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<label class="toc-overlay-icon toc-content-icon" for="__toc">
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<i class="icon"><svg><use href="#svg-toc"></use></svg></i>
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</label>
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<section id="introspection">
|
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<h1>Introspection<a class="headerlink" href="#introspection" title="Permalink to this headline">#</a></h1>
|
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<p>GObject introspection (abbreviated G-I) is a system which extracts APIs from C
|
||
code and produces binary type libraries which can be used by non-C language
|
||
bindings, and other tools, to introspect or wrap the original C libraries. It
|
||
uses a system of annotations in documentation comments in the C code to expose
|
||
extra information about the APIs which is not machine readable from the code
|
||
itself.</p>
|
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<section id="using-introspection">
|
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<h2>Using Introspection<a class="headerlink" href="#using-introspection" title="Permalink to this headline">#</a></h2>
|
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<p>The first step for using introspection is to add it to the build system. This
|
||
should be done early in the life of a project, as introspectability affects API
|
||
design. When using the Meson build system, you can use the <code class="docutils literal notranslate"><span class="pre">generate_gir()</span></code>
|
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function provided by the <a class="reference external" href="https://mesonbuild.com/Gnome-module.html#gnomegenerate_gir">gnome module</a>:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">lib</span> <span class="o">=</span> <span class="n">library</span><span class="p">(</span><span class="o">...</span><span class="p">)</span>
|
||
<span class="n">gir_files</span> <span class="o">=</span> <span class="n">gnome</span><span class="o">.</span><span class="n">generate_gir</span><span class="p">(</span><span class="n">lib</span><span class="p">,</span>
|
||
<span class="c1"># The list of files to be parsed</span>
|
||
<span class="n">sources</span><span class="p">:</span> <span class="p">[</span><span class="n">public_headers</span><span class="p">,</span> <span class="n">sources</span><span class="p">],</span>
|
||
<span class="c1"># The namespace of the library</span>
|
||
<span class="n">namespace</span><span class="p">:</span> <span class="n">ns</span><span class="p">,</span>
|
||
<span class="c1"># The version of the API</span>
|
||
<span class="n">nsversion</span><span class="p">:</span> <span class="n">ns_version</span><span class="p">,</span>
|
||
<span class="c1"># The pkg-config file exported by the library</span>
|
||
<span class="n">export_packages</span><span class="p">:</span> <span class="s1">'your-library-1'</span><span class="p">,</span>
|
||
<span class="c1"># The GIR files of the dependencies of the library</span>
|
||
<span class="n">includes</span><span class="p">:</span> <span class="n">gir_dependencies</span><span class="p">,</span>
|
||
<span class="c1"># The public header of the library</span>
|
||
<span class="n">header</span><span class="p">:</span> <span class="s1">'your-library.h'</span><span class="p">,</span>
|
||
<span class="n">extra_args</span><span class="p">:</span> <span class="p">[</span><span class="s1">'--quiet'</span><span class="p">,</span> <span class="s1">'--warn-all'</span><span class="p">],</span>
|
||
<span class="n">install</span><span class="p">:</span> <span class="n">true</span><span class="p">,</span>
|
||
<span class="p">)</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>This should result in a <code class="docutils literal notranslate"><span class="pre">.gir</span></code> and <code class="docutils literal notranslate"><span class="pre">.typelib</span></code> files being generated for the
|
||
project.</p>
|
||
<p>The GIR file is human readable, and can be inspected manually to see if the API
|
||
has been introspected correctly (although the GIR compilation process will print
|
||
error messages and warnings for any missing annotations or other problems). The
|
||
GIR file is typically used by bindings that generate code, or to generate the
|
||
API reference for your project. The <code class="docutils literal notranslate"><span class="pre">typelib</span></code> file is an efficient binary
|
||
representation of the GIR data, which can be opened at run time by dynamic
|
||
languages.</p>
|
||
<div class="admonition important">
|
||
<p class="admonition-title">Important</p>
|
||
<p>APIs with introspectable=”0” will not be exposed to language bindings as they
|
||
are missing annotations or are otherwise not representable in the GIR file.</p>
|
||
</div>
|
||
</section>
|
||
<section id="annotations">
|
||
<h2>Annotations<a class="headerlink" href="#annotations" title="Permalink to this headline">#</a></h2>
|
||
<p>The next step is to add annotations to the documentation comments for every
|
||
piece of public API. If a particular piece of API should not be exposed in the
|
||
GIR file, use the <code class="docutils literal notranslate"><span class="pre">(skip)</span></code> annotation. Documentation on the available
|
||
annotations is available on the <a class="reference external" href="https://gi.readthedocs.io/en/latest/annotations/giannotations.html">G-I website</a>.</p>
|
||
<p>If annotating the code for a program, a good approach is to split the bulk of
|
||
the code out into an internal, private convenience library. An internal API
|
||
reference manual can be built from its documentation comments. The library is
|
||
then not installed, but is linked in to the program which is itself installed.
|
||
This approach for generating internal API documentation is especially useful for
|
||
large projects where the internal code may be large and hard to navigate.</p>
|
||
<p>Annotations do not have to be added exhaustively: GIR has a set of default
|
||
annotations which it applies based on various conventions. For example, a
|
||
<code class="docutils literal notranslate"><span class="pre">const</span> <span class="pre">char*</span></code> parameter or return value does not need an explicit <code class="docutils literal notranslate"><span class="pre">(transfer</span>
|
||
<span class="pre">none)</span></code> annotation, because the <code class="docutils literal notranslate"><span class="pre">const</span></code> modifier implies this already.
|
||
Learning the defaults for annotations is a matter of practice.</p>
|
||
</section>
|
||
<section id="api-design">
|
||
<h2>API Design<a class="headerlink" href="#api-design" title="Permalink to this headline">#</a></h2>
|
||
<p>In order to be introspectable without too many annotations, APIs must follow
|
||
certain conventions, such as the standard GObject naming conventions, and the
|
||
conventions for bindable APIs. This is necessary because of the flexibility of
|
||
C: code can be written to behave in any way imaginable, but higher level
|
||
languages don’t allow this kind of freedom. So in order for a C API to be
|
||
representable in a higher level language, it has to conform to the behaviors
|
||
supported by that language.</p>
|
||
<p>Additionally, the same C API will be accessed by multiple languages, each with
|
||
potentially different behaviors; adhering to the conventions and best practices
|
||
of GObject will ensure that the API remains consistent across different
|
||
languages.</p>
|
||
<p>A quick list of conventions to follow:</p>
|
||
<dl class="simple">
|
||
<dt>Do not rely exclusively on C pre-processor macros:</dt><dd><p>The introspection scanner only understands macros that evaluates to a
|
||
constant value; if you have complex functionality, always use a real
|
||
function.</p>
|
||
</dd>
|
||
<dt>Expose a vector-based function for every variadic arguments one</dt><dd><p>Variadic arguments are not supported by every language, so you should
|
||
ensure you have a vector-based variant of any variadic arguments or
|
||
<code class="docutils literal notranslate"><span class="pre">va_list</span></code> based function in your API</p>
|
||
</dd>
|
||
<dt>Constructor functions should only call <code class="docutils literal notranslate"><span class="pre">g_object_new()</span></code></dt><dd><p>You should not have constructor functions that set internal details
|
||
of an instance, as most dynamic languages will call <code class="docutils literal notranslate"><span class="pre">g_object_new()</span></code>
|
||
directly. An exception are functions that return a singleton or
|
||
work as factory constructors, but those typically are classified as
|
||
static type functions.</p>
|
||
</dd>
|
||
<dt>Do not use the same name for properties and methods</dt><dd><p>Bindings for various programming languages expose properties and methods
|
||
in the same way, and will lead to collisions.</p>
|
||
</dd>
|
||
</dl>
|
||
<p>For a complete list of conventions to follow, please see the <a class="reference external" href="https://gi.readthedocs.io/en/latest/writingbindableapis.html">G-I website</a>.</p>
|
||
<p>The <code class="docutils literal notranslate"><span class="pre">g-ir-scanner</span></code> tool will emit warnings whenever it encounters code it
|
||
cannot understand. You should make sure to pass <code class="docutils literal notranslate"><span class="pre">--warn-all</span></code> to see a full
|
||
list of all potential warnings.</p>
|
||
<div class="admonition hint">
|
||
<p class="admonition-title">Hint</p>
|
||
<p>You may want to set <code class="docutils literal notranslate"><span class="pre">fatal_warnings:</span> <span class="pre">get_option('werror')</span></code> in the
|
||
<code class="docutils literal notranslate"><span class="pre">generate_gir()</span></code> arguments to ensure that any introspection warning
|
||
will stop the build in your continuous integration pipeline, just like
|
||
any compiler warning would; this is useful to make sure that all newly
|
||
added API is introspectable.</p>
|
||
</div>
|
||
</section>
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Contents
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<div class="toc-tree-container">
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<div class="toc-tree">
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<ul>
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<li><a class="reference internal" href="#">Introspection</a><ul>
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<li><a class="reference internal" href="#using-introspection">Using Introspection</a></li>
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<li><a class="reference internal" href="#annotations">Annotations</a></li>
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<li><a class="reference internal" href="#api-design">API Design</a></li>
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