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NAME

       CLI::Framework::Tutorial - "HOWTO" develop CLIF applications using best practices

CLIF DOCUMENTATION

       This is a guide to developing CLIF applications.  It is a supplement to the documentation
       in CLI::Framework, CLI::Framework::Application and CLI::Framework::Command, which have
       more thorough coverage of some finer points.

       It is suggested that new users start by reading this document, then use the other
       documentation for reference as necessary.

INTRODUCTION

       Developers have been reluctantly writing ad-hoc, disposable scripts for too long or
       struggling to decide how not to do so.  There is a better alternative.

       The CLI::Framework documentation enumerates many advantages to using CLIF instead of
       writing yet-another-getopt-based-script.  CLIF comes with a lot of documentation, but
       don't take that to mean that using CLIF is complicated.  CLIF apps with simple needs are
       very easy to build.  Apps with complex needs are a bit more work, but much easier to build
       (and far easier to test and maintain) than doing that work from scratch.

       This document will first demonstrate a very simple CLIF application.  Next, a complete
       application will be shown to demonstrate more advanced CLIF features.

       Think of a typical command-line script.  It needs to parse command-line options and
       arguments, check that any required external resources (files, databases, etc.) are
       available, fail nicely if something is missing or inconsistent, then do something
       application-specific that depends on the options, arguments, and external resources.

       What happens when new scripts are created to do something similar?  All too often, they
       end up with different option names for conceptually the same purpose.  It is common for
       functionality needed by several scripts to be duplicated in each similar script.  This
       rapidly gets out of hand, becoming a maintenance frustration.  Your team members are not
       "on the same page" and new people learning your tools must have lengthy, verbal, one-on-
       one code tours.

       Instead, a set of related scripts could be combined into a CLIF application.  Consistent
       naming conventions and sharing of common code is naturally encouraged.  The commands are
       easy to test.  New commands can be added with ease.

FROM P.O.S. TO CLIF IN A FEW EASY STEPS

       A "P.O.S." is a "Plain Old Script."  This section shows you how to reform an old P.O.S.,
       creating a shiny new CLIF application!

       Please see working code for this example included with the "CLI::Framework" distribution
       (examples/demo-simple.pl).

       This example demonstrates the following features:

       ·   inline application definition

       ·   basics (app, commands, command options and args)

       ·   the relationship between plain scripts and CLIF applications (including how to convert
           between them)

       To understand CLIF commands, imagine converting a legacy script to a CLIF application.
       First, create a Perl class that inherits from CLI::Framework::Command.  Place the main
       body of the script in a "run()" method.  Add the functions that the script defines, if
       any.

           # Your Command subclass...
           package Converted::Script::Command::LegacyScript;
           use base qw( CLI::Framework::Command );

           # main body of former script goes inside run():
           sub run { ... }

       Next, create a Perl class (creating a separate package file for the class is totally
       optional) that inherits from CLI::Framework::Application (or you can use "CLI::Framework"
       as a shorthand) and define a method, "command_map()", that links command names with
       classes that implement the commands:

           # Your Application class...
           package Converted::Script;
           use base qw( CLI::Framework );

           sub command_map {
               'legacy-script' => 'Converted::Script::Command::LegacyScript',
           }

       The code that provides a friendly usage message (if the legacy script provided one) can be
       replaced by defining the "usage_text" method:

           sub usage_text {
               qq{
               $0 [--verbose|v] [--help|h]: how to use this application...
               }
           }

       Back in your Command subclass, the option/argument processing code will be replaced with a
       method defining what options will be recognized (the data structure to be returned is
       exactly as documented in Getopt::Long::Descriptive):

           sub option_spec {
               [ 'help|h'      => 'show help' ],
               [ 'verbose|v'   => 'be verbose' ],
           }

       ...and that's all it takes to convert a simple script to a CLIF app.  This contrived
       example demonstrates the mechanics, but let me point out a few advantages (see DESIGN
       GOALS AND FEATURES for the long list):

       Clear division of responsibilities
           Using packages, subroutines, and separate files (if desired), CLIF apps follow
           established convention and provide a new pattern for creating tools.

       Easy to test
           Now that functional units of code are subroutines in packages, you can unit test each
           component independently.

       Easy to maintain
           Instead of puzzling over a several-thousand-line script, maintaining a CLIF
           application is like maintaining any other well-engineered application code.

       Easy to extend
           Related tools frequently occur in groups.  Instead of awkwardly forcing loosely-
           related behaviors into the same script, CLIF makes it easy to add additional commands
           in a modular way.

WHEN NOT TO USE CLIF

       CLIF could be used for the simplest of needs, but it may be overkill in very simple
       situations.

       You may want to avoid CLIF for very basic scripts that have a single behavior and are
       completely independent from other such tools.  However, if there's a chance that the
       scripts might grow to become more complex or if you would simply like a pattern to follow,
       it may still be worth considering.

CONCEPTS AND DEFINITIONS

       See CONCEPTS AND DEFINITIONS.

UNDERSTANDING THE APPLICATION RUN SEQUENCE

       See APPLICATION RUN SEQUENCE.

       Understanding this is important to building more complex apps.  You need, at the least, to
       understand how CLIF differentiates between options and arguments that are meant for the
       application itself and those options and arguments that are meant for individual commands.

       The following examples demonstrate the alternative command request forms.  Note that in
       all cases, any number of (sub)command options and arguments can be passed (these examples
       show only one of each for brevity).

       FORM #1 (without subcommands) -- command requests that involve NO subcommands take the
       following form:

           <app> [--app-opt] <cmd> [--cmd-opt] [cmd-arg] ...

       (notice how the position of options and arguments determines whether they are meant for
       the application as a whole or for the specific command).

       FORM #2 (with subcommands) -- Command requests that involve A SINGLE subcommand take this
       form:

           <app> [--app-opt] <cmd> [--cmd-opt] <subcmd> [--subcmd-opt] [subcmd-arg] ...

       Command requests that involve MULTIPLE subcommands follow the same form:

           <app> [--app-opt] <cmd> [--cmd-opt] <subcmd1> [--subcmd1-opt] <subcmd2> [--subcmd2-opt] [subcmd2-arg] ...

       (notice that the final arguments apply to the final subcommand.  The only command that can
       receive arguments is the final subcommand).

A MORE INVOLVED EXAMPLE

       Please see working code for this example included with the "CLI::Framework" distribution
       (examples/queue).

       The next example demonstrates the following features:

       ·   inline application definition

       ·   basics (app, commands, command options and args)

       ·   subcommands

       ·   validation of application and command arguments

       ·   interactive mode and non-interactive mode

       Suppose we need to write a command-line application that provides an interface to a queue.
       Strings can be added to or removed from the queue, queue contents can be displayed, and
       queue "properties" can be set to restrict the contents added to the queue.  The interface
       should work interactively.

       The following usage demonstrates the desired behavior:

           [somebody@somewhere]$ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile console

           # ---- interactive mode ----
           1) dequeue
           2) cmd-list
           3) enqueue
           4) print
           5) alias
           6) property

           > help enqueue

           enqueue [--tag=<tag1> [--tag=<tag2> [...] ] ] <item1> [<item2> ...  <itemN>]: add item(s) to queue

           > enqueue --tag=x "something"

           > property set --evens

           > e 1 21 514 937 18

       The working example in examples/queue accomplishes this goal in a single inline
       application containing the Application class and multiple Command Classes.

       This application is created in fundamentally the same way as the simple one presented
       earlier.  It uses more commands, more Application class/Command Class hooks, and
       subcommands.  The code is much longer but almost all of it is for business logic -- very
       little additional CLIF-specific code is needed.

       The example code shows how various commands can be managed by an Application subclass.
       The code is commented thoroughly to explain the various hooks that are available for
       Application class and Command Classes.

       Of course, CLIF applications can always be used in non-interactive mode:

           # ---- non-interactive mode ----
           $ examples/queue --qout=/tmp/qfile enqueue 'first'
           $ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile enqueue --tag=x --tag=y 'second'
           $ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile property list
           $ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile property set --evens
           $ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile property list
           $ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile enqueue 17
           $ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile enqueue 4
           $ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile enqueue 2
           $ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile dequeue
           $ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile dequeue
           $ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile dequeue
           $ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile enqueue 3
           $ examples/queue --qin=/tmp/qfile print

PLANNING A COMPLEX CLIF APPLICATION

       Little additional thought (beyond that needed for business logic) is required to create a
       basic CLIF app -- the strategy explained in "FROM P.O.S. TO CLIF IN A FEW EASY STEPS"
       demonstrates how CLIF differs from a "Plain Old Script".

       A more sophisticated command line application will benefit from a wider variety of the
       features CLIF provides.  The extra features are easy to use, but the additional complexity
       warrants careful planning.

       After the initial learning curve, applying interface design principles and implementing
       business rules will become the only challenging aspects to developing your CLIF
       applications.  This is as it should be -- the framework handles application-independent
       aspects, leaving you to focus on the unique features of your application.

       Here are some considerations:

       Basic interface
           What commands and subcommands should be available?
           What options and arguments will they support?
           What kind of validation should be done on the provided command requests?
           Which built-in commands will be used?
           Will an interactive mode be provided?
           If so, will a custom menu be created?
           Do any commands need to directly access or modify the application itself or the other
           commands (these will be metacommands)?
       High-level code layout
           Which components of the application will be defined in their own package files?  Which
           will be defined inline?

       Separation of concerns using MVC strategy
           How will the model be separated from the rest of the application?  What about the
           view?

       Data sharing between application and commands
           What data will data be shared between the application and the commands?  Will this be
           arranged by using the cache, using a Command superclass (a generic command class that
           all of your commands inherit from), or by some other means?

       Read on for possible answers to some of these questions.

HOW CAN I ...?

       This section briefly highlights how CLIF could be used to support various common goals.
       Even if your particular situation does not appear here, reading this short section will
       give you an understanding of how CLIF could be set up to support novel cases.

   How can I quickly create a very simple application?
       For a demonstration of how to create a very simple CLIF app, see "FROM P.O.S. TO CLIF IN A
       FEW EASY STEPS".  CLIF applications require, at the minimum:

       ·   An Application class that inherits from CLI::Framework::Application (or
           "CLI::Framework").  For anything useful to happen, it should override the
           "command_map()" hook and include a new command.

       ·   A Command Class that inherits from CLI::Framework::Command.  It should override the
           "run()" hook (or have a subcommand that overrides "run()").

       ·   An Application Script that calls the "run()" method in your application.

       These can all be defined in one file or each class can be placed in a separate file.  Do
       whatever works best for your particular needs.

   How can I add an interactive mode to my application?
       The built-in console command can be used to enable your application to run interactively.
       To do this, simply add the built-in command CLI::Framework::Command::Console to the
       command_map in your Application class.

   How can I include logging in my application?
       In your Application class, define "init()" to initialize your logging object and save the
       resulting object in the cache, where the object will be available to your application and
       command objects.

   How can I include database connectivity in my application?
       In your Application class, define "init()" to connect to your database and save the
       resulting object or database handle in the cache, where the object/handle will be
       available to your application and command objects.

       Of course, for proper Separation of Concerns, you should not simply store a connected
       database handle in the cache and use it directly in your Command classes.  You should
       instead store an object of another class that encapsulates your data model layer code.  An
       example of this is the model class for the demo journal application included with CLIF
       tests: t/lib/My/Journal/Model.pm.

   How can I support an application configuration file?
       In your Application class, define "init()" to load your configuration file and save the
       resulting configuration object in the cache using the cache, where the object will be
       available to your application and command objects.

   How can I use templates for more flexible output?
       In your Application class, override the "render()" method.

       For instance, you could write an application where all commands return a data structure to
       be used in processing a template.  Your "render()" method could determine which template
       file to process (e.g. based on which command is being run) and then process it using the
       received data structure.

   How can I create an application-aware command?
       In exceptional cases, you may need to create a command that "knows about" the application
       and needs access to some of its data (which may include the data of other commands in the
       application).

       To create an application-aware command, inherit from CLI::Framework::Command::Meta.  The
       command will then have an accessor that will provide access to the application object.

       You should generally not need to do this -- your commands should usually be decoupled from
       your application.  This will occur by default when you inherit from
       CLI::Framework::Command.

   How can I use alternative CLI prompting techniques and terminal I/O convenience functions?
       You may, for example, want to present a menu of options from a variety of choices based on
       content from a database.  Or perhaps you want to prompt the user for a list of numbers and
       you want to support a comma-separated list with ranges, etc.

       Create a CLI::Framework::Command subclass (say, "Your::Command") that implements your
       convenience functions or uses a CPAN module such as Term::Prompt.  Then all of your
       commands can inherit from "Your::Command" and will all have access to the functions.

       You may also want to override read_cmd.

   How can I create an app without a "help" command?
       The 'help' command is fundamental to most applications.  If you really want to build an
       application without a 'help' command, simply create a custom Help command with an empty
       "run" method.

   How can I dynamically determine whether or not to run interactively based on command-line
       options?
       You may wish to provide an application option ("--interactive") to start interactive mode.
       One way to do this is to use your application's "init" method to determine whether or not
       to invoke the built-in console command.  For example:

           sub init {
               my ($app, $opts) = @_;
               # imagine fancy logic to determine whether or not to run interactively...
               if( $opts->{interactive} ) {
                   $app->set_current_command('console');
               }
               return 1;
           }

       This will cause the interactive console to be launched during initialization.  This
       technique could be used to launch the built-in console command or a custom interactive
       command.

       This was considered in greater detail on the discussion forum:
       <http://cpanforum.com/posts/12426>.

TROUBLESHOOTING

       The following solutions may be helpful when working with CLIF.

       ·   Don't forget to inherit from CLI::Framework::Application in your Application class and
           CLI::Framework::Command in your command class

       ·   Don't forget to override command_map() in your Application class

       ·   Don't forget to override run() in your Command class

       ·   If in doubt, run "perl -wc <your command class file>"

           If a user-defined command class does not compile, your CLIF application will fail
           silently.  Running "perl -wc Class.pm" will report compilation problems for Class.pm.

LICENSE AND COPYRIGHT

       Copyright (c) 2009 Karl Erisman (kerisman@cpan.org). All rights reserved.

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

AUTHOR

       Karl Erisman (kerisman@cpan.org)