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    <p>The standard implementataion of the observer pattern to be found in Smalltalk would be the <code>#changed</code>/<code>#update</code> mechanism.</p> <p>It goes like this:</p> <pre><code>subject addDependent: anObserver. subject changed. </code></pre> <p>and then <code>anObserver</code> gets send <code>#update</code>:</p> <pre><code>MyObservingObject&gt;&gt;update "I got called in a #changed chain" super update. self doUpdatingStuff </code></pre> <p>You can have finer control using <code>#changed:</code> and <code>#update:</code> (note the colon):</p> <pre><code>subject addDependent: anObserver. subject changed: subject. </code></pre> <p>and</p> <pre><code>MyObservingObject&gt;&gt;update: anObject "I got called in a #changed: chain" super update: anObject. self doUpdatingStuffWith: anObject </code></pre> <p>However, it is commonly found to use a symbol to indicate what changed:</p> <pre><code>subject addDependent: anObserver. subject changed: #myNumbers. </code></pre> <p>and</p> <pre><code>MyObservingObject&gt;&gt;update: anObject "I got called in a #changed: chain" anObject == #myNumbers ifTrue: [ self doUpdatingStuffForNumbers. ^ self "inhibits the super"]. super update: anObject. </code></pre> <p>When you look at <a href="http://www.squeak.org">Squeak</a> or <a href="http://www.pharo.org">Pharo</a>, you'll find at least three other Observer implementations:</p> <ul> <li>The event handling for Morphic (see <code>Morph&gt;&gt;#on:send:to:</code>)</li> <li>A similar, more general event handling mechanism, see <code>Object&gt;&gt;#when:send:to:</code> and <code>Object&gt;&gt;#triggerEvent:</code></li> <li>the <em>Announcements</em> framework, encapsulating messages between subject and observer in classes.</li> </ul> <p>You can find a <a href="https://www.hpi.uni-potsdam.de/hirschfeld/trac/SqueakCommunityProjects/wiki/signals#FeatureComparison">comparison</a> of these at the <a href="https://www.hpi.uni-potsdam.de/hirschfeld/trac/SqueakCommunityProjects/wiki/signals">Signals project</a>, another implementation, but inspired by Qt.</p>
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