for determining the run mode with your own method. The prerun_mode() method should be used in cases where you want to use CGI::Application's normal run mode switching facility, but you want to make selective changes to the mode under specific conditions.

Note: The prerun_mode() method may ONLY be called in the context of a cgiapp_prerun() method. Your application will die() if you call prerun_mode() elsewhere, such as in setup() or a run mode method.

query()

    my $q = $webapp->query();
    my $remote_user = $q->remote_user();

This method retrieves the CGI.pm query object which has been created by instantiating your Application Module. For details on usage of this query object, refer to CGI. CGI::Application is built on the CGI module. Generally speaking, you will want to become very familiar with CGI.pm, as you will use the query object whenever you want to interact with form data.

When the new() method is called, a CGI query object is automatically created. If, for some reason, you want to use your own CGI query object, the new() method supports passing in your existing query object on construction using the QUERY attribute.

run_modes()

    $webapp->run_modes('mode1' => 'some_sub_by_name', 'mode2' => \&some_other_sub_by_ref);

This accessor/mutator expects a hash which specifies the dispatch table for the different CGI states. The run() method uses the data in this table to send the CGI to the correct function as determined by reading the CGI parameter specified by mode_param() (defaults to 'rm' for "Run Mode"). These functions are referred to as "run mode methods".

The hash table set by this method is expected to contain the mode name as a key. The value should be either a hard reference (a subref) to the run mode method which you want to be called when the CGI enters the specified run mode, or the name of the run mode method to be called:

    'mode_name_by_ref'  => \&mode_function
    'mode_name_by_name' => 'mode_function'

The run mode method specified is expected to return a block of text (e.g.: HTML) which will eventually be sent back to the web browser. The your run mode method may return its block of text as a scalar or a scalar-ref.

An advantage of specifying your run mode methods by name instead of by reference is that you can more easily create derivative application using inheritance. For instance, if you have a new application which is exactly the same as an existing application with the exception of one run mode, you could simply inherit from that other application and override the run mode method which is different. If you specified your run mode method by reference, your child class would still use the function from the parent class.

An advantage of specifying your run mode methods by reference instead of by name is performance. Dereferencing a subref is faster than eval()-ing a code block. If run-time performance is a critical issue, specify your run mode methods by reference and not by name. The speed differences are generally small, however, so specifying by name is preferred.

The run_modes() method may be called more than once. Additional values passed into run_modes() will be added to the run modes table. In the case that an existing run mode is re-defined, the new value will override the existing value. This behavior might be useful for applications which are created via inheritance from another application, or some advanced application which modifies its own capabilities based on user input.

The run_modes() method also supports a second interface for designating run modes and their methods: Via array reference:

    $webapp->run_modes([ 'mode1', 'mode2', 'mode3' ]);

This is the same as the following, via hash:

    $webapp->run_modes(
        'mode1' => 'mode1',
        'mode2' => 'mode2',
        'mode3' => 'mode3'
    );

Often, it makes good organizational sense to have your run modes map to methods of the same name. The array-ref interface provides a shortcut to that behavior while reducing verbosity of your code.

Note that another importance of specifying your run modes in either a hash or array-ref is to assure that only those Perl methods which are specifically designated may be called via your application. Application environments which don't specify allowed methods and disallow all others are insecure, potentially opening the door to allowing execution of arbitrary code. CGI::Application maintains a strict "default-deny" stance on all method invocation, thereby allowing secure applications to be built upon it.

IMPORTANT NOTE ABOUT RUN MODE METHODS

Your application should *NEVER* print() to STDOUT. Using print() to send output to STDOUT (including HTTP headers) is exclusively the domain of the inherited run() method. Breaking this rule is a common source of errors. If your program is erroneously sending content before your HTTP header, you are probably breaking this rule.

THE RUN MODE OF LAST RESORT: "AUTOLOAD"

If CGI::Application is asked to go to a run mode which doesn't exist it will usually croak() with errors. If this is not your desired behavior, it is possible to catch this exception by implementing a run mode with the reserved name "AUTOLOAD":

  $self->run_modes(
        "AUTOLOAD" => \&catch_my_exception
  );

Before CGI::Application calls croak() it will check for the existence of a run mode called "AUTOLOAD". If specified, this run mode will in invoked just like a regular run mode, with one exception: It will receive, as an argument, the name of the run mode which invoked it:

  sub catch_my_exception {
        my $self = shift;
        my $intended_runmode = shift;

        my $output = "Looking for '$intended_runmode', but found 'AUTOLOAD' instead";
        return $output;
  }

This functionality could be used for a simple human-readable error screen, or for more sophisticated application behaviors.

start_mode()

    $webapp->start_mode('mode1');

The start_mode contains the name of the mode as specified in the run_modes() table. Default mode is "start". The mode key specified here will be used whenever the value of the CGI form parameter specified by mode_param() is not defined. Generally, this is the first time your application is executed.

tmpl_path()

    $webapp->tmpl_path('/path/to/some/templates/');

This access/mutator method sets the file path to the directory (or directories) where the templates are stored. It is used by load_tmpl() to find the template files, using HTML::Template's path option. To set the path you can either pass in a text scalar or an array reference of multiple paths.

Testing

CGI::Application currently has no built-in testing methodology but we leave that up to you. There are many testing structures and frameworks that are available for testing Perl modules (ie, Test::Harness, Test::More). There are large numbers of tools and frameworks for testing live web applications (WWW::Mechanize, HTTP::Test, Apache::Test). We encourage you to look around CPAN and find the one that best meets your needs.

Since most of the module level testing frameworks require the output that is sent to STDOUT be of a certain format, and since CGI::Application will print the output of it's run modes directly to STDOUT this could cause problems. To avoid this simple set the environment variable 'CGI_APP_RETURN_ONLY' to true and you can catch your output and test it and then print your own message to STDOUT. For example

        $ENV{CGI_APP_RETURN_ONLY} = 1;
        $output = $webapp->run();
                                                                                                                                                             
        #perform whatever test on the output that you want and then print to STDOUT
        if($output =~ /GOOD/){
                print "ok 11\n";
        } else {
                print "not ok 11\n";
        }

PLUG-INS

CGI::Application has a plug-in architecture that is easy to use and easy to develop new plug-ins for.

Existing plug-ins

Here are some plug-ins that have already been developed for CGI::Application. For a current complete list, please consult CPAN:

http://search.cpan.org/search?query=CGI%3A%3AApplication%3A%3APlugin&mode=module

CGI::Application::Plugin::Config::Simple - Integration with Config::Simple.
CGI::Application::Plugin::ConfigAuto - Integration with Config::Auto.
CGI::Application::Plugin::DBH - Integration with DBI.
CGI::Application::Plugin::Session - Integration with CGI::Session
CGI::Application::Plugin::TT - Use Template::Toolkit as an alternative to HTML::Template.
CGI:::Application::Plugin::ValidateRM - Integration with Data::FormValidator and HTML::FillInForm

Consult each plug-in for the exact usage syntax.

Writing Plug-ins

Writing plug-ins is simple. Simply create a new package, and export the methods that you want to become part of a CGI::Application project. See CGI::Application::Plugin::ValidateRM for an example.

You only need to keep in mind that your extension should "play well with others". The method names exported should be unique. Also, additions to the CGI::Application object should be designed not to conflict with other potentional plug-ins. For that reason, it's recommended that you add a a hash key to the CGI::Application which reflects the name of your plug-in, such as:

 $self->{'Config::Simple'}

COMMUNITY

There a couple of primary resources available for those who wish to learn more about CGI::Application and discuss it with others.

Wiki

This is a community built and maintained resource that anyone is welcome to contribute to. It contains a number of articles of its own and links to many other CGI::Application related pages:

http://www.cgi-app.org

Support Mailing List

If you have any questions, comments, bug reports or feature suggestions, post them to the support mailing list! To join the mailing list, simply send a blank message to "cgiapp-subscribe@lists.erlbaum.net".

AUTHOR

Jesse Erlbaum <jesse@erlbaum.net>

Mark Stosberg has served as a co-maintainer since version 3.2, with the help of the numerous contributors documented in the Changes file.

More Reading

If you're interested in finding out more about CGI::Application, the following article is available on Perl.com:

    Using CGI::Application
    http://www.perl.com/pub/a/2001/06/05/cgi.html

Thanks to Simon Cozens and the O'Reilly network for publishing this article, and for the incredible value they provide to the Perl community!

CREDITS

CGI::Application is developed by The Erlbaum Group, a software engineering and consulting firm in New York City. If you are looking for a company to develop your web site or individual developers to augment your team please contact us:

    The Erlbaum Group, LLC
    826 Broadway, Suite 933
    New York, NY 10003

    Phone: 212-684-6161
    Fax: 212-684-6226
    Email: info@erlbaum.net
    Web: http://www.erlbaum.net

Thanks to Vanguard Media (http://www.vm.com) for funding the initial development of this library and for encouraging me to release it to the world.

Many thanks to Sam Tregar (author of the most excellent HTML::Template module!) for his innumerable contributions to this module over the years, and most of all for getting me off my ass to finally get this thing up on CPAN!

Many other people have contributed specific suggestions or patches, which are documented in the Changes file.

Thanks also to all the members of the CGI-App mailing list! Your ideas, suggestions, insights (and criticism!) have helped shape this module immeasurably. (To join the mailing list, simply send a blank message to "cgiapp-subscribe@lists.erlbaum.net".)

LICENSE

This module is free software; you can redistribute it and/or modify it under the terms of either:

a) the GNU General Public License as published by the Free Software Foundation; either version 1, or (at your option) any later version,

b) the "Artistic License" which comes with this module.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See either the GNU General Public License or the Artistic License for more details.

You should have received a copy of the Artistic License with this module, in the file ARTISTIC. If not, I'll be glad to provide one.

You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA

To install CGI::Application, copy and paste the appropriate command in to your terminal.

cpanm

cpanm CGI::Application

CPAN shell

perl -MCPAN -e shell
install CGI::Application

For more information on module installation, please visit the detailed CPAN module installation guide.

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