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<!DOCTYPE html>
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Module: Concurrent::Concern::Observable
— Concurrent Ruby
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<div id="content"><h1>Module: Concurrent::Concern::Observable
</h1>
<div class="box_info">
<dl>
<dt>Included in:</dt>
<dd><span class='object_link'><a href="../Agent.html" title="Concurrent::Agent (class)">Agent</a></span>, <span class='object_link'><a href="../Atom.html" title="Concurrent::Atom (class)">Atom</a></span>, <span class='object_link'><a href="../IVar.html" title="Concurrent::IVar (class)">IVar</a></span>, <span class='object_link'><a href="../TimerTask.html" title="Concurrent::TimerTask (class)">TimerTask</a></span></dd>
</dl>
<dl>
<dt>Defined in:</dt>
<dd>lib/concurrent/concern/observable.rb</dd>
</dl>
</div>
<h2>Overview</h2><div class="docstring">
<div class="discussion">
<p>The <a href="http://en.wikipedia.org/wiki/Observer_pattern">observer pattern</a> is one
of the most useful design patterns.</p>
<p>The workflow is very simple:</p>
<ul>
<li>an <code>observer</code> can register itself to a <code>subject</code> via a callback</li>
<li>many <code>observers</code> can be registered to the same <code>subject</code></li>
<li>the <code>subject</code> notifies all registered observers when its status changes</li>
<li>an <code>observer</code> can deregister itself when is no more interested to receive
event notifications</li>
</ul>
<p>In a single threaded environment the whole pattern is very easy: the
<code>subject</code> can use a simple data structure to manage all its subscribed
<code>observer</code>s and every <code>observer</code> can react directly to every event without
caring about synchronization.</p>
<p>In a multi threaded environment things are more complex. The <code>subject</code> must
synchronize the access to its data structure and to do so currently we're
using two specialized ObserverSet: CopyOnWriteObserverSet
and CopyOnNotifyObserverSet.</p>
<p>When implementing and <code>observer</code> there's a very important rule to remember:
<strong>there are no guarantees about the thread that will execute the callback</strong></p>
<p>Let's take this example</p>
<pre class="code ruby"><code class="ruby"><span class='kw'>class</span> <span class='const'>Observer</span>
<span class='kw'>def</span> <span class='id identifier rubyid_initialize'>initialize</span>
<span class='ivar'>@count</span> <span class='op'>=</span> <span class='int'>0</span>
<span class='kw'>end</span>
<span class='kw'>def</span> <span class='id identifier rubyid_update'>update</span>
<span class='ivar'>@count</span> <span class='op'>+=</span> <span class='int'>1</span>
<span class='kw'>end</span>
<span class='kw'>end</span>
<span class='id identifier rubyid_obs'>obs</span> <span class='op'>=</span> <span class='const'>Observer</span><span class='period'>.</span><span class='id identifier rubyid_new'>new</span>
<span class='lbracket'>[</span><span class='id identifier rubyid_obj1'>obj1</span><span class='comma'>,</span> <span class='id identifier rubyid_obj2'>obj2</span><span class='comma'>,</span> <span class='id identifier rubyid_obj3'>obj3</span><span class='comma'>,</span> <span class='id identifier rubyid_obj4'>obj4</span><span class='rbracket'>]</span><span class='period'>.</span><span class='id identifier rubyid_each'>each</span> <span class='lbrace'>{</span> <span class='op'>|</span><span class='id identifier rubyid_o'>o</span><span class='op'>|</span> <span class='id identifier rubyid_o'>o</span><span class='period'>.</span><span class='id identifier rubyid_add_observer'>add_observer</span><span class='lparen'>(</span><span class='id identifier rubyid_obs'>obs</span><span class='rparen'>)</span> <span class='rbrace'>}</span>
<span class='comment'># execute [obj1, obj2, obj3, obj4]
</span></code></pre>
<p><code>obs</code> is wrong because the variable <code>@count</code> can be accessed by different
threads at the same time, so it should be synchronized (using either a Mutex
or an AtomicFixum)</p>
</div>
</div>
<div class="tags">
</div>
<h2>
Instance Method Summary
<small><a href="#" class="summary_toggle">collapse</a></small>
</h2>
<ul class="summary">
<li class="public ">
<span class="summary_signature">
<a href="#add_observer-instance_method" title="#add_observer (instance method)">#<strong>add_observer</strong>(observer = nil, func = :update, &block) ⇒ Object </a>
</span>
<span class="summary_desc"><div class='inline'><p>Adds an observer to this set.</p>
</div></span>
</li>
<li class="public ">
<span class="summary_signature">
<a href="#count_observers-instance_method" title="#count_observers (instance method)">#<strong>count_observers</strong> ⇒ Integer </a>
</span>
<span class="summary_desc"><div class='inline'><p>Return the number of observers associated with this object.</p>
</div></span>
</li>
<li class="public ">
<span class="summary_signature">
<a href="#delete_observer-instance_method" title="#delete_observer (instance method)">#<strong>delete_observer</strong>(observer) ⇒ Object </a>
</span>
<span class="summary_desc"><div class='inline'><p>Remove <code>observer</code> as an observer on this object so that it will no longer receive notifications.</p>
</div></span>
</li>
<li class="public ">
<span class="summary_signature">
<a href="#delete_observers-instance_method" title="#delete_observers (instance method)">#<strong>delete_observers</strong> ⇒ Observable </a>
</span>
<span class="summary_desc"><div class='inline'><p>Remove all observers associated with this object.</p>
</div></span>
</li>
<li class="public ">
<span class="summary_signature">
<a href="#with_observer-instance_method" title="#with_observer (instance method)">#<strong>with_observer</strong>(observer = nil, func = :update, &block) ⇒ Observable </a>
</span>
<span class="summary_desc"><div class='inline'><p>As <code>#add_observer</code> but can be used for chaining.</p>
</div></span>
</li>
</ul>
<div id="instance_method_details" class="method_details_list">
<h2>Instance Method Details</h2>
<div class="method_details first">
<h3 class="signature first" id="add_observer-instance_method">
#<strong>add_observer</strong>(observer = nil, func = :update, &block) ⇒ <tt>Object</tt>
</h3><div class="docstring">
<div class="discussion">
<p>Adds an observer to this set. If a block is passed, the observer will be
created by this method and no other params should be passed.</p>
</div>
</div>
<div class="tags">
<p class="tag_title">Parameters:</p>
<ul class="param">
<li>
<span class='name'>observer</span>
<span class='type'>(<tt>Object</tt>)</span>
<em class="default">(defaults to: <tt>nil</tt>)</em>
—
<div class='inline'><p>the observer to add</p>
</div>
</li>
<li>
<span class='name'>func</span>
<span class='type'>(<tt>Symbol</tt>)</span>
<em class="default">(defaults to: <tt>:update</tt>)</em>
—
<div class='inline'><p>the function to call on the observer during notification.
Default is :update</p>
</div>
</li>
</ul>
<p class="tag_title">Returns:</p>
<ul class="return">
<li>
<span class='type'>(<tt>Object</tt>)</span>
—
<div class='inline'><p>the added observer</p>
</div>
</li>
</ul>
</div><table class="source_code">
<tr>
<td>
<pre class="lines">
61
62
63</pre>
</td>
<td>
<pre class="code"><span class="info file"># File 'lib/concurrent/concern/observable.rb', line 61</span>
<span class='kw'>def</span> <span class='id identifier rubyid_add_observer'>add_observer</span><span class='lparen'>(</span><span class='id identifier rubyid_observer'>observer</span> <span class='op'>=</span> <span class='kw'>nil</span><span class='comma'>,</span> <span class='id identifier rubyid_func'>func</span> <span class='op'>=</span> <span class='symbol'>:update</span><span class='comma'>,</span> <span class='op'>&</span><span class='id identifier rubyid_block'>block</span><span class='rparen'>)</span>
<span class='id identifier rubyid_observers'>observers</span><span class='period'>.</span><span class='id identifier rubyid_add_observer'>add_observer</span><span class='lparen'>(</span><span class='id identifier rubyid_observer'>observer</span><span class='comma'>,</span> <span class='id identifier rubyid_func'>func</span><span class='comma'>,</span> <span class='op'>&</span><span class='id identifier rubyid_block'>block</span><span class='rparen'>)</span>
<span class='kw'>end</span></pre>
</td>
</tr>
</table>
</div>
<div class="method_details ">
<h3 class="signature " id="count_observers-instance_method">
#<strong>count_observers</strong> ⇒ <tt>Integer</tt>
</h3><div class="docstring">
<div class="discussion">
<p>Return the number of observers associated with this object.</p>
</div>
</div>
<div class="tags">
<p class="tag_title">Returns:</p>
<ul class="return">
<li>
<span class='type'>(<tt>Integer</tt>)</span>
—
<div class='inline'><p>the observers count</p>
</div>
</li>
</ul>
</div><table class="source_code">
<tr>
<td>
<pre class="lines">
101
102
103</pre>
</td>
<td>
<pre class="code"><span class="info file"># File 'lib/concurrent/concern/observable.rb', line 101</span>
<span class='kw'>def</span> <span class='id identifier rubyid_count_observers'>count_observers</span>
<span class='id identifier rubyid_observers'>observers</span><span class='period'>.</span><span class='id identifier rubyid_count_observers'>count_observers</span>
<span class='kw'>end</span></pre>
</td>
</tr>
</table>
</div>
<div class="method_details ">
<h3 class="signature " id="delete_observer-instance_method">
#<strong>delete_observer</strong>(observer) ⇒ <tt>Object</tt>
</h3><div class="docstring">
<div class="discussion">
<p>Remove <code>observer</code> as an observer on this object so that it will no
longer receive notifications.</p>
</div>
</div>
<div class="tags">
<p class="tag_title">Parameters:</p>
<ul class="param">
<li>
<span class='name'>observer</span>
<span class='type'>(<tt>Object</tt>)</span>
—
<div class='inline'><p>the observer to remove</p>
</div>
</li>
</ul>
<p class="tag_title">Returns:</p>
<ul class="return">
<li>
<span class='type'>(<tt>Object</tt>)</span>
—
<div class='inline'><p>the deleted observer</p>
</div>
</li>
</ul>
</div><table class="source_code">
<tr>
<td>
<pre class="lines">
82
83
84</pre>
</td>
<td>
<pre class="code"><span class="info file"># File 'lib/concurrent/concern/observable.rb', line 82</span>
<span class='kw'>def</span> <span class='id identifier rubyid_delete_observer'>delete_observer</span><span class='lparen'>(</span><span class='id identifier rubyid_observer'>observer</span><span class='rparen'>)</span>
<span class='id identifier rubyid_observers'>observers</span><span class='period'>.</span><span class='id identifier rubyid_delete_observer'>delete_observer</span><span class='lparen'>(</span><span class='id identifier rubyid_observer'>observer</span><span class='rparen'>)</span>
<span class='kw'>end</span></pre>
</td>
</tr>
</table>
</div>
<div class="method_details ">
<h3 class="signature " id="delete_observers-instance_method">
#<strong>delete_observers</strong> ⇒ <tt><span class='object_link'><a href="" title="Concurrent::Concern::Observable (module)">Observable</a></span></tt>
</h3><div class="docstring">
<div class="discussion">
<p>Remove all observers associated with this object.</p>
</div>
</div>
<div class="tags">
<p class="tag_title">Returns:</p>
<ul class="return">
<li>
<span class='type'>(<tt><span class='object_link'><a href="" title="Concurrent::Concern::Observable (module)">Observable</a></span></tt>)</span>
—
<div class='inline'><p>self</p>
</div>
</li>
</ul>
</div><table class="source_code">
<tr>
<td>
<pre class="lines">
91
92
93
94</pre>
</td>
<td>
<pre class="code"><span class="info file"># File 'lib/concurrent/concern/observable.rb', line 91</span>
<span class='kw'>def</span> <span class='id identifier rubyid_delete_observers'>delete_observers</span>
<span class='id identifier rubyid_observers'>observers</span><span class='period'>.</span><span class='id identifier rubyid_delete_observers'>delete_observers</span>
<span class='kw'>self</span>
<span class='kw'>end</span></pre>
</td>
</tr>
</table>
</div>
<div class="method_details ">
<h3 class="signature " id="with_observer-instance_method">
#<strong>with_observer</strong>(observer = nil, func = :update, &block) ⇒ <tt><span class='object_link'><a href="" title="Concurrent::Concern::Observable (module)">Observable</a></span></tt>
</h3><div class="docstring">
<div class="discussion">
<p>As <code>#add_observer</code> but can be used for chaining.</p>
</div>
</div>
<div class="tags">
<p class="tag_title">Parameters:</p>
<ul class="param">
<li>
<span class='name'>observer</span>
<span class='type'>(<tt>Object</tt>)</span>
<em class="default">(defaults to: <tt>nil</tt>)</em>
—
<div class='inline'><p>the observer to add</p>
</div>
</li>
<li>
<span class='name'>func</span>
<span class='type'>(<tt>Symbol</tt>)</span>
<em class="default">(defaults to: <tt>:update</tt>)</em>
—
<div class='inline'><p>the function to call on the observer during notification.</p>
</div>
</li>
</ul>
<p class="tag_title">Returns:</p>
<ul class="return">
<li>
<span class='type'>(<tt><span class='object_link'><a href="" title="Concurrent::Concern::Observable (module)">Observable</a></span></tt>)</span>
—
<div class='inline'><p>self</p>
</div>
</li>
</ul>
</div><table class="source_code">
<tr>
<td>
<pre class="lines">
70
71
72
73</pre>
</td>
<td>
<pre class="code"><span class="info file"># File 'lib/concurrent/concern/observable.rb', line 70</span>
<span class='kw'>def</span> <span class='id identifier rubyid_with_observer'>with_observer</span><span class='lparen'>(</span><span class='id identifier rubyid_observer'>observer</span> <span class='op'>=</span> <span class='kw'>nil</span><span class='comma'>,</span> <span class='id identifier rubyid_func'>func</span> <span class='op'>=</span> <span class='symbol'>:update</span><span class='comma'>,</span> <span class='op'>&</span><span class='id identifier rubyid_block'>block</span><span class='rparen'>)</span>
<span class='id identifier rubyid_add_observer'>add_observer</span><span class='lparen'>(</span><span class='id identifier rubyid_observer'>observer</span><span class='comma'>,</span> <span class='id identifier rubyid_func'>func</span><span class='comma'>,</span> <span class='op'>&</span><span class='id identifier rubyid_block'>block</span><span class='rparen'>)</span>
<span class='kw'>self</span>
<span class='kw'>end</span></pre>
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