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  <div class="section" id="strategy-pattern">
<h1>Strategy-Pattern<a class="headerlink" href="#strategy-pattern" title="Permalink to this headline"></a></h1>
<p>This recipe will give you a short introduction on how to design
similar entities without using expensive (i.e. slow) inheritance
but with not more than * the well-known strategy pattern * event
listeners</p>
<div class="section" id="scenario-problem">
<h2>Scenario / Problem<a class="headerlink" href="#scenario-problem" title="Permalink to this headline"></a></h2>
<p>Given a Content-Management-System, we probably want to add / edit
some so-called &#8220;blocks&#8221; and &#8220;panels&#8221;. What are they for?</p>
<ul class="simple">
<li>A block might be a registration form, some text content, a table
with information. A good example might also be a small calendar.</li>
<li>A panel is by definition a block that can itself contain blocks.
A good example for a panel might be a sidebar box: You could easily
add a small calendar into it.</li>
</ul>
<p>So, in this scenario, when building your CMS, you will surely add
lots of blocks and panels to your pages and you will find yourself
highly uncomfortable because of the following:</p>
<ul class="simple">
<li>Every existing page needs to know about the panels it contains -
therefore, you&#8217;ll have an association to your panels. But if you&#8217;ve
got several types of panels - what do you do? Add an association to
every panel-type? This wouldn&#8217;t be flexible. You might be tempted
to add an AbstractPanelEntity and an AbstractBlockEntity that use
class inheritance. Your page could then only confer to the
AbstractPanelType and Doctrine 2 would do the rest for you, i.e.
load the right entities. But - you&#8217;ll for sure have lots of panels
and blocks, and even worse, you&#8217;d have to edit the discriminator
map <em>manually</em> every time you or another developer implements a new
block / entity. This would tear down any effort of modular
programming.</li>
</ul>
<p>Therefore, we need something that&#8217;s far more flexible.</p>
</div>
<div class="section" id="solution">
<h2>Solution<a class="headerlink" href="#solution" title="Permalink to this headline"></a></h2>
<p>The solution itself is pretty easy. We will have one base class
that will be loaded via the page and that has specific behaviour -
a Block class might render the front-end and even the backend, for
example. Now, every block that you&#8217;ll write might look different or
need different data - therefore, we&#8217;ll offer an API to these
methods but internally, we use a strategy that exactly knows what
to do.</p>
<p>First of all, we need to make sure that we have an interface that
contains every needed action. Such actions would be rendering the
front-end or the backend, solving dependencies (blocks that are
supposed to be placed in the sidebar could refuse to be placed in
the middle of your page, for example).</p>
<p>Such an interface could look like this:</p>
<div class="highlight-php"><div class="highlight"><pre><span class="cp">&lt;?php</span>
<span class="sd">/**</span>
<span class="sd"> * This interface defines the basic actions that a block / panel needs to support.</span>
<span class="sd"> *</span>
<span class="sd"> * Every blockstrategy is *only* responsible for rendering a block and declaring some basic</span>
<span class="sd"> * support, but *not* for updating its configuration etc. For this purpose, use controllers</span>
<span class="sd"> * and models.</span>
<span class="sd"> */</span>
<span class="k">interface</span> <span class="nx">BlockStrategyInterface</span> <span class="p">{</span>
    <span class="sd">/**</span>
<span class="sd">     * This could configure your entity</span>
<span class="sd">     */</span>
    <span class="k">public</span> <span class="k">function</span> <span class="nf">setConfig</span><span class="p">(</span><span class="nx">Config\EntityConfig</span> <span class="nv">$config</span><span class="p">);</span>

    <span class="sd">/**</span>
<span class="sd">     * Returns the config this strategy is configured with.</span>
<span class="sd">     * @return Core\Model\Config\EntityConfig</span>
<span class="sd">     */</span>
    <span class="k">public</span> <span class="k">function</span> <span class="nf">getConfig</span><span class="p">();</span>

    <span class="sd">/**</span>
<span class="sd">     * Set the view object.</span>
<span class="sd">     * @param  \Zend_View_Interface $view</span>
<span class="sd">     * @return \Zend_View_Helper_Interface</span>
<span class="sd">     */</span>
    <span class="k">public</span> <span class="k">function</span> <span class="nf">setView</span><span class="p">(</span><span class="nx">\Zend_View_Interface</span> <span class="nv">$view</span><span class="p">);</span>

    <span class="sd">/**</span>
<span class="sd">     * @return \Zend_View_Interface</span>
<span class="sd">     */</span>
    <span class="k">public</span> <span class="k">function</span> <span class="nf">getView</span><span class="p">();</span>

    <span class="sd">/**</span>
<span class="sd">     * Renders this strategy. This method will be called when the user</span>
<span class="sd">     * displays the site.</span>
<span class="sd">     *</span>
<span class="sd">     * @return string</span>
<span class="sd">     */</span>
    <span class="k">public</span> <span class="k">function</span> <span class="nf">renderFrontend</span><span class="p">();</span>

    <span class="sd">/**</span>
<span class="sd">     * Renders the backend of this block. This method will be called when</span>
<span class="sd">     * a user tries to reconfigure this block instance.</span>
<span class="sd">     *</span>
<span class="sd">     * Most of the time, this method will return / output a simple form which in turn</span>
<span class="sd">     * calls some controllers.</span>
<span class="sd">     *</span>
<span class="sd">     * @return string</span>
<span class="sd">     */</span>
    <span class="k">public</span> <span class="k">function</span> <span class="nf">renderBackend</span><span class="p">();</span>

    <span class="sd">/**</span>
<span class="sd">     * Returns all possible types of panels this block can be stacked onto</span>
<span class="sd">     *</span>
<span class="sd">     * @return array</span>
<span class="sd">     */</span>
    <span class="k">public</span> <span class="k">function</span> <span class="nf">getRequiredPanelTypes</span><span class="p">();</span>

    <span class="sd">/**</span>
<span class="sd">     * Determines whether a Block is able to use a given type or not</span>
<span class="sd">     * @param string $typeName The typename</span>
<span class="sd">     * @return boolean</span>
<span class="sd">     */</span>
    <span class="k">public</span> <span class="k">function</span> <span class="nf">canUsePanelType</span><span class="p">(</span><span class="nv">$typeName</span><span class="p">);</span>

    <span class="k">public</span> <span class="k">function</span> <span class="nf">setBlockEntity</span><span class="p">(</span><span class="nx">AbstractBlock</span> <span class="nv">$block</span><span class="p">);</span>

    <span class="k">public</span> <span class="k">function</span> <span class="nf">getBlockEntity</span><span class="p">();</span>
<span class="p">}</span>
</pre></div>
</div>
<p>As you can see, we have a method &#8220;setBlockEntity&#8221; which ties a potential strategy to an object of type AbstractBlock. This type will simply define the basic behaviour of our blocks and could potentially look something like this:</p>
<div class="highlight-php"><div class="highlight"><pre><span class="cp">&lt;?php</span>
<span class="sd">/**</span>
<span class="sd"> * This is the base class for both Panels and Blocks.</span>
<span class="sd"> * It shouldn&#39;t be extended by your own blocks - simply write a strategy!</span>
<span class="sd"> */</span>
<span class="k">abstract</span> <span class="k">class</span> <span class="nc">AbstractBlock</span> <span class="p">{</span>
    <span class="sd">/**</span>
<span class="sd">     * The id of the block item instance</span>
<span class="sd">     * This is a doctrine field, so you need to setup generation for it</span>
<span class="sd">     * @var integer</span>
<span class="sd">     */</span>
    <span class="k">private</span> <span class="nv">$id</span><span class="p">;</span>

    <span class="c1">// Add code for relation to the parent panel, configuration objects, ....</span>

    <span class="sd">/**</span>
<span class="sd">     * This var contains the classname of the strategy</span>
<span class="sd">     * that is used for this blockitem. (This string (!) value will be persisted by Doctrine 2)</span>
<span class="sd">     *</span>
<span class="sd">     * This is a doctrine field, so make sure that you use an @column annotation or setup your</span>
<span class="sd">     * yaml or xml files correctly</span>
<span class="sd">     * @var string</span>
<span class="sd">     */</span>
    <span class="k">protected</span> <span class="nv">$strategyClassName</span><span class="p">;</span>

    <span class="sd">/**</span>
<span class="sd">     * This var contains an instance of $this-&gt;blockStrategy. Will not be persisted by Doctrine 2.</span>
<span class="sd">     *</span>
<span class="sd">     * @var BlockStrategyInterface</span>
<span class="sd">     */</span>
    <span class="k">protected</span> <span class="nv">$strategyInstance</span><span class="p">;</span>

    <span class="sd">/**</span>
<span class="sd">     * Returns the strategy that is used for this blockitem.</span>
<span class="sd">     *</span>
<span class="sd">     * The strategy itself defines how this block can be rendered etc.</span>
<span class="sd">     *</span>
<span class="sd">     * @return string</span>
<span class="sd">     */</span>
    <span class="k">public</span> <span class="k">function</span> <span class="nf">getStrategyClassName</span><span class="p">()</span> <span class="p">{</span>
        <span class="k">return</span> <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">strategyClassName</span><span class="p">;</span>
    <span class="p">}</span>

    <span class="sd">/**</span>
<span class="sd">     * Returns the instantiated strategy</span>
<span class="sd">     *</span>
<span class="sd">     * @return BlockStrategyInterface</span>
<span class="sd">     */</span>
    <span class="k">public</span> <span class="k">function</span> <span class="nf">getStrategyInstance</span><span class="p">()</span> <span class="p">{</span>
        <span class="k">return</span> <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">strategyInstance</span><span class="p">;</span>
    <span class="p">}</span>

    <span class="sd">/**</span>
<span class="sd">     * Sets the strategy this block / panel should work as. Make sure that you&#39;ve used</span>
<span class="sd">     * this method before persisting the block!</span>
<span class="sd">     *</span>
<span class="sd">     * @param BlockStrategyInterface $strategy</span>
<span class="sd">     */</span>
    <span class="k">public</span> <span class="k">function</span> <span class="nf">setStrategy</span><span class="p">(</span><span class="nx">BlockStrategyInterface</span> <span class="nv">$strategy</span><span class="p">)</span> <span class="p">{</span>
        <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">strategyInstance</span>  <span class="o">=</span> <span class="nv">$strategy</span><span class="p">;</span>
        <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">strategyClassName</span> <span class="o">=</span> <span class="nb">get_class</span><span class="p">(</span><span class="nv">$strategy</span><span class="p">);</span>
        <span class="nv">$strategy</span><span class="o">-&gt;</span><span class="na">setBlockEntity</span><span class="p">(</span><span class="nv">$this</span><span class="p">);</span>
    <span class="p">}</span>
</pre></div>
</div>
<p>Now, the important point is that $strategyClassName is a Doctrine 2
field, i.e. Doctrine will persist this value. This is only the
class name of your strategy and not an instance!</p>
<p>Finishing your strategy pattern, we hook into the Doctrine postLoad
event and check whether a block has been loaded. If so, you will
initialize it - i.e. get the strategies classname, create an
instance of it and set it via setStrategyBlock().</p>
<p>This might look like this:</p>
<div class="highlight-php"><div class="highlight"><pre><span class="cp">&lt;?php</span>
<span class="k">use</span> <span class="nx">\Doctrine\ORM</span><span class="p">,</span>
    <span class="nx">\Doctrine\Common</span><span class="p">;</span>

<span class="sd">/**</span>
<span class="sd"> * The BlockStrategyEventListener will initialize a strategy after the</span>
<span class="sd"> * block itself was loaded.</span>
<span class="sd"> */</span>
<span class="k">class</span> <span class="nc">BlockStrategyEventListener</span> <span class="k">implements</span> <span class="nx">Common\EventSubscriber</span> <span class="p">{</span>

    <span class="k">protected</span> <span class="nv">$view</span><span class="p">;</span>

    <span class="k">public</span> <span class="k">function</span> <span class="nf">__construct</span><span class="p">(</span><span class="nx">\Zend_View_Interface</span> <span class="nv">$view</span><span class="p">)</span> <span class="p">{</span>
        <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">view</span> <span class="o">=</span> <span class="nv">$view</span><span class="p">;</span>
    <span class="p">}</span>

    <span class="k">public</span> <span class="k">function</span> <span class="nf">getSubscribedEvents</span><span class="p">()</span> <span class="p">{</span>
       <span class="k">return</span> <span class="k">array</span><span class="p">(</span><span class="nx">ORM\Events</span><span class="o">::</span><span class="na">postLoad</span><span class="p">);</span>
    <span class="p">}</span>

    <span class="k">public</span> <span class="k">function</span> <span class="nf">postLoad</span><span class="p">(</span><span class="nx">ORM\Event\LifecycleEventArgs</span> <span class="nv">$args</span><span class="p">)</span> <span class="p">{</span>
        <span class="nv">$blockItem</span> <span class="o">=</span> <span class="nv">$args</span><span class="o">-&gt;</span><span class="na">getEntity</span><span class="p">();</span>

        <span class="c1">// Both blocks and panels are instances of Block\AbstractBlock</span>
        <span class="k">if</span> <span class="p">(</span><span class="nv">$blockItem</span> <span class="nx">instanceof</span> <span class="nx">Block\AbstractBlock</span><span class="p">)</span> <span class="p">{</span>
            <span class="nv">$strategy</span>  <span class="o">=</span> <span class="nv">$blockItem</span><span class="o">-&gt;</span><span class="na">getStrategyClassName</span><span class="p">();</span>
            <span class="nv">$strategyInstance</span> <span class="o">=</span> <span class="k">new</span> <span class="nv">$strategy</span><span class="p">();</span>
            <span class="k">if</span> <span class="p">(</span><span class="k">null</span> <span class="o">!==</span> <span class="nv">$blockItem</span><span class="o">-&gt;</span><span class="na">getConfig</span><span class="p">())</span> <span class="p">{</span>
                <span class="nv">$strategyInstance</span><span class="o">-&gt;</span><span class="na">setConfig</span><span class="p">(</span><span class="nv">$blockItem</span><span class="o">-&gt;</span><span class="na">getConfig</span><span class="p">());</span>
            <span class="p">}</span>
            <span class="nv">$strategyInstance</span><span class="o">-&gt;</span><span class="na">setView</span><span class="p">(</span><span class="nv">$this</span><span class="o">-&gt;</span><span class="na">view</span><span class="p">);</span>
            <span class="nv">$blockItem</span><span class="o">-&gt;</span><span class="na">setStrategy</span><span class="p">(</span><span class="nv">$strategyInstance</span><span class="p">);</span>
        <span class="p">}</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>In this example, even some variables are set - like a view object
or a specific configuration object.</p>
</div>
</div>


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