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<!DOCTYPE html>  <html lang="en"> <head> <meta charset="utf-8"> <title>CanJS - can-observation</title> <meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=no"> <link rel="stylesheet" type="text/css" href="./static/bundles/bit-docs-site/static.css"> <link rel="shortcut icon" sizes="16x16 24x24 32x32 48x48 64x64" href="/docs/images/canjs_favicon.ico"> <link rel="apple-touch-icon" sizes="57x57" href="../docs/images/canjs_favicon_57x57.png"> <link rel="apple-touch-icon-precomposed" sizes="57x57" href="../docs/images/canjs_favicon_57x57.png"> <link rel="apple-touch-icon" sizes="72x72" href="../docs/images/canjs_favicon_72x72.png"> <link rel="apple-touch-icon" sizes="114x114" href="../docs/images/canjs_favicon_114x114.png"> <link rel="apple-touch-icon" sizes="120x120" href="../docs/images/canjs_favicon_128x128.png"> <link rel="apple-touch-icon" sizes="144x144" href="../docs/images/canjs_favicon_144x144.png"> <link rel="apple-touch-icon" sizes="152x152" href="../docs/images/canjs_favicon_152x152.png"> <meta content="yes" name="apple-mobile-web-app-capable"> <meta name="apple-mobile-web-app-status-bar-style" content="white-translucent"> <script> (function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){ (i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o), m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m) })(window,document,'script','https://www.google-analytics.com/analytics.js','ga'); ga('create', 'UA-2302003-11', 'auto'); ga('send', 'pageview'); </script> </head> <body> <input type="checkbox" id="nav-trigger" class="nav-trigger"/> <label for="nav-trigger">Menu</label> <div id="everything"> <div id="left" class="column"> <div class="top-left"> <div class="brand"> <div class="logo"> <a href="../index.html" alt="CanJS"></a> <div class="dropdown project-dropdown"> <a href="https://donejs.com/">DoneJS</a> <a href="http://stealjs.com/">StealJS</a> <a href="http://jquerypp.com/">jQuery ++</a> <a href="https://funcunit.com/">FuncUnit</a> <a href="http://documentjs.com/">DocumentJS</a> </div> </div> <div class="version"> <div class="version-number"> 3.0.0 </div> <div class="dropdown version-dropdown"> <a href="https://v2.canjs.com">2.3.27</a> </div> </div> </div> <div class="search-bar"> <p>   </p> </div> </div> <div class="bottom-left"> <div class="social-side-container"> <ul class="social-side"> <li> <a class="header-mobile github" href="https://github.com/canjs/canjs" target="_blank"><img class="social-icon-small" src="../docs/images/github.png">Github</a> </li> <li> <a class="header-mobile twitter" href="https://twitter.com/canjs" target="_blank"><img class="social-icon-small" src="../docs/images/twitter.png">Twitter</a> </li> </ul> <ul class="social-side"> <li> <a class="header-mobile" href="https://gitter.im/canjs/canjs" target="_blank">Chat</a> </li> <li> <a class="header-mobile" href="http://forums.donejs.com/c/canjs" target="_blank">Forum</a> </li> </ul> </div> <ul> <li class=" "> <a class="page" href="guides.html" title="Welcome to CanJS! 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Calls back <code>compute</code> when the return value of <code>func</code> changes.</p> <div class="parameters"> <h3 class="parameters-title">Parameters</h3> <ol> <li><b>func</b> <code>{function}</code>: <p>The function whose value is being observed.</p> </li> <li><b>context</b> <code>{*}</code>: <p>What <code>this</code> should be when <code>func</code> is called.</p> </li> <li><b>updated</b> <code>{function(newValue, oldValue, batchNum)}</code>: <p>A function to call when <code>func</code>'s return value changes.</p> </li> </ol> </div> </div> <section class="body"> <h2>Use</h2> <p>Instances of <code>Observation</code> are rarely created directly. Instead, use <a href="can-compute.html" title="Create an observable value.">can-compute</a>'s more friendly API to observe when a function's value changes. <a href="can-compute.html" title="Create an observable value.">can-compute</a> uses <code>can-observation</code> internally.</p> <p><code>Observation</code>'s static methods like: <a href="can-observation.add.html" title="Signals that an object's property is being observed, so that any functions that are recording observations will see that this object is a dependency.">add</a>, <a href="can-observation.ignore.html" title="can-observation.ignore">ignore</a>, and <a href="can-observation.trap.html" title="can-observation.trap">trap</a> are used more commonly to control which observable events a compute will listen to.</p> <p>To use <code>can-observation</code> directly, create something observable (supports <code>addEventListener</code>) and calls <a href="can-observation.add.html" title="Signals that an object's property is being observed, so that any functions that are recording observations will see that this object is a dependency.">add</a> like:</p> <pre><code class="language-js">var Observation = require("can-observation"); var assign = require("can-util/js/assign/assign"); var canEvent = require("can-event"); var me = assign({}, canEvent); var name = "Justin"; Object.defineProperty(me,"name",{ get: function(){ Observation.add(this,"name"); return name; }, set: function(newVal) { var oldVal = name; name = newVal; this.dispatch("name", newVal, oldVal); } }) </code></pre> <p>Next, create an observation instance with a function that reads the observable value:</p> <pre><code class="language-js">var observation = new Observation(function(){ return "Hello "+me.name; }, null, function(newVal, oldVal, batchNum){ console.log(newVal); }) </code></pre> <p>Finally, call <code>observation.start()</code> to start listening and be notified of changes:</p> <pre><code class="language-js">observation.start(); 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