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It is typically used to:</p> <ul> <li>Add or remove other attributes as side effects</li> <li>Coerce the set value into an appropriate action</li> </ul> <p>The behavior of the setter depends on the number of arguments specified. This means that a setter like:</p> <pre><code>define: { prop: { set: function(){} } } </code></pre> <p>behaves differently than:</p> <pre><code>define: { prop: { set: function(newVal){} } } </code></pre> <div class="parameters"> <h3 class="parameters-title">Parameters</h3> <ol> <li><b>newVal</b> <code>{*}</code>: <p>The <a href="can-map-define._type.html" title="Converts a value passed to attr into an appropriate value.">type function</a> coerced value the user intends to set on the <a href="can-map.html" title="Create observable objects.">Map</a>.</p> </li> <li><b>setValue</b> <code>{function(newValue)}</code>: <p>A callback that can set the value of the property asyncronously.</p> </li> </ol> </div> <div class="returns"> <h3 class="returns-title">Returns</h3> <p> <code>{*|undefined}</code>: <p>If a non-undefined value is returned, that value is set as the attribute value.</p> <p>If an <code>undefined</code> value is returned, the behavior depends on the number of arguments the setter declares:</p> <ul> <li>If the setter <em>does not</em> specify the <code>newValue</code> argument, the attribute value is set to whatever was passed to <a href="can-map.prototype.attr.html" title="Get or set properties on a Map.">attr</a>.</li> <li>If the setter specifies the <code>newValue</code> argument only, the attribute value will be removed.</li> <li>If the setter specifies both <code>newValue</code> and <code>setValue</code>, the value of the property will not be updated until <code>setValue</code> is called.</li> </ul> </p> </div> </div> <section class="body"> <h2>Use</h2> <p>An attribute's <code>set</code> function can be used to customize the behavior of when an attribute value is set via <a href="can-map.prototype.attr.html" title="Get or set properties on a Map."><code>.attr()</code></a>. Lets see some common cases:</p> <h4>Side effects</h4> <p>The following makes setting a <code>page</code> property update the <code>offset</code>:</p> <pre><code>define: { page: { set: function(newVal){ this.attr('offset', (parseInt(newVal) - 1) * this.attr('limit')); } } } </code></pre> <p>The following makes changing <code>makeId</code> remove the <code>modelId</code> property:</p> <pre><code>define: { makeId: { set: function(newValue){ // Check if we are changing. if(newValue !== this.attr("makeId")) { this.removeAttr("modelId"); } // Must return value to set as we have a `newValue` argument. return newValue; } } } </code></pre> <h4>Asynchronous Setter</h4> <p>The following shows an async setter:</p> <pre><code>define: { prop: { set: function( newVal, setVal){ $.get("/something", {}, setVal ); } } } </code></pre> <h2>Behavior depends on the number of arguments.</h2> <p>When a setter returns <code>undefined</code>, its behavior changes depending on the number of arguments.</p> <p>With 0 arguments, the original set value is set on the attribute.</p> <pre><code>MyMap = Map.extend({ define: { prop: {set: function(){}} } }) var map = new MyMap({prop : "foo"}); map.attr("prop") //-> "foo" </code></pre> <p>With 1 argument, <code>undefined</code> will remove the property.</p> <pre><code>MyMap = Map.extend({ define: { prop: {set: function(newVal){}} } }) var map = new MyMap({prop : "foo"}); Map.keys(map) //-> [] </code></pre> <p>With 2 arguments, <code>undefined</code> leaves the property in place. It is expected that <code>setValue</code> will be called:</p> <pre><code>MyMap = Map.extend({ define: { prop: {set: function(newVal, setValue){}} } }) var map = new MyMap({prop : "foo"}); map.attr("prop") //-> "foo" </code></pre> <h2>Side effects</h2> <p>A set function provides a useful hook for performing side effect logic as a certain property is being changed.</p> <p>For example, in the example below, Paginator can.Map includes a <code>page</code> property, which derives its value entirely from other properties (limit and offset). If something tries to set the <code>page</code> directly, the set method will set the value of <code>offset</code>:</p> <pre><code>var Paginate = Map.extend({ define: { page: { set: function (newVal) { this.attr('offset', (parseInt(newVal) - 1) * this.attr('limit')); }, get: function () { return Math.floor(this.attr('offset') / this.attr('limit')) + 1; } } } }); var p = new Paginate({limit: 10, offset: 20}); </code></pre> <h2>Merging</h2> <p>By default, if a value returned from a setter is an object, array, can.Map, or can.List, the effect will be to replace the property with the new object completely.</p> <pre><code>Contact = Map.extend({ define: { info: { set: function(newVal){ return newVal; } } } }) var alice = new Contact({ info: {name: 'Alice Liddell', email: 'alice@liddell.com'} }); alice.attr(); // {name: 'Alice Liddell', 'email': 'alice@liddell.com'} alice.info._cid; // '.map1' alice.attr('info', {name: 'Allison Wonderland', phone: '888-888-8888'}); alice.attr(); // {name: 'Allison Wonderland', 'phone': '888-888-8888'} alice.info._cid; // '.map2' </code></pre> <p>By contrast, if you access a property of a Map using <code>.attr</code>, then change it by calling <code>.attr</code> on it directly, the new properties will be merged with the existing nested Map, not replaced.</p> <pre><code>var contact = new Map({ 'info' : {'breath' : 'smells like roses'} }); var newInfo = {'teeth' : 'shiny and clean'}; contact.attr('info').attr(newInfo); // info is now a merged object </code></pre> <p>If you would rather have the new Map or List merged into the current value, not replaced, call <code>.attr</code> inside the setter:</p> <pre><code>Contact = Map.extend({ define: { info: { set: function(newVal){ this.info.attr(newVal); return this.info; } } } }) var alice = new Contact({ info: {name: 'Alice Liddell', email: 'alice@liddell.com'} }); alice.attr(); // {name: 'Alice Liddell', 'email': 'alice@liddell.com'} alice.info._cid; // '.map1' alice.attr('info', {name: 'Allison Wonderland', phone: '888-888-8888'}); alice.attr(); //{ // name: 'Allison Wonderland', // email: 'alice@liddell.com', // phone: '888-888-8888' //} alice.info._cid; // '.map1' </code></pre> <h2>Batched Changes</h2> <p>By default, calls to set methods are wrapped in a call to <a href="can-event/batch/batch.start.html" title="Begin an event batch.">batch.start</a> and <a href="can-event/batch/batch.stop.html" title="End an event batch.">batch.stop</a>, so if a set method has side effects that set more than one property, all these sets are wrapped in a single batch for better performance.</p> </section> <script type="text/javascript"> var docObject = {"src":{"path":"node_modules/can-map-define/docs/set.md"},"description":"\nSpecify what happens when a value is set on a map attribute.\n","title":"set","name":"can-map-define.set","type":"function","parent":"can-map-define","signatures":[{"code":"set( [newVal,] [setValue] )","description":"\n\nA set function defines the behavior of what happens when a value is set on a\n[can-map Map]. 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