Build Widgets/UI Elements

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Previous recipes have demonstrated how to change page content and introduced event handling. The following recipes will introduce can.Component, which allows for straightforward widget construction by packaging template, state, and event handling code in one place.

While similar behavior can be accomplished with can.Control, building a Component enables building reusable widgets using custom HTML tags.

Create a Component

The previous recipe that displays a list of people can instead be represented as a component.


By specifying people as the tag, a component is created wherever <people></people> appears in a template.

    tag: 'people',

The scope object on a Component contains the component's state, data, and behavior. Here, it specifies how to remove a person from the list:

    scope: {
        people: people,
        remove: function( person ) {
            var people = this.attr("people");
            var index = people.indexOf(person);
            people.splice(index, 1);

The template for the component itself is passed via the template property. This can either be an external file or a string. Each li uses can-click, which declares an event binding. Here, remove inside the component's scope will be called with the relevant people object as an argument.

scope: {
    template: '<ul>' +
                '{{#each people}}' +
                '<li can-click="remove">' +
                    '{{lastname}}, {{firstname}}' +
                '</li>' +
                '{{/each}}' +

This behaves similarly to the can.Control from above. However, the <people> tag can be used without having any knowledge about the inner workings of the widget. Using declarative HTML tags, a component can be used without writing any javascript. The template, state, and behavior are all combined into one Component.

Build a Tabs Widget

A tabs widget could be instantiated with the following HTML:

    <panel title="Fruit">Oranges, Apples, Pears</panel>
    <panel title="Vegetable">Carrot, Lettuce, Rutabega</panel>
    <panel title="Grains">Bread, Pasta, Rice</panel>

A designer that understands HTML can put together a template for a tabs widget without understanding anything other than the syntax. This is one of the most useful features of components.

Tabs Widget Behavior

Before implementing the component itself, we’ll define an observable view model--the scope object of the UI element. This makes the code modular and easier to manage (and also allows for unit testing).

In order to accurately represent a tabs widget, a TabsViewModel needs:

  • An observable list of panels
  • A state variable with the active panel
  • Helper methods to add, remove, and activate panels

Since TabsViewModel is a can.Map, the panels property is automatically converted to a can.List. The active property references the panel object that should currently be displayed.

var TabsViewModel = can.Map.extend({
    panels: [],
    active: null,
    addPanel: function( panel ){
        var panels = this.attr("panels");
        panel.attr("visible", false);
        //activate panel if it is the first one
        if ( panels.attr("length") === 1 ){
            this.activate( panel );
    removePanel: function( panel ){
        var panels = this.attr("panels");
        var index = panels.indexOf(panel);
        panels.splice(index, 1);
        //activate a new panel if panel being removed was the active panel
        if( this.attr("active") === panel ){
            panels.attr("length") ? this.activate(panels[0]) : this.attr("active", null)
    activate: function( panel ){
        var active = this.attr("active")
        if( active !== panel ){
            active && active.attr("visible", false);
            this.attr("active", panel.attr("visible", true));

Tabs Widget Component

Now that the view model is defined, making a component is simply a matter of defining the way the tabs widget is displayed.

The template for a tabs component needs a list of panel titles that will activate that panel when clicked. By calling activate with a panel as the argument, the properties of the panel can be manipulated. By changing the visible property of a panel, a template can be used to display or hide the panel accordingly.

For this component, our template should look something like this:

    <panel title="Fruits">Apples, Oranges</panel>
    <panel title="Vegetables">Carrots, Celery</panel>

A designer can create a tabs component with panel components inside it. The template object on the tabs component's scope needs to be able to render the content that is inside of the <tabs> tag. To do this, we simply use the <content> tag, which will render everything within the component's tags:

    tag: "tabs",
    scope: TabsViewModel,
    template: "<ul>\
                {{#each panels}}\
                    <li can-click='activate'>{{title}}</li>\
                <content />"

The tabs component contains panels, which are also defined as components. The tabs template contains the logic for whether the panel is visible (visible is controlled by the tabs component's activate method).

Each panel's scope contains a title, which should be taken from the title attribute in the <panel> tag. If you want to set the string value of a Component's attribute as a scope variable, use @'.

tag: "panel",
template: "{{#if visible}}<content />{{/if}}",
scope: {
    title: "@"

In addition to the scope property, a component has an events property. This events property uses a can.Control instantiated inside the component to handle events.

Since we defined behavior for adding panels on the parent tabs component, we should use this method whenever a panel is inserted into the page (and an inserted event is triggered). To add the panel to the tabs component's scope, we call the addPanel method by accessing the parent scope with this.element.parent().scope():

    events: {
        inserted: function() {
            this.element.parent().scope().addPanel( this.scope )
        removed: function() {
            this.element.parent().scope().addPanel( this.scope )

With this component, any time a <tabs> element with <panel> elements is put in a page, a tabs widget will automatically be created. This allows application behavior and design to be compartmentalized from each other.