Introduction to Web Components. Examples can be found at https://github.com/alexcheng1982/learn-web-components.

1. Introduction to Web Components
HTML Templates, Custom Elements & Shadow DOM
Fu Cheng
@alexcheng1982

2. 2
Agenda
Software Components
HTML Templates
Custom Elements
Shadow DOM
HTML Imports
Polymer Project

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Web Components is
Component Model of the Web

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Components
http://www.flickr.com/photos/bdesham/2432400623

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Software Component
An individual software component is a software package,
a web service, a web resource, or a module that
encapsulates a set of related functions (or data).
Components are modular and cohesive.
Components communicate with each other via interfaces.
A component model is a definition of standards for
component implementation, documentation and
deployment.
– EJB, COM, COBRA
http://en.wikipedia.org/wiki/Software_component

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Software Component is for
 Reuse
 Sharing
http://www.flickr.com/photos/wheatfields/118700012/
Don't Reinvent the Wheels

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Components for the Web
We now have different components for the web
– Dojo widgets
– jQuery plugins
– YUI widget
– Sencha components
Different frameworks have their own solutions
No common standards

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What's the Problem?
Bad encapsulation
http://www.flickr.com/photos/27898848@N06/3174611184

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In the Web We Have
HTML CSS JavaScript
Structure
Content
Presentation Behaviour

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After You Add a Component
You may introduce
– Additional DOM nodes
• Statically inlined
• Dynamically created using JavaScript
– CSS rules
– JavaScript objects
These may conflict with your own application
– Duplicated DOM element IDs
– Component's styles mingles with existing styles
– Conflicted global JavaScript object names

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Add a Twitter Badge to Your Page (1)
Go to https://twitter.com/about/resources/buttons
Select type
You get code like this

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Add a Twitter Badge to Your Page (2)
You get DOM like this
iframe is the primary tool we have for encapsulation

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Components Contribute to Current Page
HTML CSS JS
HTML CSS JS HTML CSS JS HTML CSS JS
Widget 1 Widget 2 Widget 3

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Web Components
Component model standard for the Web
W3C working draft
(http://www.w3.org/standards/techs/components)
A very new standard

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Web Components
Web Components standards consist of
– HTML Templates (http://www.w3.org/TR/html-templates/)
– Decorators
– Custom Elements (http://www.w3.org/TR/custom-elements/)
– Shadow DOM (http://www.w3.org/TR/shadow-dom/)
– HTML Imports (http://www.w3.org/TR/html-imports/)

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HTML Templates
Templates = basic structure + place holders
Dynamic web applications use templates to generate
HTML fragments
Current template techniques
– Inline invisible markup
• Set CSS style 'display:none'
– Embed HTML markup in string
• In JavaScript files

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HTML Templates
Client-side templates are widely used today
– Server-side exposes REST API over JSON
– Combine data with template strings
– Use innerHTML to render
Problems
– Invisible markup
• Resources are still loaded
– Template strings
• XSS attacks
• Not intuitive

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HTML Templates
<template> element
Declarative way to create HTML fragments
Use content attribute to access the fragment
Content of templates are not part of the document and are
inactive
– Not display
– Cannot query
– Resources not load
Move or clone template to current DOM tree to render

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Custom Elements
Create your own HTML elements
Use <element> to create
– <element name="x-my-element"><p>My Element</p></element>
Extend existing element
– <element name="x-my-button" extends="button"></element>
How to use in the page
– New element
• <x-my-element></x-my-element>
– Extended element
• <button is="x-my-button>Push</button>

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Custom Elements
Support lifecyle callbacks
– ready
– inserted
– removed
Use HTML Imports to import custom elements
– <link rel="import" href="x-nice-image.html">

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Shadow DOM
Shadow DOM is the key of encapsulation within the DOM
tree
 Establish functional boundaries in a document tree

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Typical Web Application

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Shadow Tree
Shadow DOM allows elements in the document tree to
host other DOM trees
A set of rules establish encapsulation boundaries
between document tree and shadow trees

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Document Tree & Shadow Tree
Shadow root is the root of shadow tree

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Shadow Tree Rendered
When rendered, the shadow tree takes place of the shadow host's content

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Insertion Point
Insertion point is where shadow host's children are transposed when rendering

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Insertion Point Rendered
When rendered, shadow host's children are distributed to different
insertion points

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Encapsulation
All nodes in the shadow tree are NOT accessible in
shadow host's document
Node ids are NOT addressable in shadow host's
document
Style sheets are NOT accessible using shadow host's
document's CSSOM

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Encapsulation
Styles
– CSS rules declared in an enclosing tree not apply in a shadow tree,
except when the apply-author-styles flag is set for this tree
– CSS selectors cannot cross the shadow boundary
– Use reset-style-inheritance flag to set inheritable CSS properties
to the initial value
Use @host to add styles to shadow host

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Encapsulation
Events dispatched in the shadow tree may
– Cross the shadow boundary or
– Be terminated at the shadow boundary
Events crossing the shadow boundary are retargetted
– Event's information is adjusted to hide shadow tree

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Simple Sample

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Simple Sample
 Use JavaScript to create a shadow
host
 Use <content> to specify insertion
point
 Shadow host's chidren and shadow
tree are composited together when
rendered
 CSS selectors only match wholly
inside or outside of the shadow tree
Style 'color: #f00' not applied to shadow tree

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HTML Imports
HTML Imports are a way to include and reuse HTML
documents in other HTML documents.
<link rel="import" href="x-person-badge.html">

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Polymer Project
Polymer is a new type of library for the web, built on top of
Web Components, and designed to leverage the evolving
web platform on modern browsers.
– Created by Google
– http://www.polymer-project.org/
– Announced in Google I/O 2013
Embrace latest standards
Provide polyfills for old browsers
Going to replace Web UI in Dart

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Polymer Architecture

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Examples
Web Components examples in my GitHub repository:
https://github.com/alexcheng1982/learn-web-components