can.Component

Use

To create a can.Component, you must first extend can.Component with the methods and properties of how your component behaves:

can.Component.extend({
  tag: "hello-world",
  template: can.stache("{{#if visible}}{{message}}{{else}}Click me{{/if}}"),
  viewModel: {
    visible: false,
    message: "Hello There!"
  },
  events: {
    click: function(){
        this.viewModel.attr("visible", !this.viewModel.attr("visible") );
    }
  }
});

This element says "Click me" until a user clicks it and then says "Hello There!". To create a a instance of this component on the page, add <hello-world></hello-world> to a mustache template, render the template and insert the result in the page like:

var template = can.stache("<hello-world></hello-world>");
$(document.body).append( template() );

Check this out here:

Typically, you do not append a single component at a time. Instead, you'll render a template with many custom tags like:

<srchr-app>
  <srchr-search models="models">
    <input name="search"/>
  </srchr-search>
  <ui-panel>
    <srchr-history/>
    <srchr-results models="models"/>
  </ui-panel>
</srchr-app>

Creating a can.Component

Use can.Component.extend to create a can.Component constructor function that will automatically get initialized whenever the component's tag is found.

Note that inheriting from components works differently than other CanJS APIs. You can't call .extend on a particular component to create a "subclass" of that component.

Instead, components work more like HTML elements. To reuse functionality from a base component, build on top of it with parent components that wrap other components in their template and pass any needed viewModel properties via attributes.

Tag

A component's tag is the element node name that the component will be created on.

The following matches <hello-world> elements.

can.Component.extend({
  tag: "hello-world"
});

Template

A component's template is rendered as the element's innerHTML.

The following component:

can.Component.extend({
  tag: "hello-world",
  template: can.stache("<h1>Hello World</h1>")
});

Changes <hello-world></hello-world> elements into:

<hello-world><h1>Hello World</h1></hello-world>

Use the <content/> tag to position the custom element's source HTML.

The following component:

can.Component.extend({
  tag: "hello-world",
  template: can.stache("<h1><content/></h1>")
});

Changes <hello-world>Hi There</hello-world> into:

<hello-world><h1>Hi There</h1></hello-world>

viewModel

A component's viewModel defines a can.Map that is used to render the component's template. The maps properties are typically set by attribute bindings on the custom element. By default, every attribute's value is looked up in the parent viewModel of the custom element and added to the viewModel object.

The following component:

can.Component.extend({
  tag: "hello-world",
  template: can.stache("<h1>{{message}}</h1>")
});

Changes the following rendered template:

var template = can.stache("<hello-world {message}='greeting'/>");
template({
  greeting: "Salutations"
})

Into:

<hello-world><h1>Salutations</h1></hello-world>

Default values can be provided. The following component:

can.Component.extend({
  tag: "hello-world",
  template: can.stache("<h1>{{message}}</h1>"),
  viewModel: {
    message: "Hi"
  }
});

Changes the following rendered template:

var template = can.stache("<hello-world/>");
template({})

Into:

<hello-world><h1>Hi</h1></hello-world>

If you want to set the string value of the attribute on viewModel, set an attribute without any binding syntax.

The following template, with the previous "hello-world" component:

var template = can.stache("<hello-world message='Howdy'/>");
template({})

Renders to:

<hello-world><h1>Howdy</h1></hello-world>

Events

A component's events object is used to listen to events (that are not listened to with view bindings). The following component adds "!" to the message every time <hello-world> is clicked:

can.Component.extend({
  tag: "hello-world",
  template: can.stache("<h1>{{message}}</h1>"),
  events: {
    "click" : function(){
      var currentMessage = this.viewModel.attr("message");
      this.viewModel.attr("message", currentMessage+ "!")
    }
  }
});

Components have the ability to bind to special inserted and removed events that are called when a component's tag has been inserted into or removed from the page.

Helpers

A component's helpers object provides mustache helper functions that are available within the component's template. The following component only renders friendly messages:

can.Component.extend({
  tag: "hello-world",
  template: can.stache("{{#isFriendly message}}"+
              "<h1>{{message}}</h1>"+
            "{{/isFriendly}}"),
  helpers: {
    isFriendly: function(message, options){
      if( /hi|hello|howdy/.test(message) ) {
        return options.fn();
      } else {
        return options.inverse();
      }
    }
  }
});

Differences between components in can.mustache and can.stache

A can.mustache template passes values from the viewModel to a can.Component by specifying the key of the value in the attribute directly. For example:

can.Component.extend({
  tag: "my-tag",
  template: "<h1>{{greeting}}</h1>"
});
var template = can.mustache("<my-tag greeting='message'></my-tag>");

var frag = template({
  message: "Hi"
});

frag //-> <my-tag greeting='message'><h1>Hi</h1></my-tag>

With can.stache, you wrap the attribute name with {} for parent to child binding. For example:

can.Component.extend({
  tag: "my-tag",
  template: can.stache("<h1>{{greeting}}</h1>")
});
var template = can.stache("<my-tag {greeting}='message'></my-tag>");

var frag = template({
  message: "Hi"
});

frag //-> <my-tag {greeting}='message'><h1>Hi</h1></my-tag>

If the key was not wrapped, the template would render:

frag //-> <my-tag greeting='message'><h1>message</h1></my-tag>

Because the attribute value would be passed as the value of greeting.

Examples

Check out the following examples built with can.Component.

Tabs

The following demos a tabs widget. Click "Add Vegetables" to add a new tab.

An instance of the tabs widget is created by creating <tabs> and <panel> elements like:

<tabs>
  {{#each foodTypes}}
    <panel title='title'>{{content}}</panel>
  {{/each}}
</tabs>

To add another panel, all we have to do is add data to foodTypes like:

foodTypes.push({
  title: "Vegetables",
  content: "Carrots, peas, kale"
})

The secret is that the <panel> element listens to when it is inserted and adds its data to the tabs' list of panels with:

this.element.parent().viewModel().addPanel( this.viewModel );

TreeCombo

The following tree combo lets people walk through a hierarchy and select locations.

The secret to this widget is the viewModel's breadcrumb property, which is an array of items the user has navigated through, and selectableItems, which represents the children of the last item in the breadcrub. These are defined on the viewModel like:

breadcrumb: [],
selectableItems: function(){
  var breadcrumb = this.attr("breadcrumb");
        
  // if there's an item in the breadcrumb
  if(breadcrumb.attr('length')){
        
    // return the last item's children
    return breadcrumb.attr(""+(breadcrumb.length-1)+'.children');
  } else{
        
    // return the top list of items
    return this.attr('items');
  }
}

When the "+" icon is clicked next to each item, the viewModel's showChildren method is called, which adds that item to the breadcrumb like:

showChildren: function( item, ev ) {
  ev.stopPropagation();
  this.attr('breadcrumb').push(item)
},

Paginate

The following example shows 3 widget-like components: a grid, next / prev buttons, and a page count indicator. And, it shows an application component that puts them all together.

This demo uses a Paginate can.Map to assist with maintaining a paginated state:

var Paginate = can.Map.extend({
...
});

The app component, using the [can.Map.define define plugin], creates an instance of the Paginate model and a websitesPromise that represents a request for the Websites that should be displayed.

viewModel: {
  define: {
    paginate: {
      value: function() {
        return new Paginate({
          limit: 5
        });
      }
    },
    websitesPromise: {
      get: function() {
        var params = {
              limit: this.attr('paginate.limit'),
              offset: this.attr('paginate.offset')
          },
          websitesPromise = Website.findAll(params),
          self = this;

        websitesPromise.then(function(websites) {
          self.attr('paginate.count', websites.count);
        });

        return websitesPromise;
      }
    }
  }
}

The app control passes paginate, paginate's values, and websitesPromise to its sub-components:

<app>
  <grid {promise-data}='websitesPromise'>
    {{#each items}}
      <tr>
        <td width='40%'>{{name}}</td>
        <td width='70%'>{{url}}</td>
      </tr>
    {{/each}}
  </grid>
  <next-prev {paginate}='paginate'></next-prev>
  <page-count {page}='paginate.page' {count}='paginate.pageCount'/>
</app>

IE 8 Support

While CanJS does support Internet Explorer 8 out of the box, if you decide to use can.Component then you will need to include HTML5 Shiv in order for your custom tags to work properly.

For namespaced tag names (e.g. <can:example>) and hyphenated tag names (e.g. <can-example>) to work properly, you will need to use version 3.7.2 or later.

Videos

Watch this video for an overview of can.Component, why you should use it, and a hello world example:

This video provides a more in depth overview of the API and goes over several examples of can.Components:

Note: the videos above reference the scope property, which was replaced by the viewModel property in 2.2.