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<h3>Table of Contents</h3>
<ul class="link-list" role="directory">
<li><a href="#module-Observable-label-Mechanism">Mechanism</a>
<li><a href="#module-Observable-label-Example">Example</a>
</ul>
</div>
<div id="class-metadata">
<!-- Method Quickref -->
<div id="method-list-section" class="nav-section">
<h3>Methods</h3>
<ul class="link-list" role="directory">
<li ><a href="#method-i-add_observer">#add_observer</a>
<li ><a href="#method-i-changed">#changed</a>
<li ><a href="#method-i-changed-3F">#changed?</a>
<li ><a href="#method-i-count_observers">#count_observers</a>
<li ><a href="#method-i-delete_observer">#delete_observer</a>
<li ><a href="#method-i-delete_observers">#delete_observers</a>
<li ><a href="#method-i-notify_observers">#notify_observers</a>
</ul>
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<main role="main" aria-labelledby="module-Observable">
<h1 id="module-Observable" class="module">
module Observable
</h1>
<section class="description">
<p>The Observer pattern (also known as publish/subscribe) provides a simple mechanism for one object to inform a set of interested third-party objects when its state changes.</p>
<h2 id="module-Observable-label-Mechanism">Mechanism<span><a href="#module-Observable-label-Mechanism">¶</a> <a href="#top">↑</a></span></h2>
<p>The notifying class mixes in the <code>Observable</code> module, which provides the methods for managing the associated observer objects.</p>
<p>The observable object must:</p>
<ul><li>
<p>assert that it has <code>#changed</code></p>
</li><li>
<p>call <code>#notify_observers</code></p>
</li></ul>
<p>An observer subscribes to updates using <a href="Observable.html#method-i-add_observer"><code>Observable#add_observer</code></a>, which also specifies the method called via <a href="Observable.html#method-i-notify_observers"><code>notify_observers</code></a>. The default method for <a href="Observable.html#method-i-notify_observers"><code>notify_observers</code></a> is update.</p>
<h3 id="module-Observable-label-Example">Example<span><a href="#module-Observable-label-Example">¶</a> <a href="#top">↑</a></span></h3>
<p>The following example demonstrates this nicely. A <code>Ticker</code>, when run, continually receives the stock <code>Price</code> for its <code>@symbol</code>. A <code>Warner</code> is a general observer of the price, and two warners are demonstrated, a <code>WarnLow</code> and a <code>WarnHigh</code>, which print a warning if the price is below or above their set limits, respectively.</p>
<p>The <code>update</code> callback allows the warners to run without being explicitly called. The system is set up with the <code>Ticker</code> and several observers, and the observers do their duty without the top-level code having to interfere.</p>
<p>Note that the contract between publisher and subscriber (observable and observer) is not declared or enforced. The <code>Ticker</code> publishes a time and a price, and the warners receive that. But if you don't ensure that your contracts are correct, nothing else can warn you.</p>
<pre class="ruby"><span class="ruby-identifier">require</span> <span class="ruby-string">"observer"</span>
<span class="ruby-keyword">class</span> <span class="ruby-constant">Ticker</span> <span class="ruby-comment">### Periodically fetch a stock price.</span>
<span class="ruby-identifier">include</span> <span class="ruby-constant">Observable</span>
<span class="ruby-keyword">def</span> <span class="ruby-identifier ruby-title">initialize</span>(<span class="ruby-identifier">symbol</span>)
<span class="ruby-ivar">@symbol</span> = <span class="ruby-identifier">symbol</span>
<span class="ruby-keyword">end</span>
<span class="ruby-keyword">def</span> <span class="ruby-identifier ruby-title">run</span>
<span class="ruby-identifier">last_price</span> = <span class="ruby-keyword">nil</span>
<span class="ruby-identifier">loop</span> <span class="ruby-keyword">do</span>
<span class="ruby-identifier">price</span> = <span class="ruby-constant">Price</span>.<span class="ruby-identifier">fetch</span>(<span class="ruby-ivar">@symbol</span>)
<span class="ruby-identifier">print</span> <span class="ruby-node">"Current price: #{price}\n"</span>
<span class="ruby-keyword">if</span> <span class="ruby-identifier">price</span> <span class="ruby-operator">!=</span> <span class="ruby-identifier">last_price</span>
<span class="ruby-identifier">changed</span> <span class="ruby-comment"># notify observers</span>
<span class="ruby-identifier">last_price</span> = <span class="ruby-identifier">price</span>
<span class="ruby-identifier">notify_observers</span>(<span class="ruby-constant">Time</span>.<span class="ruby-identifier">now</span>, <span class="ruby-identifier">price</span>)
<span class="ruby-keyword">end</span>
<span class="ruby-identifier">sleep</span> <span class="ruby-value">1</span>
<span class="ruby-keyword">end</span>
<span class="ruby-keyword">end</span>
<span class="ruby-keyword">end</span>
<span class="ruby-keyword">class</span> <span class="ruby-constant">Price</span> <span class="ruby-comment">### A mock class to fetch a stock price (60 - 140).</span>
<span class="ruby-keyword">def</span> <span class="ruby-keyword">self</span>.<span class="ruby-identifier ruby-title">fetch</span>(<span class="ruby-identifier">symbol</span>)
<span class="ruby-value">60</span> <span class="ruby-operator">+</span> <span class="ruby-identifier">rand</span>(<span class="ruby-value">80</span>)
<span class="ruby-keyword">end</span>
<span class="ruby-keyword">end</span>
<span class="ruby-keyword">class</span> <span class="ruby-constant">Warner</span> <span class="ruby-comment">### An abstract observer of Ticker objects.</span>
<span class="ruby-keyword">def</span> <span class="ruby-identifier ruby-title">initialize</span>(<span class="ruby-identifier">ticker</span>, <span class="ruby-identifier">limit</span>)
<span class="ruby-ivar">@limit</span> = <span class="ruby-identifier">limit</span>
<span class="ruby-identifier">ticker</span>.<span class="ruby-identifier">add_observer</span>(<span class="ruby-keyword">self</span>)
<span class="ruby-keyword">end</span>
<span class="ruby-keyword">end</span>
<span class="ruby-keyword">class</span> <span class="ruby-constant">WarnLow</span> <span class="ruby-operator"><</span> <span class="ruby-constant">Warner</span>
<span class="ruby-keyword">def</span> <span class="ruby-identifier ruby-title">update</span>(<span class="ruby-identifier">time</span>, <span class="ruby-identifier">price</span>) <span class="ruby-comment"># callback for observer</span>
<span class="ruby-keyword">if</span> <span class="ruby-identifier">price</span> <span class="ruby-operator"><</span> <span class="ruby-ivar">@limit</span>
<span class="ruby-identifier">print</span> <span class="ruby-node">"--- #{time.to_s}: Price below #@limit: #{price}\n"</span>
<span class="ruby-keyword">end</span>
<span class="ruby-keyword">end</span>
<span class="ruby-keyword">end</span>
<span class="ruby-keyword">class</span> <span class="ruby-constant">WarnHigh</span> <span class="ruby-operator"><</span> <span class="ruby-constant">Warner</span>
<span class="ruby-keyword">def</span> <span class="ruby-identifier ruby-title">update</span>(<span class="ruby-identifier">time</span>, <span class="ruby-identifier">price</span>) <span class="ruby-comment"># callback for observer</span>
<span class="ruby-keyword">if</span> <span class="ruby-identifier">price</span> <span class="ruby-operator">></span> <span class="ruby-ivar">@limit</span>
<span class="ruby-identifier">print</span> <span class="ruby-node">"+++ #{time.to_s}: Price above #@limit: #{price}\n"</span>
<span class="ruby-keyword">end</span>
<span class="ruby-keyword">end</span>
<span class="ruby-keyword">end</span>
<span class="ruby-identifier">ticker</span> = <span class="ruby-constant">Ticker</span>.<span class="ruby-identifier">new</span>(<span class="ruby-string">"MSFT"</span>)
<span class="ruby-constant">WarnLow</span>.<span class="ruby-identifier">new</span>(<span class="ruby-identifier">ticker</span>, <span class="ruby-value">80</span>)
<span class="ruby-constant">WarnHigh</span>.<span class="ruby-identifier">new</span>(<span class="ruby-identifier">ticker</span>, <span class="ruby-value">120</span>)
<span class="ruby-identifier">ticker</span>.<span class="ruby-identifier">run</span>
</pre>
<p>Produces:</p>
<pre>Current price: 83
Current price: 75
--- Sun Jun 09 00:10:25 CDT 2002: Price below 80: 75
Current price: 90
Current price: 134
+++ Sun Jun 09 00:10:25 CDT 2002: Price above 120: 134
Current price: 134
Current price: 112
Current price: 79
--- Sun Jun 09 00:10:25 CDT 2002: Price below 80: 79</pre>
</section>
<section id="5Buntitled-5D" class="documentation-section">
<section id="public-instance-5Buntitled-5D-method-details" class="method-section">
<header>
<h3>Public Instance Methods</h3>
</header>
<div id="method-i-add_observer" class="method-detail ">
<div class="method-heading">
<span class="method-name">add_observer</span><span
class="method-args">(observer, func=:update)</span>
<span class="method-click-advice">click to toggle source</span>
</div>
<div class="method-description">
<p>Add <code>observer</code> as an observer on this object. So that it will receive notifications.</p>
<dl class="rdoc-list note-list"><dt><code>observer</code>
<dd>
<p>the object that will be notified of changes.</p>
</dd><dt><code>func</code>
<dd>
<p><a href="Symbol.html"><code>Symbol</code></a> naming the method that will be called when this <a href="Observable.html"><code>Observable</code></a> has changes.</p>
<p>This method must return true for <code>observer.respond_to?</code> and will receive <code>*arg</code> when <a href="Observable.html#method-i-notify_observers"><code>notify_observers</code></a> is called, where <code>*arg</code> is the value passed to <a href="Observable.html#method-i-notify_observers"><code>notify_observers</code></a> by this <a href="Observable.html"><code>Observable</code></a></p>
</dd></dl>
<div class="method-source-code" id="add_observer-source">
<pre><span class="ruby-comment"># File lib/observer.rb, line 128</span>
<span class="ruby-keyword">def</span> <span class="ruby-identifier ruby-title">add_observer</span>(<span class="ruby-identifier">observer</span>, <span class="ruby-identifier">func</span>=<span class="ruby-value">:update</span>)
<span class="ruby-ivar">@observer_peers</span> = {} <span class="ruby-keyword">unless</span> <span class="ruby-keyword">defined?</span> <span class="ruby-ivar">@observer_peers</span>
<span class="ruby-keyword">unless</span> <span class="ruby-identifier">observer</span>.<span class="ruby-identifier">respond_to?</span> <span class="ruby-identifier">func</span>
<span class="ruby-identifier">raise</span> <span class="ruby-constant">NoMethodError</span>, <span class="ruby-node">"observer does not respond to `#{func}'"</span>
<span class="ruby-keyword">end</span>
<span class="ruby-ivar">@observer_peers</span>[<span class="ruby-identifier">observer</span>] = <span class="ruby-identifier">func</span>
<span class="ruby-keyword">end</span></pre>
</div>
</div>
</div>
<div id="method-i-changed" class="method-detail ">
<div class="method-heading">
<span class="method-name">changed</span><span
class="method-args">(state=true)</span>
<span class="method-click-advice">click to toggle source</span>
</div>
<div class="method-description">
<p><a href="Set.html"><code>Set</code></a> the changed state of this object. Notifications will be sent only if the changed <code>state</code> is <code>true</code>.</p>
<dl class="rdoc-list note-list"><dt><code>state</code>
<dd>
<p>Boolean indicating the changed state of this <a href="Observable.html"><code>Observable</code></a>.</p>
</dd></dl>
<div class="method-source-code" id="changed-source">
<pre><span class="ruby-comment"># File lib/observer.rb, line 169</span>
<span class="ruby-keyword">def</span> <span class="ruby-identifier ruby-title">changed</span>(<span class="ruby-identifier">state</span>=<span class="ruby-keyword">true</span>)
<span class="ruby-ivar">@observer_state</span> = <span class="ruby-identifier">state</span>
<span class="ruby-keyword">end</span></pre>
</div>
</div>
</div>
<div id="method-i-changed-3F" class="method-detail ">
<div class="method-heading">
<span class="method-name">changed?</span><span
class="method-args">()</span>
<span class="method-click-advice">click to toggle source</span>
</div>
<div class="method-description">
<p>Returns true if this object's state has been changed since the last <a href="Observable.html#method-i-notify_observers"><code>notify_observers</code></a> call.</p>
<div class="method-source-code" id="changed-3F-source">
<pre><span class="ruby-comment"># File lib/observer.rb, line 177</span>
<span class="ruby-keyword">def</span> <span class="ruby-identifier ruby-title">changed?</span>
<span class="ruby-keyword">if</span> <span class="ruby-keyword">defined?</span> <span class="ruby-ivar">@observer_state</span> <span class="ruby-keyword">and</span> <span class="ruby-ivar">@observer_state</span>
<span class="ruby-keyword">true</span>
<span class="ruby-keyword">else</span>
<span class="ruby-keyword">false</span>
<span class="ruby-keyword">end</span>
<span class="ruby-keyword">end</span></pre>
</div>
</div>
</div>
<div id="method-i-count_observers" class="method-detail ">
<div class="method-heading">
<span class="method-name">count_observers</span><span
class="method-args">()</span>
<span class="method-click-advice">click to toggle source</span>
</div>
<div class="method-description">
<p>Return the number of observers associated with this object.</p>
<div class="method-source-code" id="count_observers-source">
<pre><span class="ruby-comment"># File lib/observer.rb, line 155</span>
<span class="ruby-keyword">def</span> <span class="ruby-identifier ruby-title">count_observers</span>
<span class="ruby-keyword">if</span> <span class="ruby-keyword">defined?</span> <span class="ruby-ivar">@observer_peers</span>
<span class="ruby-ivar">@observer_peers</span>.<span class="ruby-identifier">size</span>
<span class="ruby-keyword">else</span>
<span class="ruby-value">0</span>
<span class="ruby-keyword">end</span>
<span class="ruby-keyword">end</span></pre>
</div>
</div>
</div>
<div id="method-i-delete_observer" class="method-detail ">
<div class="method-heading">
<span class="method-name">delete_observer</span><span
class="method-args">(observer)</span>
<span class="method-click-advice">click to toggle source</span>
</div>
<div class="method-description">
<p>Remove <code>observer</code> as an observer on this object so that it will no longer receive notifications.</p>
<dl class="rdoc-list note-list"><dt><code>observer</code>
<dd>
<p>An observer of this <a href="Observable.html"><code>Observable</code></a></p>
</dd></dl>
<div class="method-source-code" id="delete_observer-source">
<pre><span class="ruby-comment"># File lib/observer.rb, line 141</span>
<span class="ruby-keyword">def</span> <span class="ruby-identifier ruby-title">delete_observer</span>(<span class="ruby-identifier">observer</span>)
<span class="ruby-ivar">@observer_peers</span>.<span class="ruby-identifier">delete</span> <span class="ruby-identifier">observer</span> <span class="ruby-keyword">if</span> <span class="ruby-keyword">defined?</span> <span class="ruby-ivar">@observer_peers</span>
<span class="ruby-keyword">end</span></pre>
</div>
</div>
</div>
<div id="method-i-delete_observers" class="method-detail ">
<div class="method-heading">
<span class="method-name">delete_observers</span><span
class="method-args">()</span>
<span class="method-click-advice">click to toggle source</span>
</div>
<div class="method-description">
<p>Remove all observers associated with this object.</p>
<div class="method-source-code" id="delete_observers-source">
<pre><span class="ruby-comment"># File lib/observer.rb, line 148</span>
<span class="ruby-keyword">def</span> <span class="ruby-identifier ruby-title">delete_observers</span>
<span class="ruby-ivar">@observer_peers</span>.<span class="ruby-identifier">clear</span> <span class="ruby-keyword">if</span> <span class="ruby-keyword">defined?</span> <span class="ruby-ivar">@observer_peers</span>
<span class="ruby-keyword">end</span></pre>
</div>
</div>
</div>
<div id="method-i-notify_observers" class="method-detail ">
<div class="method-heading">
<span class="method-name">notify_observers</span><span
class="method-args">(*arg)</span>
<span class="method-click-advice">click to toggle source</span>
</div>
<div class="method-description">
<p>Notify observers of a change in state <strong>if</strong> this object's changed state is <code>true</code>.</p>
<p>This will invoke the method named in <a href="Observable.html#method-i-add_observer"><code>add_observer</code></a>, passing <code>*arg</code>. The changed state is then set to <code>false</code>.</p>
<dl class="rdoc-list note-list"><dt><code>*arg</code>
<dd>
<p>Any arguments to pass to the observers.</p>
</dd></dl>
<div class="method-source-code" id="notify_observers-source">
<pre><span class="ruby-comment"># File lib/observer.rb, line 193</span>
<span class="ruby-keyword">def</span> <span class="ruby-identifier ruby-title">notify_observers</span>(<span class="ruby-operator">*</span><span class="ruby-identifier">arg</span>)
<span class="ruby-keyword">if</span> <span class="ruby-keyword">defined?</span> <span class="ruby-ivar">@observer_state</span> <span class="ruby-keyword">and</span> <span class="ruby-ivar">@observer_state</span>
<span class="ruby-keyword">if</span> <span class="ruby-keyword">defined?</span> <span class="ruby-ivar">@observer_peers</span>
<span class="ruby-ivar">@observer_peers</span>.<span class="ruby-identifier">each</span> <span class="ruby-keyword">do</span> <span class="ruby-operator">|</span><span class="ruby-identifier">k</span>, <span class="ruby-identifier">v</span><span class="ruby-operator">|</span>
<span class="ruby-identifier">k</span>.<span class="ruby-identifier">send</span> <span class="ruby-identifier">v</span>, <span class="ruby-operator">*</span><span class="ruby-identifier">arg</span>
<span class="ruby-keyword">end</span>
<span class="ruby-keyword">end</span>
<span class="ruby-ivar">@observer_state</span> = <span class="ruby-keyword">false</span>
<span class="ruby-keyword">end</span>
<span class="ruby-keyword">end</span></pre>
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