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<div class="navheader"><table width="100%" summary="Navigation header"><tr><td width="20%" align="left"><a accesskey="p" href="plugins.html">Prev</a> </td><th width="60%" align="center"> </th><td width="20%" align="right"> <a accesskey="n" href="about.html">Next</a></td></tr></table><hr /></div><div class="chapter"><div class="titlepage"><div><div><h2 class="title"><a id="plugins_usage"></a>Chapter 22. External plugins</h2></div></div></div><p>It is often convenient to be able to keep the build files
used by all your projects in one place. Those files could
be Makefiles, configuration files, templates and more.</p><p>Erlang.mk allows you to automatically load plugins from
dependencies. Plugins can do anything, including defining
new variables, defining file templates, hooking themselves
inside the normal Erlang.mk processing or even adding new
rules.</p><p>You can load plugins using one of two methods. You can
either load all plugins from a dependency, or just one.
We will also cover conventions about writing external
plugins.</p><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="_loading_all_plugins_from_a_dependency"></a>22.1. Loading all plugins from a dependency</h2></div></div></div><p>To load plugins from a dependency, all you need to do is add
the dependency name to <code class="literal">DEP_PLUGINS</code> in addition to the list
of dependencies.</p><p>For example, if you have <code class="literal">cowboy</code> in <code class="literal">DEPS</code>, add <code class="literal">cowboy</code> in
<code class="literal">DEP_PLUGINS</code> also:</p><pre class="programlisting">DEPS = cowboy
DEP_PLUGINS = cowboy</pre><p>This will load the file <span class="emphasis"><em>plugins.mk</em></span> in the root folder of
the Cowboy repository.</p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="_loading_one_plugin_from_a_dependency"></a>22.2. Loading one plugin from a dependency</h2></div></div></div><p>Now that we know how to load all plugins, let’s take a look
at how to load one specific plugin from a dependency.</p><p>To do this, instead of writing only the name of the dependency,
we will write its name and the path to the plugin file. This
means that writing <code class="literal">DEP_PLUGINS = cowboy</code> is equivalent to
writing <code class="literal">DEP_PLUGINS = cowboy/plugins.mk</code>.</p><p>Knowing this, if we were to load the plugin <span class="emphasis"><em>mk/dist.mk</em></span>
from Cowboy and no other, we would write the following in
our Makefile:</p><pre class="programlisting">DEPS = cowboy
DEP_PLUGINS = cowboy/mk/dist.mk</pre></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="_writing_external_plugins"></a>22.3. Writing external plugins</h2></div></div></div><p>The <span class="emphasis"><em>plugins.mk</em></span> file is a convention. It is meant to load
all the plugins from the dependency. The code for the plugin
can be written directly in <span class="emphasis"><em>plugins.mk</em></span> or be separate.</p><p>If you are providing more than one plugin with your repository,
the recommended way is to create one file per plugin in the
<span class="emphasis"><em>mk/</em></span> folder in your repository, and then include those
individual plugins in <span class="emphasis"><em>plugins.mk</em></span>.</p><p>For example, if you have two plugins <span class="emphasis"><em>mk/dist.mk</em></span> and
<span class="emphasis"><em>mk/templates.mk</em></span>, you could write the following <span class="emphasis"><em>plugins.mk</em></span>
file:</p><pre class="programlisting">THIS := $(dir $(realpath $(lastword $(MAKEFILE_LIST))))
include $(THIS)/mk/dist.mk
include $(THIS)/mk/templates.mk</pre><p>The <code class="literal">THIS</code> variable is required to relatively include files.</p><p>This allows users to not only be able to select individual
plugins, but also select all plugins from the dependency
in one go if they wish to do so.</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="plugins.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="plugins.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="about.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top"> </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> </td></tr></table></div>
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