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Used to provide properties that derive their value from other properties on the object, or the property value that was set on the object.</p> <p>Specify <code>get</code> like:</p> <pre><code class="language-js">propertyName: { get: function(){ ... } }, propertyName: { get: function(lastSetValue) { ... } } </code></pre> <div class="parameters"> <h3 class="parameters-title">Parameters</h3> <ol> <li><b>lastSetValue</b> <code>{*}</code>: <p>The value last set by <code>instance.propertyName = value</code>. Typically, <em>lastSetValue</em> should be an observable value, like a <a href="can-compute.html" title="Create an observable value.">can-compute</a> or promise. If it's not, it's likely that a <a href="can-define.types.set.html" title="Specify what happens when a property value is set.">set</a> should be used instead.</p> </li> </ol> </div> <div class="returns"> <h3 class="returns-title">Returns</h3> <p> <code>{*}</code>: <p>The value of the property.</p> </p> </div> </div> <div class="signature"> <h2 class="signature-title"> <code>get( lastSetValue, resolve(value) )</code> </h2> <p>Asynchronously defines the behavior when a value is read on an instance. Used to provide property values that are available asynchronously.</p> <p>Only observed properties (via <a href="can-event.on.html" title="can-event.on">on</a>, <a href="can-event.addEventListener.html" title="can-event.addEventListener">addEventListener</a>, etc) will be passed the <code>resolve</code> function. It will be <code>undefined</code> if the value is not observed. This is for memory safety.</p> <p>Specify <code>get</code> like:</p> <pre><code class="language-js">propertyName: { get: function(lastSetValue, resolve){ ... } } </code></pre> <div class="parameters"> <h3 class="parameters-title">Parameters</h3> <ol> <li><b>lastSetValue</b> <code>{*}</code>: <p>The value last set by <code>instance.propertyName = value</code>.</p> </li> <li><b>resolve</b> <code>{function(value)|undefined}</code>: <p>Updates the value of the property. This can be called multiple times if needed. Will be <code>undefined</code> if the value is not observed.</p> </li> </ol> </div> <div class="returns"> <h3 class="returns-title">Returns</h3> <p> <code>{*}</code>: <p>The value of the property before <code>resolve</code> is called. Or a value for unobserved property reads to return.</p> </p> </div> </div> <section class="body"> <h2>Use</h2> <p>Getter methods are useful for:</p> <ul> <li>Defining virtual properties on a map.</li> <li>Defining property values that change with their <em>internal</em> set value.</li> </ul> <h2>Virtual properties</h2> <p>Virtual properties are properties that don't actually store any value, but derive their value from some other properties on the map.</p> <p>Whenever a getter is provided, it is wrapped in a <a href="can-compute.html" title="Create an observable value.">can-compute</a>, which ensures that whenever its dependent properties change, a change event will fire for this property also.</p> <pre><code class="language-js">var Person = DefineMap.extend({ first: "string", last: "string", fullName: { get: function () { return this.first + " " + this.last; } } }); var p = new Person({first: "Justin", last: "Meyer"}); p.fullName; // "Justin Meyer" p.on("fullName", function(ev, newVal){ newVal //-> "Lincoln Meyer"; }); p.first = "Lincoln"; </code></pre> <h2>Asynchronous virtual properties</h2> <p>Often, a virtual property's value only becomes available after some period of time. For example, given a <code>personId</code>, one might want to retrieve a related person:</p> <pre><code class="language-js">var AppState = DefineMap.extend({ personId: "number", person: { get: function(lastSetValue, resolve){ Person.get({id: this.personId}) .then(function(person){ resolve(person); }); } } }); </code></pre> <p>Asynchronous properties should be bound to before reading their value. If they are not bound to, the <code>get</code> function will be called each time.</p> <p>The following example will make multiple <code>Person.get</code> requests:</p> <pre><code>var state = new AppState({personId: 5}); state.person //-> undefined // called sometime later ... state.person //-> undefined </code></pre> <p>However, by binding, the compute only reruns the <code>get</code> function once <code>personId</code> changes:</p> <pre><code>var state = new AppState({personId: 5}); state.on("person", function(){}) state.person //-> undefined // called sometime later state.person //-> Person<{id: 5}> </code></pre> <p>A template like <a href="can-stache.html" title="Live binding Mustache and Handlebars-compatible templates.">can-stache</a> will automatically bind for you, so you can pass <code>state</code> to the template like the following without binding:</p> <pre><code>var template = stache("<span>{{person.fullName}}</span>"); var state = new AppState({}); var frag = template(state); state.personId = 5; frag.childNodes[0].innerHTML //=> "" // sometime later frag.childNodes[0].innerHTML //=> "Lincoln Meyer" </code></pre> <p>The magic tags are updated as <code>personId</code>, <code>person</code>, and <code>fullName</code> change.</p> <h2>Properties values that change with their <em>internal</em> set value</h2> <p>A getter can be used to derive a value from a set value. A getter's <code>lastSetValue</code> argument is the last value set by <code>instance.propertyName = value</code>.</p> <p>For example, a property might be set to a compute, but when read, provides the value of the compute.</p> <pre><code>var MyMap = DefineMap.extend({ value: { get: function( lastSetValue ){ return lastSetValue(); } } }); var map = new MyMap(); var compute = compute(1); map.value = compute; map.value //-> 1 compute(2); map.value //-> 2 </code></pre> <p>This technique should only be used when the <code>lastSetValue</code> is some form of observable, that when it changes, can update the <code>getter</code> value.</p> <p>For simple conversions, <a href="can-define.types.set.html" title="Specify what happens when a property value is set.">set</a> or <a href="can-define.types.type.html" title="Converts a value set on an instance into an appropriate value.">type</a> should be used.</p> <h2>Updating the virtual property value</h2> <p>It's common to update virtual property values instead of replacing it.</p> <p>The following example creates an empty <code>locationIds</code> <a href="can-define/list/list.html" title="Create observable lists.">can-define/list/list</a> when a new instance of <code>Store</code> is created. However, as <code>locations</code> change, the <a href="can-define/list/list.html" title="Create observable lists.">can-define/list/list</a> will be updated with the <code>id</code>s of the <code>locations</code>.</p> <pre><code>var Store = DefineMap.extend({ locations: DefineList, locationIds: { Value: DefineList, get: function(initialValue){ var ids = this.locations.map(function(location){ ids.push(location.id); }); return initialValue.replace(ids); } } }); </code></pre> </section> <script type="text/javascript"> var docObject = {"src":{"path":"node_modules/can-define/docs/types.get.md"},"description":"\nSpecify what happens when a certain property is read on a map. `get` functions\nwork like a [can-compute] and automatically update themselves when a dependent\nobservable value is changed.\n","title":"get","name":"can-define.types.get","type":"function","parent":"can-define.behaviors","signatures":[{"code":"get( [lastSetValue] )","description":"\n\nDefines the behavior when a property value is read on a instance. 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