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<!DOCTYPE html>  <html lang="en"> <head> <meta charset="utf-8"> <title>CanJS - Observables</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'); 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Observables are the subjects in the <a href="http://en.wikipedia.org/wiki/Observer_pattern">observer pattern</a>. CanJS comes with three forms of observables:</p> <ul> <li><a href="../docs/can.Map.html">can.Map</a> - Used for Objects.</li> <li><a href="../docs/can.List.html">can.List</a> - Used for Arrays.</li> <li><a href="../docs/can.compute.html">can.compute</a> - Used for values.</li> </ul> <p>can.Map and can.List are often extended to create observable types. <a href="../docs/can.Model.html">Models</a> and <a href="../docs/can.route.html">can.route</a> are based on can.Map, and can.Component's <a href="../docs/can.Component.prototype.scope.html">scope</a> is a can.Map, but observables are useful on their own too.</p> <p>To create a Map, call <code>new can.Map(obj)</code>. This will give you a map with the same properties and values as <em>obj</em>. To create a List, call <code>new can.List(array)</code>. This will give you a List with the same elements as <em>array</em>.</p> <pre><code>var pagination = new can.Map({page: 1, perPage: 25, count: 1388}); pagination.attr('perPage'); // 25 var hobbies = new can.List(['programming', 'bball', 'party rocking']); hobbies.attr(2); // 'partying' </code></pre> <h2>Manipulating properties</h2> <p>The <a href="../docs/can.Map.prototype.attr.html">attr</a> method is used to read and write a property or properties from a Map or List.</p> <pre><code>pagination.attr('perPage'); // 25 pagination.attr('perPage', 50); pagination.attr('perPage'); // 50 pagination.attr({page: 10, lastVisited: 1}); pagination.attr(); // {page: 10, perPage: 50, count: 1388, lastVisited: 1} </code></pre> <p>Properties can be removed from Observes with <code>removeAttr</code>, which is equivalent to the <code>delete</code> keyword:</p> <pre><code>pagination.removeAttr('count'); pagination.attr(); // {page: 10, perPage: 50, lastVisited: 1} </code></pre> <h2>Listening to events</h2> <p>When a property on a Map is changed with <code>attr</code>, the Map will emit two events: A <em>change</em> event and an event with the same name as the property that was changed. You can listen for these events by using <a href="../docs/can.Map.prototype.bind.html">bind</a>:</p> <pre><code>paginate.bind('change', function(event, attr, how, newVal, oldVal) { attr; // 'perPage' how; // 'set' newVal; // 30 oldVal; // 50 }); paginate.bind('perPage', function(event, newVal, oldVal) { newVal; // 30 oldVal; // 50 }); paginate.attr('perPage', 30); </code></pre> <p>You can similarly stop listening to these events by using <a href="../docs/can.Map.prototype.unbind.html">unbind</a>:</p> <pre><code>var timesChanged = 0, changeHandler = function() { timesChanged++; }, obs = new can.Map({value: 10}); obs.bind('change', changeHandler); obs.attr('value', 20); timesChanged; // 1 obs.unbind('change', changeHandler); obs.attr('value', 30); timesChanged; // 1 </code></pre> <h2>Iterating though a Map</h2> <p>If you want to iterate through the properties on a Map, use <code>each</code>:</p> <pre><code>paginate.each(function(val, key) { console.log(key + ': ' + val); }); // The console shows: // page: 10 // perPage: 30 // lastVisited: 1 </code></pre> <h2>Extending a Map</h2> <p>Extending a can.Map (or can.List) lets you create custom observable types. The following extends can.Map to create a Paginate type that has a <code>.next()</code> method to change its state:</p> <pre><code>Paginate = can.Map.extend({ limit: 100, offset: 0, count: Infinity, page: function() { return Math.floor(this.attr('offset') / this.attr('limit')) + 1; }, next: function() { this.attr('offset', this.attr('offset') + this.attr('limit') ); } }); var pageInfo = new Paginate(); pageInfo.attr("offset") //-> 0 pageInfo.next(); pageInfo.attr("offset") //-> 100 pageInfo.page() //-> 2 </code></pre> <h2>Observable Arrays</h2> <p>As mentioned above, CanJS also provides observable arrays with can.List. can.List inherits from can.Map, so a List works much the same way an Map does, with the addition of several methods useful for working with arrays:</p> <ul> <li><a href="../docs/can.List.prototype.indexOf.html">indexOf</a>, which looks for an item in a List.</li> <li><a href="../docs/can.List.prototype.pop.html">pop</a>, which removes the last item from a List.</li> <li><a href="../docs/can.List.prototype.push.html">push</a>, which adds an item to the end of a List.</li> <li><a href="../docs/can.List.prototype.shift.html">shift</a>, which removes the first item from a List.</li> <li><a href="../docs/can.List.prototype.unshift.html">unshift</a>, which adds an item to the front of a List.</li> <li><a href="../docs/can.List.prototype.splice.html">splice</a>, which removes and inserts items anywhere in a List.</li> </ul> <p>When these methods are used to modify a List, the appropriate events are emitted. See <a href="../docs/can.List.html">the API for Lists</a> for more information on the arguments passed to those event handlers.</p> <h2>Computed values</h2> <p>CanJS also provides a way to make values themselves observable with <a href="../docs/can.compute.html">can.compute</a>. A Compute represents a dynamic value that can be read, set, and listened to just like a Map.</p> <h3>Static Computes</h3> <p>A simple static Compute contains a single value, and is created by calling <code>can.compute(value)</code>. This value can be read, set, and listened to:</p> <pre><code>// create a Compute var age = can.compute(25), previousAge = 0; // read the Compute's value age(); // 25 // listen for changes in the Compute's value age.bind('change', function(ev, newVal, oldVal) { previousAge = oldVal; }); // set the Compute's value age(26); age(); // 26 previousAge; // 25 </code></pre> <h3>Composite Computes</h3> <p>Computes can also be used to generate a unique value based on values derived from other observable properties. This type of compute is created by calling <code>can.compute(getterFunction)</code>. When the observable properties that the compute is derived from change, the value of the compute changes:</p> <pre><code>var name = new can.Map({ first: 'Alice', last: 'Liddell' }); var fullName = can.compute(function() { // We use attr to read the values // so the compute knows what to listen to. return name.attr('first') + ' ' + name.attr('last'); }); var previousName = ''; fullName(); // 'Alice Liddell' fullName.bind('change', function(ev, newVal, oldVal) { previousName = oldVal; }); name.attr({ first: 'Allison', last: 'Wonderland' }); fullname(); // 'Allison Wonderland' previousName; // 'Alice Liddell' </code></pre> <p>Since the value of the Compute is cached any time a derived value is changed, reading the value is fast.</p> <h3>Converted Computes</h3> <p>Computes are also useful for creating links to properties within Observes. One of the most frequent examples of this is when converting from one unit to another.</p> <pre><code>// progress ranges from 0 to 1. var project = new can.Map({ progress: 0.3 }); var progressPercentage = can.compute(function(newVal) { if(newVal !== undefined) { // set a value project.attr('progress', newVal / 100); } else { // get the value return project.attr('progress') * 100; } }); percentage(); // 30 // Setting percentage... percentage(75); // ...updates project.progress! project.attr('progress'); // .75 </code></pre> </section> <script type="text/javascript"> var docObject = {"src":{"path":"docs/can-guides/observables.md"},"description":"\n","name":"Observables","title":"Observables","type":"page","parent":"Tutorial","order":1,"comment":" ","pathToRoot":".."}; </script> </article> <footer><p>CanJS is part of <a href="http://donejs.com" target="_blank">DoneJS</a>. 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