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<section id="variable-watch">
<span id="command:variable_watch"></span><h1>variable_watch<a class="headerlink" href="#variable-watch" title="Permalink to this heading">ΒΆ</a></h1>
<p>Watch the CMake variable for change.</p>
<div class="highlight-cmake notranslate"><div class="highlight"><pre><span></span><span class="nf">variable_watch(</span><span class="nv"><variable></span><span class="w"> </span><span class="p">[</span><span class="nv"><command></span><span class="p">]</span><span class="nf">)</span><span class="w"></span>
</pre></div>
</div>
<p>If the specified <code class="docutils literal notranslate"><span class="pre"><variable></span></code> changes and no <code class="docutils literal notranslate"><span class="pre"><command></span></code> is given,
a message will be printed to inform about the change.</p>
<p>If <code class="docutils literal notranslate"><span class="pre"><command></span></code> is given, this command will be executed instead.
The command will receive the following arguments:
<code class="docutils literal notranslate"><span class="pre">COMMAND(<variable></span> <span class="pre"><access></span> <span class="pre"><value></span> <span class="pre"><current_list_file></span> <span class="pre"><stack>)</span></code></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre"><variable></span></code></dt><dd><p>Name of the variable being accessed.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre"><access></span></code></dt><dd><p>One of <code class="docutils literal notranslate"><span class="pre">READ_ACCESS</span></code>, <code class="docutils literal notranslate"><span class="pre">UNKNOWN_READ_ACCESS</span></code>, <code class="docutils literal notranslate"><span class="pre">MODIFIED_ACCESS</span></code>,
<code class="docutils literal notranslate"><span class="pre">UNKNOWN_MODIFIED_ACCESS</span></code>, or <code class="docutils literal notranslate"><span class="pre">REMOVED_ACCESS</span></code>. The <code class="docutils literal notranslate"><span class="pre">UNKNOWN_</span></code>
values are only used when the variable has never been set. Once set,
they are never used again during the same CMake run, even if the
variable is later unset.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre"><value></span></code></dt><dd><p>The value of the variable. On a modification, this is the new
(modified) value of the variable. On removal, the value is empty.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre"><current_list_file></span></code></dt><dd><p>Full path to the file doing the access.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre"><stack></span></code></dt><dd><p>List of absolute paths of all files currently on the stack of file
inclusion, with the bottom-most file first and the currently
processed file (that is, <code class="docutils literal notranslate"><span class="pre">current_list_file</span></code>) last.</p>
</dd>
</dl>
<p>Note that for some accesses such as <span class="target" id="index-0-command:list"></span><a class="reference internal" href="list.html#command:list" title="list"><code class="xref cmake cmake-command docutils literal notranslate"><span class="pre">list(APPEND)</span></code></a>, the watcher
is executed twice, first with a read access and then with a write one.
Also note that an <span class="target" id="index-0-command:if"></span><a class="reference internal" href="if.html#command:if" title="if"><code class="xref cmake cmake-command docutils literal notranslate"><span class="pre">if(DEFINED)</span></code></a> query on the variable does not
register as an access and the watcher is not executed.</p>
<p>Only non-cache variables can be watched using this command. Access to
cache variables is never watched. However, the existence of a cache
variable <code class="docutils literal notranslate"><span class="pre">var</span></code> causes accesses to the non-cache variable <code class="docutils literal notranslate"><span class="pre">var</span></code> to
not use the <code class="docutils literal notranslate"><span class="pre">UNKNOWN_</span></code> prefix, even if a non-cache variable <code class="docutils literal notranslate"><span class="pre">var</span></code>
has never existed.</p>
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