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Editing: PluginAuthor.pod

# PODNAME: Alien::Build::Manual::PluginAuthor
# ABSTRACT: Alien::Build plugin author documentation
# VERSION

__END__

=pod

=encoding UTF-8

=head1 NAME

Alien::Build::Manual::PluginAuthor - Alien::Build plugin author documentation

=head1 VERSION

version 2.80

=head1 SYNOPSIS

your plugin:

package Alien::Build::Plugin::Build::MyPlugin;
 
 use strict;
 use warnings;
 use Alien::Build::Plugin;
 
 has arg1 => 'default_for arg1';
 has arg2 => sub { [ 'default', 'for', 'arg2' ] };
 
 sub init
 {
   my($self, $meta) = @_;
   ...
 }
 
 1;

and then from L<alienfile>:

use alienfile;
 plugin 'Build::MyPlugin' => (
   arg1 => 'override for arg1',
   arg2 => [ 'something', 'else' ],
 );

=for html <p>flowchart</p>
<div style="display: flex"><div style="margin: 3px; flex: 1 1 50%">
<img src="image/PluginAuthor-flowchart.png" style="max-width: 100%">
</div></div>
<p><b>Notes</b>: The colored blocks indicate <tt>alienfile</tt> blocks.
Hooks are indicated as predefined process (rectangle with double struck
vertical edges).  Hooks that can easily be implemented from an
<tt>alienfile</tt> are indicated in blue (Note that <tt>[]</tt> is used
to indicate passing in an array reference, but a subroutine
reference can also be used).  For simplicity, the the flowchart does
not include when required modules are loaded.  Except for configure
time requirements, they are loaded when the corresponding <tt>alienfile</tt>
blocks are entered.  It is not shown, but generally any plugin can cause
a <b>Fail</b> by throwing an exception with <tt>die</tt>.</p>

Perlish pseudo code for how plugins are called:

my $probe;
 my $override = override();
 
 if($override eq 'system') {
 
   $probe = probe();
 
   if($probe ne 'system') {
     die 'system tool or library not found';
   }
 
 }
 
 elsif($override eq 'default') {
   $probe = probe();
 
 } else { # $override eq 'share'
   # note that in this instance the
   # probe hook is never called
   $probe = 'share';
 }
 
 if($probe eq 'system') {
   gather_system();
 
 } else { # $probe eq 'share'
 
   download();
   extract();
   patch();
   build();
   gather_share();
 
   # Check to see if there isa build_ffi hook
   if(defined &build_ffi) {
     patch_ffi();
     build_ffi();
     gather_ffi();
   }
 }
 
 # By default this just returns the value of $ENV{ALIEN_INSTALL_TYPE}
 sub override {
   return $ENV{ALIEN_INSTALL_TYPE};
 }
 
 # Default download implementation; can be
 # replaced by specifying a different download
 # hook.  See Alien::Build::Plugin::Core::Download
 # for detailed implementation.
 sub download {
 
   my $response = fetch();
 
   if($response->{type} eq 'html' || $response->{type} eq 'dir_listing') {
     # decode will transform an HTML listing (html) or a FTP directory
     # listing (dir_listing) into a regular list
     $response = decode($response);
   }
 
   if($response->{type} eq 'list') {
 
     # prefer will filter bad entries in the list
     # and sort them so that the first one is
     # the one that we want
     $response = prefer($response);
 
     my $first_preferred = $res->{list}->[0];
 
     # prefer can sometimes infer the version from the
     # filename.
     if(defined $first_preferred->{version}) {
       # not a hook
       runtime_prop->{version} = $first_preferred->{version};
     }
 
     $response = fetch($first_preferred);
 
   }
 
   if($response->{type} eq 'file') {
     # not a hook
     write_file_to_disk $response;
   }
 
 }

=head1 DESCRIPTION

This document explains how to write L<Alien::Build> plugins using the
L<Alien::Build::Plugin> base class.

=head2 Writing plugins

Plugins use L<Alien::Build::Plugin>, which sets the appropriate base
class, and provides you with the C<has> property builder.  C<has> takes
two arguments, the name of the property and the default value.  (As
with L<Moose> and L<Moo>, you should use a code reference to specify
default values for non-string defaults).  No B<not> set this as your
plugin's base class directly:

use parent qw( Alien::Build::Plugin );  # wrong
 use Alien::Build::Plugin;               # right

The only method that you need to implement is C<init>.  From this method
you can add hooks to change the behavior of the L<alienfile> recipe.
This is a very simple example of a probe hook, with the actual probe
logic removed:

sub init
 {
   my($self, $meta) = @_;
   $meta->register_hook(
     probe => sub {
       my($build) = @_;
       if( ... )
       {
         return 'system';
       }
       else
       {
         return 'share';
       }
     },
   );
 }

Hooks get the L<Alien::Build> instance as their first argument, and depending
on the hook may get additional arguments.

=head2 Modifying hooks

You can also modify hooks using C<before_hook>, C<around_hook> and C<after_hook>,
similar to L<Moose> modifiers:

sub init
 {
   my($self, $meta) = @_;
 
   $meta->before_hook(
     build => sub {
       my($build) = @_;
       $build->log('this runs before the build');
     },
   );
 
   $meta->after_hook(
     build => sub {
       my($build) = @_;
       $build->log('this runs after the build');
     },
   );
 
   $meta->around_hook(
     build => sub {
       my $orig = shift;
 
       # around hooks are useful for setting environment variables
       local $ENV{CPPFLAGS} = '-I/foo/include';
 
       $orig->(@_);
     },
   );
 }

=head2 Testing plugins

You can and should write tests for your plugin.  The best way to do
this is using L<Test::Alien::Build>, which allows you to write an
inline L<alienfile> in your test.  Here is an example:

use Test::V0;
 use Test::Alien::Build;
 
 my $build = alienfile_ok q{
   use alienfile;
   plugin 'Build::MyPlugin' => (
     arg1 => 'override for arg1',
     arg2 => [ 'something', 'else' ],
   );
   ...
 };
 
 # you can interrogate $build, it is an instance of L<Alien::Build>.
 
 my $alien = alien_build_ok;
 
 # you can interrogate $alien, it is an instance of L<Alien::Base>.

=head2 Negotiator plugins

A Negotiator plugin doesn't itself typically implement anything on
its own, but picks the best plugin to achieve a particular goal.

The "best" plugin can in some cases vary depending on the platform
or tools that are available.  For example The
L<download negotiator|Alien::Build::Plugin::Download::Negotiate>
might choose to use the fetch plugin that relies on the command line
C<curl>, or it might choose the fetch plugin that relies on the Perl
module L<HTTP::Tiny> depending on the platform and what is already
installed.  (For either to be useful they have to support SSL).

The Negotiator plugin is by convention named something like
C<Alien::Build::Plugin::*::Negotiate>, but is typically invoked
without the C<::Negotiate> suffix.  For example:

plugin 'Download'; # is short for Alien::Build::Plugin::Download::Negotiator

Here is a simple example of a negotiator which picks C<curl> if already
installed and L<HTTP::Tiny> otherwise.  (The actual download plugin
is a lot smarter and complicated than this, but this is a good
simplified example).

package Alien::Build::Plugin::Download::Negotiate;
 
 use strict;
 use warnings;
 use Alien::Build::Plugin;
 use File::Which qw( which );
 
 sub init
 {
   my($self, $meta) = @_;
 
   if(which('curl')) {
     $meta->apply_plugin('Fetch::Curl');
   } else {
     $meta->apply_plugin('Fetch::HTTPTiny');
   }
 }

=head2 Hooks

The remainder of this document is a reference for the hooks that you
can register.  Generally speaking you can register any hook that you
like, but some care must be taken as some hooks have default behavior
that will be overridden when you register a hook.  The hooks are
presented in alphabetical order.  The execution order is shown
in the flowchart above (if you are browsing the HTML version of this
document), or the Perlish pseudo code in the synopsis section.

=head1 HOOKS

=head2 build hook

$meta->register_hook( build => sub {
   my($build) = @_;
   ...
 });

This does the main build of the alienized project and installs it into
the staging area.  The current directory is the build root.  You need
to run whatever tools are necessary for the project, and install them
into C<$build->install_prop->{prefix}> (C<%{.install.prefix}>).

=head2 build_ffi hook

$meta->register_hook( build_ffi => sub {
   my($build) = @_;
   ...
 });

This is the same as L<build|/"build hook">, except it fires only on a FFI build.

=head2 decode hook

$meta->register_hook( decode => sub {
   my($build, $res) = @_;
   ...
 }

This hook takes a response hash reference from the C<fetch> hook above
with a type of C<html> or C<dir_listing> and converts it into a response
hash reference of type C<list>.  In short it takes an HTML or FTP file
listing response from a fetch hook and converts it into a list of filenames
and links that can be used by the prefer hook to choose the correct file to
download.  See the L<fetch hook|/"fetch hook"> for the specification of the
input and response hash references.

=head2 check_digest hook

# implement the well known FOO-92 digest
 $meta->register_hook( check_digest => sub {
   my($build, $file, $algorithm, $digest) = @_;
   if($algorithm ne 'FOO92') {
     return 0;
   }
   my $actual = foo92_hex_digest($file);
   if($actual eq $digest) {
     return 1;
   } else {
     die "Digest FOO92 does not match: got $actual, expected $digest";
   }
 });

This hook should check the given C<$file> (the format is the same as used by
L<the fetch hook|/"fetch hook">) matches the given C<$digest> using the
given C<$algorithm>.  If the plugin does not support the given algorithm,
then it should return a false value.  If the digest does not match, it
should throw an exception.  If the digest matches, it should return a
true value.

=head2 clean_install

$meta->register_hook( clean_install => sub {
   my($build) = @_;
 });

This hook allows you to remove files from the final install location before
the files are installed by the installer layer (examples: L<Alien::Build::MM>,
L<Alien::Build::MB> or L<App::af>).  This hook is not called by default,
and must be enabled via the interface to the installer layer
(example: L<Alien::Build::MM/clean_install>).

This hook SHOULD NOT remove the C<_alien> directory or its content from the
install location.

The default implementation removes all the files EXCEPT the C<_alien> directory
and its content.

=head2 download hook

$meta->register_hook( download => sub {
   my($build) = @_;
   ...
 });

This hook is used to download from the internet the source.  Either as
an archive (like tar, zip, etc), or as a directory of files (C<git clone>,
etc).  When the hook is called, the current working directory will be a
new empty directory, so you can save the download to the current
directory.  If you store a single file in the directory, L<Alien::Build>
will assume that it is an archive, which will be processed by the
L<extract hook|/"extract hook">.  If you store multiple files, L<Alien::Build> will
assume the current directory is the source root.  If no files are stored
at all, an exception with an appropriate diagnostic will be thrown.

B<Note>: If you register this hook, then the fetch, decode and prefer
hooks will NOT be called, unless you call them yourself from this hook.

=head2 extract hook

$meta->register_hook( extract => sub {
   my($build, $archive) = @_;
   ...
 });

This hook is used to extract an archive that has already been downloaded.
L<Alien::Build> already has plugins for the most common archive formats,
so you will likely only need this to add support for new or novel archive
formats.  When this hook is called, the current working directory will
be a new empty directory, so you can save the content of the archive to
the current directory.  If a single directory is written to the current
directory, L<Alien::Build> will assume that is the root directory of the
package.  If multiple files and/or directories are present, that will
indicate that the current working directory is the root of the package.
The logic typically handles correctly the default behavior for tar
(where packages are typically extracted to a subdirectory) and for
zip (where packages are typically extracted to the current directory).

=head2 fetch hook

package Alien::Build::Plugin::MyPlugin;
 
 use strict;
 use warnings;
 use Alien::Build::Plugin;
 use Carp ();
 
 has '+url' => sub { Carp::croak "url is required property" };
 
 sub init
 {
   my($self, $meta) = @_;
 
   $meta->register_hook( fetch => sub {
     my($build, $url, %options) = @_;
     ...
   }
 }
 
 1;

Used to fetch a resource.  The first time it will be called without an
argument (or with C<$url> set to C<undef>, so the configuration used to
find the resource should be specified by the plugin's properties.  On
subsequent calls the first argument will be a URL.

The C<%options> hash may contain these options:

=over 4

=item http_headers

HTTP request headers, if an appropriate protocol is being used.  The
headers are provided as an array reference of key/value pairs, which
allows for duplicate header keys with multiple values.

If a non-HTTP protocol is used, or if the plugin cannot otherwise
send HTTP request headers, the plugin SHOULD issue a warning using
the C<< $build->log >> method, but because this option wasn't part
of the original spec, the plugin MAY no issue that warning while
ignoring it.

=back

Note that versions of L<Alien::Build> prior to 2.39 did not pass the
options hash into the fetch plugin.

Normally the first fetch will be to either a file or a directory listing.
If it is a file then the content should be returned as a hash reference
with the following keys:

# content of file stored in Perl
 return {
   type     => 'file',
   filename => $filename,
   content  => $content,
   version  => $version,  # optional, if known
   protocol => $protocol, # AB 2.60 optional, but recommended
 };
 
 # content of file stored in the filesystem
 return {
   type     => 'file',
   filename => $filename,
   path     => $path,     # full file system path to file
   version  => $version,  # optional, if known
   tmp      => $tmp,      # optional
   protocol => $protocol, # AB 2.60 optional, but recommended
 };

C<$tmp> if set will indicate if the file is temporary or not, and can
be used by L<Alien::Build> to save a copy in some cases.  The default
is true, so L<Alien::Build> assumes the file or directory is temporary
if you don't tell it otherwise.  Probably the most common situation
when you would set C<tmp> to false, is when the file is bundled inside
the L<Alien> distribution.  See L<Alien::Build::Plugin::Fetch::Local>
for example.

If the URL points to a directory listing you should return it as either
a hash reference containing a list of files:

return {
   type => 'list',
   list => [
     # filename: each filename should be just the
     #   filename portion, no path or url.
     # url: each url should be the complete url
     #   needed to fetch the file.
     # version: OPTIONAL, may be provided by some fetch or prefer
     { filename => $filename1, url => $url1, version => $version1 },
     { filename => $filename2, url => $url2, version => $version2 },
   ],
   protocol => $protocol, # AB 2.60 optional, but recommended
 };

or if the listing is in HTML format as a hash reference containing the
HTML information:

return {
   type => 'html',
   charset  => $charset, # optional
   base     => $base,    # the base URL: used for computing relative URLs
   content  => $content, # the HTML content
   protocol => $protocol, # optional, but recommended
 };

or a directory listing (usually produced by an FTP servers) as a hash
reference:

return {
   type     => 'dir_listing',
   base     => $base,
   content  => $content,
   protocol => $protocol, # AB 2.60 optional, but recommended
 };

[version 2.60]

For all of these responses C<$protocol> is optional, since it was not part
of the original spec, however it is strongly recommended that you include
this field, because future versions of L<Alien::Build> will use this to
determine if a file was downloaded securely (that is via a secure protocol
such as SSL).

Some plugins (like L<decode plugins |Alien::Build::Plugin::Decode>) trans
late a file hash from one type to another, they should maintain the
C<$protocol> from the old to the new representation of the file.

=head2 gather_ffi hook

$meta->register_hook( gather_ffi => sub {
   my($build) = @_;
   $build->runtime_prop->{cflags}  = ...;
   $build->runtime_prop->{libs}    = ...;
   $build->runtime_prop->{version} = ...;
 });

This hook is called for a FFI build to determine the properties
necessary for using the library or tool.  These properties should be
stored in the L<runtime_prop|Alien::Build/runtime_prop> hash as shown above.
Typical properties that are needed for libraries are cflags and libs.
If at all possible you should also try to determine the version of the
library or tool.

=head2 gather_share hook

$meta->register_hook( gather_share => sub {
   my($build) = @_;
   $build->runtime_prop->{cflags}  = ...;
   $build->runtime_prop->{libs}    = ...;
   $build->runtime_prop->{version} = ...;
 });

This hook is called for a share install to determine the properties
necessary for using the library or tool.  These properties should be
stored in the L<runtime_prop|Alien::Build/runtime_prop> hash as shown above.
Typical properties that are needed for libraries are cflags and libs.
If at all possible you should also try to determine the version of the
library or tool.

=head2 gather_system hook

$meta->register_hook( gather_system => sub {
   my($build) = @_;
   $build->runtime_prop->{cflags}  = ...;
   $build->runtime_prop->{libs}    = ...;
   $build->runtime_prop->{version} = ...;
 });

This hook is called for a system install to determine the properties
necessary for using the library or tool.  These properties should be
stored in the L<runtime_prop|Alien::Build/runtime_prop> hash as shown above.
Typical properties that are needed for libraries are cflags and libs.
If at all possible you should also try to determine the version of the
library or tool.

=head2 override hook

$meta->register_hook( override => sub {
   my($build) = @_;
   return $ENV{ALIEN_INSTALL_TYPE} || '';
 });

This allows you to alter the override logic.  It should return one of
C<share>, C<system>, C<default> or C<''>.  The default implementation
is shown above.  L<Alien::Build::Plugin::Probe::Override> and
L<Alien::Build::Plugin::Probe::OverrideCI> are examples of how you
can use this hook.

=head2 patch hook

$meta->register_hook( patch => sub {
   my($build) = @_;
   ...
 });

This hook is completely optional.  If registered, it will be triggered after
extraction and before build.  It allows you to apply any patches or make any
modifications to the source if they are necessary.

=head2 patch_ffi hook

$meta->register_hook( patch_ffi => sub {
   my($build) = @_;
   ...
 });

This hook is exactly like the L<patch hook|/"patch hook">, except it fires only on an
FFI build.

=head2 prefer hook

$meta->register_hook( prefer => sub {
   my($build, $res) = @_;
   return {
     type => 'list',
     list => [sort @{ $res->{list} }],
   };
 }

This hook sorts candidates from a listing generated from either the C<fetch>
or C<decode> hooks.  It should return a new list hash reference with the
candidates sorted from best to worst.  It may also remove candidates
that are totally unacceptable.

=head2 probe hook

$meta->register_hook( probe => sub {
   my($build) = @_;
   return 'system' if ...; # system install
   return 'share';         # otherwise
 });
 
 $meta->register_hook( probe => [ $command ] );

This hook should return the string C<system> if the operating
system provides the library or tool.  It should return C<share>
otherwise.

You can also use a command that returns true when the tool
or library is available.  For example for use with C<pkg-config>:

$meta->register_hook( probe =>
   [ '%{pkgconf} --exists libfoo' ] );

Or if you needed a minimum version:

$meta->register_hook( probe =>
   [ '%{pkgconf} --atleast-version=1.00 libfoo' ] );

Note that this hook SHOULD NOT gather system properties, such as
cflags, libs, versions, etc, because the probe hook will be skipped
in the event the environment variable C<ALIEN_INSTALL_TYPE> is set.
The detection of these properties should instead be done by the
L<gather_system|/"gather_system hook"> hook.

Multiple probe hooks can be given.  These will be used in sequence,
stopping at the first that detects a system installation.

=head1 SEE ALSO

=over 4

=item L<Alien::Build::Manual>

Other L<Alien::Build> manuals.

=back

=head1 AUTHOR

Author: Graham Ollis E<lt>plicease@cpan.orgE<gt>

Contributors:

Diab Jerius (DJERIUS)

Roy Storey (KIWIROY)

Ilya Pavlov

David Mertens (run4flat)

Mark Nunberg (mordy, mnunberg)

Christian Walde (Mithaldu)

Brian Wightman (MidLifeXis)

Zaki Mughal (zmughal)

mohawk (mohawk2, ETJ)

Vikas N Kumar (vikasnkumar)

Flavio Poletti (polettix)

Salvador Fandiño (salva)

Gianni Ceccarelli (dakkar)

Pavel Shaydo (zwon, trinitum)

Kang-min Liu (劉康民, gugod)

Nicholas Shipp (nshp)

Juan Julián Merelo Guervós (JJ)

Joel Berger (JBERGER)

Petr Písař (ppisar)

Lance Wicks (LANCEW)

Ahmad Fatoum (a3f, ATHREEF)

José Joaquín Atria (JJATRIA)

Duke Leto (LETO)

Shoichi Kaji (SKAJI)

Shawn Laffan (SLAFFAN)

Paul Evans (leonerd, PEVANS)

Håkon Hægland (hakonhagland, HAKONH)

nick nauwelaerts (INPHOBIA)

Florian Weimer

=head1 COPYRIGHT AND LICENSE

This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.

=cut

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