ViewModel.md

@property {Object|function} can-component.prototype.ViewModel ViewModel @parent can-component.prototype

@description

Provides or describes a constructor function that provides values and methods to the component’s [can-component::view view]. The constructor function is initialized with values specified by the component element’s [can-stache-bindings data bindings].

@type {Object} An object that will be passed to [can-define/map/map.extend DefineMap.extend] and used to create a new observable instance accessible by the component’s [can-component::view].

For example, every time <my-tag> is found, a new [can-define/map/map DefineMap] instance will be created:

<my-tag></my-tag>

<script type="module">
import {Component} from "can";

Component.extend({
  tag: "my-tag",
  ViewModel: {
    message: {default: "Hello there!"}
  },
  view: `<h1>{{message}}</h1>`
});

var viewModelInstance = document.querySelector("my-tag").viewModel;
console.log(viewModelInstance) //-> MyTagVM{message: "Hello there!"}
</script>

@codepen

@type {function} A constructor function (usually defined by [can-define/map/map.extend DefineMap.extend], or [can-observe.Object observe.Object]) that will be used to create a new observable instance accessible by the component’s [can-component::view].

For example, every time <my-tag> is found, a new instance of MyTagViewModel will be created:

<my-tag></my-tag>

<script type="module">
import {Component, DefineMap} from "can";

const MyTagViewModel = DefineMap.extend( "MyTagViewModel", {
	message: {default: "Hello there!"}
} );

Component.extend({
  tag: "my-tag",
  ViewModel: MyTagViewModel,
  view: `<h1>{{message}}</h1>`
});

var viewModelInstance = document.querySelector("my-tag").viewModel;
console.log(viewModelInstance) //-> MyTagViewModel{message: "Hello there!"}
</script>

@codepen

Use element.viewModel to read a component’s view-model instance.

@param {Object} properties The initial properties that are passed by the [can-stache-bindings data bindings].

The view bindings on a tag control the properties and values used to instantiate the ViewModel. For example, calling <my-tag> as follows invokes MyTagViewModel as shown in the following example:

<my-tag/> <!-- new MyTagViewModel({}) -->

<my-tag
	message:from="'Hi There'"/> <!-- new MyTagViewModel({message: "Hi There"}) -->

@return {Object} A new instance of the corresponding constructor function. This instance is added to the top of the [can-view-scope] the component’s [can-component::view] is rendered with.

@body

Background

Before reading this documentation, it's useful to have read the [guides/technology-overview] and [guides/html] guides.

Use

[can-component]’s ViewModel property is used to create an object, typically an instance of a [can-define/map/map], that will be used to render the component’s view. This is most easily understood with an example. The following component shows the current page number based off a limit and offset value:

<my-paginate></my-paginate>

<script type="module">
import {DefineMap, Component} from "can";

const MyPaginateViewModel = DefineMap.extend( {
	offset: { default: 0 },
	limit: { default: 20 },
	get page() {
		return Math.floor( this.offset / this.limit ) + 1;
	}
} );

Component.extend( {
	tag: "my-paginate",
	ViewModel: MyPaginateViewModel,
	view: "Page {{page}}."
} );
</script>

@codepen

This will result in:

<my-paginate>Page 1</my-paginate>

This is because the provided ViewModel object is used to create an instance of [can-define/map/map] like:

const viewModel = new MyPaginateViewModel();

The [can-define.types.value] property definition makes offset default to 0 and limit default to 20.

Next, the values are passed into viewModel from the [can-stache-bindings data bindings] within <my-paginate> (in this case there is none).

And finally, that data is used to render the component’s view and inserted into the element using [can-view-scope] and [can-stache]:

const newViewModel = new Scope( viewModel );
const result = stache( "Page {{page}}." )( newViewModel );
element.innerHTML = result;

There is a short-hand for the prototype methods and properties used to extend [can-define/map/map DefineMap] by setting the Component’s ViewModel to an object and using that anonymous type as the view model.

The following does the same as above:

<my-paginate></my-paginate>

<script type="module">
import {Component} from "can";

Component.extend( {
	tag: "my-paginate",
	ViewModel: {
		offset: { default: 0 },
		limit: { default: 20 },
		get page() {
			return Math.floor( this.offset / this.limit ) + 1;
		}
	},
	view: "Page {{page}}."
} );
</script>

@codepen

Values passed from attributes

Values can be "passed" into the viewModel instance of a component, similar to passing arguments into a function. Using [can-stache-bindings], the following binding types can be setup:

• [can-stache-bindings.toChild] — Update the component’s viewModel instance when the parent scope value changes.
• [can-stache-bindings.toParent] — Update the parent scope when the component’s viewModel instance changes.
• [can-stache-bindings.twoWay] — Update the parent scope or the component’s viewModel instance when the other changes.

Using [can-stache], values are passed into components like this:

<my-paginate offset:from='index' limit:from='size' />

The above creates an offset and limit property on the component that are initialized to whatever index and size are.

The following component requires an offset and limit:

Component.extend( {
	tag: "my-paginate",
	ViewModel: {
		offset: { default: 0 },
		limit: { default: 20 },
		get page() {
			return Math.floor( this.offset / this.limit ) + 1;
		}
	},
	view: "Page {{page}}."
} );

If <my-paginate> is used like:

const renderer = stache( "<my-paginate offset:from='index' limit:from='size' />" );

const pageInfo = new DefineMap( { index: 0, size: 20 } );

document.body.appendChild( renderer( pageInfo ) );

…pageInfo’s index and size are set as the component’s offset and limit attributes. If we were to change the value of pageInfo’s index like:

pageInfo.index = 20;

…the component’s offset value will change and its view will update to:

<my-paginate>Page 2</my-paginate>

Using attribute values

You can also pass a literal string value of the attribute. To do this in [can-stache], simply pass a quoted value not wrapped in single brackets, and the viewModel instance property will be initialized to this string value:

<my-tag title:from="'hello'" />

The above will set the title property on the component’s viewModel instance to the string hello.

If the tag’s title attribute is changed, it does not update the viewModel instance property automatically. Instead, you can use [can-view-model] to get a reference to the viewModel instance and modify it. This can be seen in the following example:

@demo demos/can-component/accordion.html @codepen

Clicking the Change title button sets a <my-panel> element’s title attribute like:

import canViewModel from "can-view-model";

out.addEventListener( "click", function( ev ) {
	const el = ev.target;
	const parent = canViewModel( el.parentNode );
	if ( el.nodeName === "BUTTON" ) {
		parent.title = "Users";
	}
} );

Calling methods on ViewModel from events within the view

Using html attributes like can-EVENT-METHOD, you can directly call a ViewModel method from a view. For example, we can make <my-paginate> elements include a next button that calls the ViewModel’s next method like:

Component.extend( {
	tag: "my-paginate",
	ViewModel: {
		offset: { default: 0 },
		limit: { default: 20 },
		next: function() {
			this.offset = this.offset + this.limit;
		},
		get page() {
			return Math.floor( this.offset / this.limit ) + 1;
		}
	},
	view: "Page {{page}} <button on:click='next()'>Next</button>"
} );

ViewModel methods get called back with the current context, the element that you are listening to and the event that triggered the callback.

@demo demos/can-component/paginate_next.html @codepen

Publishing events on ViewModels

DefineMaps can publish events on themselves. For instance, the following <player-edit> component, dispatches a "close" event when its close method is called:

Component.extend( {
	tag: "player-edit",
	view: document.getElementById( "player-edit-stache" ).innerHTML,
	ViewModel: DefineMap.extend( {
		player: Player,
		close: function() {
			this.dispatch( "close" );
		}
	} )
} );

These can be listened to with [can-stache-bindings.event] bindings like:

<player-edit
	on:close="removeEdit()"
	player:from="editingPlayer" />

The following demo uses this ability to create a close button that hides the player editor:

@demo demos/can-component/paginate_next_event.html @codepen