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  <section id="introduction">
<h1>Introduction<a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h1>
<section id="units">
<h2>Units<a class="headerlink" href="#units" title="Permalink to this headline">¶</a></h2>
<p>Units are the basic building block in Crown. A Unit represents a single
logical object in the game world: a character, a light, a prop, and so on.</p>
<figure class="align-center" id="id1">
<img alt="../_images/units.png" src="../_images/units.png" />
<figcaption>
<p><span class="caption-text">A unit imported from a scene, a camera unit, and a unit representing a sprite.</span><a class="headerlink" href="#id1" title="Permalink to this image">¶</a></p>
</figcaption>
</figure>
<p>At the most fundamental level, Units are just numeric IDs that identify a
specific object in a World. Units on their own are empty. You add Components
to a Unit to give it behaviour and appearance.</p>
</section>
<section id="components">
<h2>Components<a class="headerlink" href="#components" title="Permalink to this headline">¶</a></h2>
<p>Contrary to some other engines, in Crown all Unit Components are optional,
even the Transform Component can be omitted. This way you can have purely
functional Units with no runtime cost.</p>
<figure class="align-center" id="id2">
<img alt="../_images/components.png" src="../_images/components.png" />
<figcaption>
<p><span class="caption-text">A “light” unit with its transform component displayed in the Inspector.</span><a class="headerlink" href="#id2" title="Permalink to this image">¶</a></p>
</figcaption>
</figure>
<p>Units can have any number of components attached to them, but only one
component per type is allowed.</p>
</section>
<section id="hierarchies">
<h2>Hierarchies<a class="headerlink" href="#hierarchies" title="Permalink to this headline">¶</a></h2>
<p>Units can be organised in parent/child hierarchies. This allows the creation
of complex units from a number of simpler ones. To create a Unit with two
lights, for example, you will create separate units each with their own Light
Component, and then link the two together via a Transform Component.</p>
</section>
<section id="prefabs">
<h2>Prefabs<a class="headerlink" href="#prefabs" title="Permalink to this headline">¶</a></h2>
<p>A unit saved in the Project as <code class="docutils literal notranslate"><span class="pre">.unit</span></code> resource is called Prefab. Prefabs
are a powerful way to reuse units. Prefabs can store not just a single unit,
but entire hierarchies with all their components and properties.</p>
<p>A prefab inherits all units and components of its base prefab, if any.
Inherited units and components in the derived prefab can be removed,
overridden, or complemented with new units and components.</p>
<p>Modifications to a base prefab are automatically propagated to all derived
prefabs.</p>
<p>The prebab system can be used to implement many popular patterns and
techniques. The most common use for a prefab is to leverage its inheritance
system to implement OOP-style behavior; you could have:</p>
<ul class="simple">
<li><p>a base <code class="docutils literal notranslate"><span class="pre">enemy</span></code> prefab that defines the common geometry, animations and
scripts;</p></li>
<li><p>derived prefabs such as <code class="docutils literal notranslate"><span class="pre">enemy_heavy</span></code> and <code class="docutils literal notranslate"><span class="pre">enemy_fast</span></code> that override
only materials, health values or behaviour parameters.</p></li>
</ul>
</section>
</section>


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