angular-cn/public/docs/ts/latest/guide/component-styles.jade

include ../_util-fns

:marked
  Angular 2 applications are styled with regular CSS. That means we can apply
  everything we know about CSS stylesheets, selectors, rules, and media queries
  to our Angular applications directly.
  
  On top of this, Angular has the ability to bundle *component styles*
  with our components enabling a more modular design than regular stylesheets.
  
  In this chapter we learn how to load and apply these *component styles*.
  
  # Table Of Contents

  * [Using Component Styles](#using-component-styles)
  * [Special selectors](#special-selectors)
  * [Loading Styles into Components](#loading-style)
  * [Controlling View Encapsulation: Emulated, Native, and None](#controlling-view-encapsulation-native-emulated-and-none)
  * [Appendix 1: Inspecting the generated runtime component styles](#inspect-generated-css)
  * [Appendix 2: Loading Styles with Relative URLs](#relative-urls)
  
  **[Run the live code](/resources/live-examples/component-styles/ts/plnkr.html)
    shown in this chapter.**

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  ## Using Component Styles
  
  For every Angular 2 component we write, we may define not only an HTML template,
  but also the CSS styles that go with that template, 
  specifying any selectors, rules, and media queries that we need.

  One way to do this is to set the `styles` property in the component metadata.
  The `styles` property takes an array of strings that contain CSS code.
  Usually we give it one string as in this example:
  
+makeExample('component-styles/ts/app/hero-app.component.ts')(format='.')

:marked
  Component styles differ from traditional, global styles in a couple of ways.
  
  Firstly, the selectors we put into a component's styles *only apply withing the template
  of that component*. The `h1 { }` selector in the example above only applies to the `<h1>` tag
  in the template of `HeroAppComponent`. Any `<h1>` elements elsewhere in
  the application are unaffected.
  
  This is a big improvement in modularity compared to how CSS traditionally works:
  
  1. We can use the CSS class names and selectors that make the most sense in the context of each component. 
     
  1. Class names and selectors are local to the component and won't collide with 
  classes and selectors used elsewhere in the application.
     
  1. Our component's styles *cannot* be changed by changes to styles elsewhere in the application.
     
  1. We can co-locate the CSS code of each component with the TypeScript and HTML code of the component,
     which leads to a neat and tidy project structure.
     
  1. We can change or remove component CSS code in the future without trawling through the
     whole application to see where else it may have been used. We just look at the component we're in.

a(id="special-selectors")
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  ## Special selectors
  
  Component styles have a few special *selectors* from the world of 
  <a href="https://www.w3.org/TR/css-scoping-1/" target="_blank">shadow DOM style scoping</a>.
  
  ### :host

  Use the `:host` pseudo-class selector to target styles in the element that *hosts* the component (as opposed to
  targeting elements *inside* the component's template):
  
+makeExample('component-styles/ts/app/hero-details.component.css', 'host')(format='.')

:marked
  This is the *only* way we can target the host element. We cannot reach
  it from inside the component with other selectors, because it is not part of the
  component's own template. It is in a parent component's template.
  
  Use the *function form* to apply host styles conditionally by 
  including another selector inside parentheses after `:host`.
  
  In the next example we target the host element again, but only when it also has the `active` CSS class.
  
+makeExample('component-styles/ts/app/hero-details.component.css', 'hostfunction')(format=".")
  
:marked
  ### :host-context
  
  Sometimes it is useful to apply styles based on some condition *outside* a component's view.
  For example, there may be a CSS theme class applied to the document `<body>` element, and
  we want to change how our component looks based on that.
  
  Use the `:host-context()` pseudo-class selector. It works just like the function
  form of `:host()`. It looks for a CSS class in *any ancestor* of the component host element, all the way
  up to the document root. It's useful when combined with another selector.
  
  In the following example, we apply a `background-color` style to all `<h2>` elements *inside* the component, only
  if some ancestor element has the CSS class `theme-light`.
  
+makeExample('component-styles/ts/app/hero-details.component.css', 'hostcontext')(format='.')
  
:marked
  ### /deep/
  
  Component styles normally apply only to the HTML in the component's own template. 
  
  We can use the `/deep/` selector to force a style down through the child component tree into all the child component views.
  The `/deep/` selector works to any depth of nested components, and it applies *both to the view
  children and the content children* of the component. 
  
  In this example, we target all `<h3>` elements, from the host element down 
  through this component to all of its child elements in the DOM: 
+makeExample('component-styles/ts/app/hero-details.component.css', 'deep')(format=".")

:marked
  The `/deep/` selector also has the alias `>>>`. We can use either of the two interchangeably.

.alert.is-important
  :marked
    The `/deep/` and `>>>` selectors should only be used with **emulated** view encapsulation.
    This is the default and it is what we use most of the time. See the
    [Controlling View Encapsulation](#controlling-view-encapsulation-native-emulated-and-none)
    section for more details.

a(id='loading-styles')
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  ## Loading Styles into Components
  
  We have several ways to add styles to a component: 
  * inline in the template HTML
  * by setting `styles` or `styleUrls` metadata
  * with CSS imports
  
  The scoping rules outlined above apply to each of these loading patterns.
  
  ### Styles in Metadata
  
  We can add a `styles` array property to the `@Component` decorator.
  Each string in the array (usually just one string) defines the css.
  
+makeExample('component-styles/ts/app/hero-app.component.ts')
  
:marked
  ### Template Inline Styles
  
  We can embed styles directly into the HTML template by putting them
  inside `<style>` tags.
  
+makeExample('component-styles/ts/app/hero-controls.component.ts', 'inlinestyles')

:marked
  ### Style URLs in Metadata
  
  We can load styles from external CSS files by adding a `styleUrls` attribute
  into a component's `@Component` or `@View` decorator:
  
+makeExample('component-styles/ts/app/hero-details.component.ts', 'styleurls')
  
.alert.is-important
  :marked
    The URL is ***relative to the application root*** which is usually the
    location of the `index.html` web page that hosts the application. 
    The style file URL is *not* relative to the component file.
    That's why the example URL begins `app/`.
    See [Appendix 2](#relative-urls) to specify a URL relative to the component file.
  
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  :marked
    Users of module bundlers like Webpack may also use the `styles` attribute to load 
    styles from external files at build time. They could write:
    
    `styles: [require('my.component.css')]`

    We set the `styles` property, **not** `styleUrls` property! The module bundler is loading the CSS strings, not Angular. 
    Angular only sees the CSS strings *after* the bundler loads them. To Angular it is as if
    we wrote the `styles` array by hand. 
    Refer to the module bundler's documentation for information on loading CSS in this manner.
  
:marked
  ### Template Link Tags
  
  We can also embed `<link>` tags into the component's HTML template. 
  
  As with `styleUrls`, the link tag's `href` URL is relative to the HTML host page of the application, 
  not relative to the component file.
  
+makeExample('component-styles/ts/app/hero-team.component.ts', 'stylelink')
  
:marked
  ### CSS @imports
  
  We can also import CSS files into our CSS files by using the standard CSS
  [`@import` rule](https://developer.mozilla.org/en/docs/Web/CSS/@import).
  
  In *this* case the URL is relative to the CSS file into which we are importing.
  
+makeExample('component-styles/ts/app/hero-details.component.css', 'import', 'app/hero-details.component.css (excerpt)')
  
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:marked
  ## Controlling View Encapsulation: Native, Emulated, and None
  
  As discussed above, component CSS styles are *encapsulated* into the component's own view and do
  not affect the rest of the application.
  
  We can control how this encapsulation happens on a *per
  component* basis by setting the *view encapsulation mode* in the component metadata. There
  are three modes to choose from:
  
  * `Native` view encapsulation uses the browser's native [Shadow DOM](https://developer.mozilla.org/en-US/docs/Web/Web_Components/Shadow_DOM)
    implementation to attach a Shadow DOM to the component's host element, and then puts the component
    view inside that Shadow DOM. The component's styles are included within the Shadow DOM.
    
  * `Emulated` view encapsulation (**the default**) emulates the behavior of Shadow DOM by preprocessing
    (and renaming) the CSS code to effectively scope the CSS to the component's view.
    See [Appendix 1](#inspect-generated-css) for details.
    
  * `None` means that Angular does no view encapsulation. 
    Angular adds the CSS to the global styles. 
    The scoping rules, isolations, and protections discussed earlier do not apply. 
    This is essentially the same as pasting the component's styles into the HTML.
    
  Set the components encapsulation mode using the `encapsulation` property in the component metadata:
  
+makeExample('component-styles/ts/app/quest-summary.component.ts', 'encapsulation.native')(format='.')
:marked
  `Native` view encapsulation only works on [browsers that have native support
  for Shadow DOM](http://caniuse.com/#feat=shadowdom). The support is still limited,
  which is why `Emulated` view encapsulation is the default mode and recommended
  in most cases.

a(id="inspect-generated-css")
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  ## Appendix 1: Inspecting The CSS Generated in Emulated View Encapsulation
  
  When using the default emulated view encapsulation, Angular preprocesses
  all component styles so that they approximate the standard Shadow CSS scoping rules.
  
  When we inspect the DOM of a running Angular application with emulated view
  encapsulation enabled, we see that each DOM element has some extra attributes
  attached to it:
  
code-example(format="").
  &lt;hero-details _nghost-pmm-5>
    &lt;h2 _ngcontent-pmm-5>Mister Fantastic&lt;/h2>
    &lt;hero-team _ngcontent-pmm-5 _nghost-pmm-6>
      &lt;h3 _ngcontent-pmm-6>Team&lt;/h3>
    &lt;/hero-team>
  &lt;/hero-detail>

:marked
  We see two kinds of generated attributes:
  * An element that would be a Shadow DOM host in native encapsulation has a
    generated `_nghost` attribute. This is typically the case for component host elements.
    
  * An element within a component's view has a `_ngcontent` attribute 
  that identifies to which host's emulated Shadow DOM this element belongs.
  
  The exact values of these attributes are not important. They are automatically
  generated and we never refer to them in application code. But they are targeted
  by the generated component styles, which we'll find in the `<head>` section of the DOM:
  
code-example(format="").
  [_nghost-pmm-5] {
    display: block;
    border: 1px solid black;
  }

  h3[_ngcontent-pmm-6] {
    background-color: white;
    border: 1px solid #777;
  }
  
:marked
  These are the styles we wrote, post-processed so that each selector is augmented
  with `_nghost` or `_ngcontent` attribute selectors. 
  These extra selectors enable the scoping rules described in this guide.
  
  We'll likely live with *emulated* mode until shadow DOM gains traction.
  
a(id="relative-urls")
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:marked
  ## Appendix 2: Loading Styles with Relative URLs
  
  It's common practice to split a component's code, HTML, and CSS into three separate files in the same directory:
  
code-example(format='').
  quest-summary.component.ts
  quest-summary.component.html
  quest-summary.component.css
:marked
  We include the template and CSS files by setting the `templateUrl` and `styleUrls` metadata properties respectively.
  Because these files are co-located with the component,
  it would be nice to refer to them by name without also having to specify a path back to the root of the application.
  
  We'd *prefer* to write this:
+makeExample('component-styles/ts/app/quest-summary.component.ts', 'urls')(format='.')
:marked
  We can't do that by default. Angular can't find the files and throws an error:

  `EXCEPTION: Failed to load quest-summary.component.html`

  Why can't Angular calculate the HTML and CSS URLs from the component file's location? 
  
  Unfortunately, that location is not readily known. 
  Angular apps can be loaded in many ways: from individual files, from SystemJS packages, or
  from CommonJS packages, to name a few. 
  With this diversity of load strategies, it's not easy to tell at runtime where these files actually reside.
  
  The only location Angular can be sure of is the URL of the `index.html` home page.
  So by default it resolves template and style paths relative to the URL of `index.html`.
  That's why we previously wrote our CSS file URLs with an `app/` base path prefix.
  
  Although this works with any code loading scheme, it is very inconvenient.
  We move file folders around all the time during the evolution of our applications. 
  It's no fun patching the style and template URLs when we do.
  
  ### *moduleId*
  
  We can change the way Angular calculates the full URL be setting the component metadata's `moduleId` property.
  
  If we knew the component file's base path, we'd set `moduleId` to that and 
  let Angular construct the full URL from this base path plus the CSS and template file names.
  
  Our challenge is to calculate the base path with minimal effort. 
  If it's too hard, we shouldn't bother; we should just write the full path to the root and move on.

  Fortunately, *certain* module loaders make it easy.
  SystemJS (starting in v.0.19.19) sets a special `__moduleName` variable to the URL of the component file.
.alert.is-important
  :marked
    Caution: we currently regard the *__moduleName* feature as experimental.
:marked
  Now it's trivial to set the `moduleId` to the `__moduleName` and write module-relative paths for style and template URLs.
  
+makeExample('component-styles/ts/app/quest-summary.component.ts','', 'app/quest-summary.component.ts')

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  :marked
    With a module bundler like Webpack we are more likely to set the `styles` and `template` properties with the bundler's
    `require` mechanism rather than bother with `styleUrls` and `templateUrl`.