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Edits to "Component Styles" and "Pipes" (#3266) 

* Edits to "Component Styles"

* Edits to "Pipes".

* Updated with feedback from Jules and a few minor edits.
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cfranger 2017-02-23 18:54:19 -08:00 committed by Jules Kremer
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public/docs/ts/latest/guide/component-styles.jade
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@ -2,102 +2,98 @@ block includes
include ../_util-fns
:marked
Angular 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.
Angular applications are styled with standard CSS. That means you can apply
everything you know about CSS stylesheets, selectors, rules, and media queries
directly to Angular applications.
On top of this, Angular has the ability to bundle *component styles*
with our components enabling a more modular design than regular stylesheets.
Additionally, Angular can bundle *component styles*
with components, enabling a more modular design than regular stylesheets.
In this chapter we learn how to load and apply these *component styles*.
This page describes how to load and apply these component styles.
## Table Of Contents
* [Using Component Styles](#using-component-styles)
* [Using component styles](#using-component-styles)
* [Special selectors](#special-selectors)
* [Loading Styles into Components](#loading-styles)
* [Controlling View Encapsulation: Emulated, Native, and None](#view-encapsulation)
* [Appendix 1: Inspecting the generated runtime component styles](#inspect-generated-css)
* [Appendix 2: Loading Styles with Relative URLs](#relative-urls)
* [Loading styles into components](#loading-styles)
* [Controlling view encapsulation: native, emulated, and none](#view-encapsulation)
* [Appendix 1: Inspecting the CSS generated in emulated view encapsulation](#inspect-generated-css)
* [Appendix 2: Loading styles with relative URLs](#relative-urls)
Run the <live-example></live-example> of the code shown in this chapter.
You can run the <live-example></live-example> in Plunker and download the code from there.
.l-main-section
:marked
## Using Component Styles
## Using component styles
For every Angular component we write, we may define not only an HTML template,
For every Angular component you write, you 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.
specifying any selectors, rules, and media queries that you 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:
Usually you give it one string, as in the following example:
+makeExample('component-styles/ts/src/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 within the template
of that component*. The `h1` selector in the example above only applies to the `<h1>` tag
The selectors you put into a component's styles apply only within the template
of that component. The `h1` selector in the preceding example applies only 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:
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
* You can use the CSS class names and selectors that make the most sense in the context of each component.
* Class names and selectors are local to the component and don'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,
* Changes to styles elsewhere in the application don't affect the component's styles.
* You 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.
* You can change or remove component CSS code without searching through the
whole application to find where else the code is used.
a(id="special-selectors")
.l-main-section
:marked
## Special selectors
Component styles have a few special *selectors* from the world of
[shadow DOM style scoping](https://www.w3.org/TR/css-scoping-1):
Component styles have a few special *selectors* from the world of shadow DOM style scoping
(described in the [CSS Scoping Module Level 1](https://www.w3.org/TR/css-scoping-1) page on the
[W3C](https://www.w3.org) site).
The following sections describe these selectors.
### :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):
targeting elements *inside* the component's template).
+makeExample('component-styles/ts/src/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.
The `:host` selector is the only way to target the host element. You can't reach
the host element from inside the component with other selectors because it's not part of the
component's own template. The host element 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.
The next example targets the host element again, but only when it also has the `active` CSS class.
+makeExample('component-styles/ts/src/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.
Sometimes it's useful to apply styles based on some condition *outside* of a component's view.
For example, a CSS theme class could be applied to the document `<body>` element, and
you want to change how your 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.
Use the `:host-context()` pseudo-class selector, which works just like the function
form of `:host()`. The `:host-context()` selector looks for a CSS class in any ancestor of the component host element,
up to the document root. The `:host-context()` selector is useful when combined with another selector.
In the following example, we apply a `background-color` style to all `<h2>` elements *inside* the component, only
The following example applies 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/src/app/hero-details.component.css', 'hostcontext')(format='.')
@ -107,55 +103,46 @@ a(id="special-selectors")
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.
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 to both the view
children and 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:
The following example targets all `<h3>` elements, from the host element down
through this component to all of its child elements in the DOM.
+makeExample('component-styles/ts/src/app/hero-details.component.css', 'deep')(format=".")
:marked
The `/deep/` selector also has the alias `>>>`. We can use either of the two interchangeably.
The `/deep/` selector also has the alias `>>>`. You can use either 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](#view-encapsulation)
section for more details.
Use the `/deep/` and `>>>` selectors only with *emulated* view encapsulation.
Emulated is the default and most commonly used view encapsulation. For more information, see the
[Controlling view encapsulation](#view-encapsulation) section.
a(id='loading-styles')
.l-main-section
:marked
## Loading Styles into Components
## 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
There are several ways to add styles to a component:
* By setting `styles` or `styleUrls` metadata.
* Inline in the template HTML.
* With CSS imports.
The scoping rules outlined above apply to each of these loading patterns.
The scoping rules outlined earlier apply to each of these loading patterns.
### Styles in Metadata
### Styles in metadata
We can add a `styles` #{_array} property to the `@Component` #{_decorator}.
You 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/src/app/hero-app.component.ts')
:marked
### Template Inline Styles
### Style URLs in metadata
We can embed styles directly into the HTML template by putting them
inside `<style>` tags.
+makeExample('component-styles/ts/src/app/hero-controls.component.ts', 'inlinestyles')
:marked
### Style URLs in Metadata
We can load styles from external CSS files by adding a `styleUrls` attribute
You can load styles from external CSS files by adding a `styleUrls` attribute
into a component's `@Component` #{_decorator}:
+makeExample('component-styles/ts/src/app/hero-details.component.ts', 'styleurls')
@ -163,95 +150,103 @@ a(id='loading-styles')
block style-url
.alert.is-important
:marked
The URL is ***relative to the application root*** which is usually the
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 `src/app/`.
See [Appendix 2](#relative-urls) to specify a URL relative to the
component file.
To specify a URL relative to the component file, see [Appendix 2](#relative-urls).
block module-bundlers
.l-sub-section
: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:
If you use module bundlers like Webpack, you can also use the `styles` attribute
to load styles from external files at build time. You 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.
Set the `styles` property, not the `styleUrls` property. The module
bundler loads the CSS strings, not Angular.
Angular sees the CSS strings only after the bundler loads them.
To Angular, it's as if you wrote the `styles` array by hand.
For information on loading CSS in this manner, refer to the module bundler's documentation.
:marked
### Template Link Tags
### Template inline styles
We can also embed `<link>` tags into the component's HTML template.
You can embed styles directly into the HTML template by putting them
inside `<style>` tags.
+makeExample('component-styles/ts/src/app/hero-controls.component.ts', 'inlinestyles')
:marked
### Template link tags
You can also embed `<link>` tags into the component's HTML template.
As with `styleUrls`, the link tag's `href` URL is relative to the
application root, not relative to the component file.
application root, not the component file.
+makeExample('component-styles/ts/src/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).
You can also import CSS files into the CSS files using the standard CSS `@import` rule.
For details, see [`@import`](https://developer.mozilla.org/en/docs/Web/CSS/@import)
on the [MDN](https://developer.mozilla.org) site.
block css-import-url
:marked
In *this* case the URL is relative to the CSS file into which we are importing.
In this case, the URL is relative to the CSS file into which you're importing.
+makeExample('component-styles/ts/src/app/hero-details.component.css', 'import', 'src/app/hero-details.component.css (excerpt)')
a#view-encapsulation
.l-main-section
:marked
## Controlling View Encapsulation: Native, Emulated, and None
## 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.
As discussed earlier, component CSS styles are encapsulated into the component's view and don't
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:
To control how this encapsulation happens on a *per
component* basis, you can set the *view encapsulation mode* in the component metadata.
Choose from the following modes:
* `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
* `Native` view encapsulation uses the browser's native shadow DOM implementation (see
[Shadow DOM](https://developer.mozilla.org/en-US/docs/Web/Web_Components/Shadow_DOM)
on the [MDN](https://developer.mozilla.org) site)
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.
For details, see [Appendix 1](#inspect-generated-css).
* `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.
The scoping rules, isolations, and protections discussed earlier don't 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:
To set the components encapsulation mode, use the `encapsulation` property in the component metadata:
+makeExample('component-styles/ts/src/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,
`Native` view encapsulation only works on browsers that have native support
for shadow DOM (see [Shadow DOM v0](http://caniuse.com/#feat=shadowdom) on the
[Can I use](http://caniuse.com) site). The support is still limited,
which is why `Emulated` view encapsulation is the default mode and recommended
in most cases.
a#inspect-generated-css
.l-main-section
:marked
## Appendix 1: Inspecting The CSS Generated in Emulated View Encapsulation
## 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 using 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
In the DOM of a running Angular application with emulated view
encapsulation enabled, each DOM element has some extra attributes
attached to it:
code-example(format="").
@ -263,16 +258,15 @@ code-example(format="").
&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
There are 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.
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:
The exact values of these attributes aren't important. They are automatically
generated and you never refer to them in application code. But they are targeted
by the generated component styles, which are in the `<head>` section of the DOM:
code-example(format="").
[_nghost-pmm-5] {
@ -286,16 +280,14 @@ code-example(format="").
}
:marked
These are the styles we wrote, post-processed so that each selector is augmented
These styles are 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.
These extra selectors enable the scoping rules described in this page.
a#relative-urls
.l-main-section
:marked
## Appendix 2: Loading Styles with Relative URLs
## 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="nocode").
@ -304,16 +296,16 @@ code-example(format="nocode").
quest-summary.component.css
:marked
We include the template and CSS files by setting the `templateUrl` and `styleUrls` metadata properties respectively.
You 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.
block module-id
:marked
We can change the way Angular calculates the full URL be setting the component metadata's `moduleId` property to `module.id`.
You can change the way Angular calculates the full URL by setting the component metadata's `moduleId` property to `module.id`.
+makeExample('src/app/quest-summary.component.ts')
:marked
Learn more about `moduleId` in the [Component-Relative Paths](../cookbook/component-relative-paths.html) chapter.
Learn more about `moduleId` in the [Component-Relative Paths](../cookbook/component-relative-paths.html) page.

336
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@ -3,28 +3,29 @@ block includes
:marked
Every application starts out with what seems like a simple task: get data, transform them, and show them to users.
Getting data could be as simple as creating a local variable or as complex as streaming data over a Websocket.
Getting data could be as simple as creating a local variable or as complex as streaming data over a WebSocket.
Once data arrive, we could push their raw `toString` values directly to the view.
That rarely makes for a good user experience.
E.g., almost everyone prefers a simple birthday date like
<samp>April 15, 1988</samp> to the original raw string format
&mdash; <samp>Fri Apr 15 1988 00:00:00 GMT-0700 (Pacific Daylight Time)</samp>.
Once data arrive, you could push their raw `toString` values directly to the view,
but that rarely makes for a good user experience.
For example, in most use cases, users prefer to see a date in a simple format like
<samp>April 15, 1988</samp> rather than the raw string format
<samp>Fri Apr 15 1988 00:00:00 GMT-0700 (Pacific Daylight Time)</samp>.
Clearly some values benefit from a bit of massage. We soon discover that we
Clearly, some values benefit from a bit of editing. You may notice that you
desire many of the same transformations repeatedly, both within and across many applications.
We almost think of them as styles.
In fact, we'd like to apply them in our HTML templates as we do styles.
You can almost think of them as styles.
In fact, you might like to apply them in your HTML templates as you do styles.
Introducing Angular pipes, a way to write display-value transformations that we can declare in our HTML!
Try the <live-example></live-example>.
Introducing Angular pipes, a way to write display-value transformations that you can declare in your HTML.
You can run the <live-example></live-example> in Plunker and download the code from there.
.l-main-section
:marked
## Using Pipes
## Using pipes
A pipe takes in data as input and transforms it to a desired output.
We'll illustrate by transforming a component's birthday property into
In this page, you'll use pipes to transform a component's birthday property into
a human-friendly date.
+makeExample('pipes/ts/src/app/hero-birthday1.component.ts', null, 'src/app/hero-birthday1.component.ts')(format='.')
@ -35,14 +36,14 @@ block includes
+makeExample('pipes/ts/src/app/app.component.html', 'hero-birthday-template')(format=".")
:marked
Inside the interpolation expression we flow the component's `birthday` value through the
Inside the interpolation expression, you flow the component's `birthday` value through the
[pipe operator](./template-syntax.html#pipe) ( | ) to the [Date pipe](../api/common/index/DatePipe-pipe.html)
function on the right. All pipes work this way.
.l-sub-section
:marked
The `Date` and `Currency` pipes need the **ECMAScript Internationalization API**.
Safari and other older browsers don't support it. We can add support with a polyfill.
The `Date` and `Currency` pipes need the *ECMAScript Internationalization API*.
Safari and other older browsers don't support it. You can add support with a polyfill.
code-example(language="html").
&lt;script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=Intl.~locale.en"&gt;&lt;/script&gt;
@ -52,48 +53,49 @@ block includes
## Built-in pipes
Angular comes with a stock of pipes such as
`DatePipe`, `UpperCasePipe`, `LowerCasePipe`, `CurrencyPipe`, and `PercentPipe`.
They are all immediately available for use in any template.
They are all available for use in any template.
.l-sub-section
:marked
Learn more about these and many other built-in pipes in the [API Reference](../api/#!?query=pipe);
filter for entries that include the word "pipe".
Read more about these and many other built-in pipes in the [pipes topics](../api/#!?query=pipe) of the
[API Reference](../api); filter for entries that include the word "pipe".
Angular doesn't have a `FilterPipe` or an `OrderByPipe` for reasons explained in an [appendix below](#no-filter-pipe).
Angular doesn't have a `FilterPipe` or an `OrderByPipe` for reasons explained in the [Appendix](#no-filter-pipe) of this page.
.l-main-section
:marked
## Parameterizing a Pipe
## Parameterizing a pipe
A pipe may accept any number of optional parameters to fine-tune its output.
We add parameters to a pipe by following the pipe name with a colon ( : ) and then the parameter value
(e.g., `currency:'EUR'`). If our pipe accepts multiple parameters, we separate the values with colons (e.g. `slice:1:5`)
A pipe can accept any number of optional parameters to fine-tune its output.
To add parameters to a pipe, follow the pipe name with a colon ( : ) and then the parameter value
(such as `currency:'EUR'`). If the pipe accepts multiple parameters, separate the values with colons (such as `slice:1:5`)
We'll modify our birthday template to give the date pipe a format parameter.
After formatting the hero's April 15th birthday, it should render as **<samp>04/15/88</samp>**:
Modify the birthday template to give the date pipe a format parameter.
After formatting the hero's April 15th birthday, it renders as **<samp>04/15/88</samp>**:
+makeExample('pipes/ts/src/app/app.component.html', 'format-birthday')(format=".")
:marked
The parameter value can be any valid
[template expression](./template-syntax.html#template-expressions)
The parameter value can be any valid template expression,
(see the [Template expressions](./template-syntax.html#template-expressions) section of the
[Template Syntax](./template-syntax.html) page)
such as a string literal or a component property.
In other words, we can control the format through a binding the same way we control the birthday value through a binding.
In other words, you can control the format through a binding the same way you control the birthday value through a binding.
Let's write a second component that *binds* the pipe's format parameter
Write a second component that *binds* the pipe's format parameter
to the component's `format` property. Here's the template for that component:
+makeExample('pipes/ts/src/app/hero-birthday2.component.ts', 'template', 'src/app/hero-birthday2.component.ts (template)')(format=".")
:marked
We also added a button to the template and bound its click event to the component's `toggleFormat()` method.
You also added a button to the template and bound its click event to the component's `toggleFormat()` method.
That method toggles the component's `format` property between a short form
(`'shortDate'`) and a longer form (`'fullDate'`).
+makeExample('pipes/ts/src/app/hero-birthday2.component.ts', 'class', 'src/app/hero-birthday2.component.ts (class)')(format='.')
:marked
As we click the button, the displayed date alternates between
As you click the button, the displayed date alternates between
"**<samp>04/15/1988</samp>**" and
"**<samp>Friday, April 15, 1988</samp>**".
@ -103,61 +105,58 @@ figure.image-display
.l-sub-section
:marked
Learn more about the `DatePipe` format options in the [API Docs](../api/common/index/DatePipe-pipe.html).
Read more about the `DatePipe` format options in the [Date Pipe](../api/common/index/DatePipe-pipe.html)
API Reference page.
:marked
## Chaining pipes
We can chain pipes together in potentially useful combinations.
In the following example, we chain the birthday to the `DatePipe` and on to the `UpperCasePipe`
so we can display the birthday in uppercase. The following birthday displays as
**<samp>APR 15, 1988</samp>**.
You can chain pipes together in potentially useful combinations.
In the following example, to display the birthday in uppercase,
the birthday is chained to the `DatePipe` and on to the `UpperCasePipe`.
The birthday displays as **<samp>APR 15, 1988</samp>**.
+makeExample('pipes/ts/src/app/app.component.html', 'chained-birthday')(format=".")
:marked
This example &mdash; which displays **<samp>FRIDAY, APRIL 15, 1988</samp>** &mdash;
chains the same pipes as above, but passes in a parameter to `date` as well.
This example&mdash;which displays **<samp>FRIDAY, APRIL 15, 1988</samp>**&mdash;chains
the same pipes as above, but passes in a parameter to `date` as well.
+makeExample('pipes/ts/src/app/app.component.html', 'chained-parameter-birthday')(format=".")
.l-main-section
:marked
## Custom Pipes
## Custom pipes
We can write our own custom pipes.
You can write your own custom pipes.
Here's a custom pipe named `ExponentialStrengthPipe` that can boost a hero's powers:
+makeExample('pipes/ts/src/app/exponential-strength.pipe.ts', null, 'src/app/exponential-strength.pipe.ts')(format=".")
:marked
This pipe definition reveals several key points:
This pipe definition reveals the following key points:
* A pipe is a class decorated with pipe metadata.
* The pipe class implements the `PipeTransform` interface's `transform` method that
accepts an input value followed by optional parameters and returns the transformed value.
* There will be one additional argument to the `transform` method for each parameter passed to the pipe.
Our pipe has one such parameter: the `exponent`.
* We tell Angular that this is a pipe by applying the
`@Pipe` #{_decorator} which we import from the core Angular library.
* The `@Pipe` #{_decorator} allows us to define the
pipe name that we'll use within template expressions. It must be a valid JavaScript identifier.
Our pipe's name is `exponentialStrength`.
Your pipe has one such parameter: the `exponent`.
* To tell Angular that this is a pipe, you apply the
`@Pipe` #{_decorator}, which you import from the core Angular library.
* The `@Pipe` #{_decorator} allows you to define the
pipe name that you'll use within template expressions. It must be a valid JavaScript identifier.
Your pipe's name is `exponentialStrength`.
.l-sub-section
:marked
### The *PipeTransform* Interface
### The *PipeTransform* interface
The `transform` method is essential to a pipe.
The `PipeTransform` *interface* defines that method and guides both tooling and the compiler.
It is technically optional; Angular looks for and executes the `transform` method regardless.
Technically, it's optional; Angular looks for and executes the `transform` method regardless.
:marked
Now we need a component to demonstrate our pipe.
Now you need a component to demonstrate the pipe.
+makeExample('pipes/ts/src/app/power-booster.component.ts',null,'src/app/power-booster.component.ts')(format='.')
figure.image-display
img(src='/resources/images/devguide/pipes/power-booster.png' alt="Power Booster")
@ -165,28 +164,28 @@ figure.image-display
- var _decls = _docsFor == 'dart' ? 'pipes' : 'declarations';
- var _appMod = _docsFor == 'dart' ? '@Component' : 'AppModule';
:marked
Two things to note:
Note the following:
1. We use our custom pipe the same way we use built-in pipes.
1. We must include our pipe in the `!{_decls}` #{_array} of the `!{_appMod}`.
* You use your custom pipe the same way you use built-in pipes.
* You must include your pipe in the `!{_decls}` #{_array} of the `!{_appMod}`.
.callout.is-helpful
header Remember the !{_decls} #{_array}!
header Remember the !{_decls} #{_array}
:marked
Angular reports an error if we neglect to list our custom pipe.
We didn't list the `DatePipe` in our previous example because all
You must manually register custom pipes.
If you don't, Angular reports an error.
In the previous example, you didn't list the `DatePipe` because all
Angular built-in pipes are pre-registered.
Custom pipes must be registered manually.
:marked
If we try the <live-example></live-example>,
we can probe its behavior by changing the value and the optional exponent in the template.
To probe the behavior in the <live-example></live-example>,
change the value and optional exponent in the template.
## Power Boost Calculator (extra-credit)
## Power Boost Calculator
It's not much fun updating the template to test our custom pipe.
We could upgrade the example to a "Power Boost Calculator" that combines
our pipe and two-way data binding with `ngModel`.
It's not much fun updating the template to test the custom pipe.
Upgrade the example to a "Power Boost Calculator" that combines
your pipe and two-way data binding with `ngModel`.
+makeExample('src/app/power-boost-calculator.component.ts')
@ -196,140 +195,140 @@ figure.image-display
.l-main-section
a#change-detection
:marked
## Pipes and Change Detection
## Pipes and change detection
Angular looks for changes to data-bound values through a *change detection* process that runs after every JavaScript event:
every keystroke, mouse move, timer tick, and server response. This could be expensive.
Angular strives to lower the cost whenever possible and appropriate.
Angular picks a simpler, faster change detection algorithm when we use a pipe. Let's see how.
Angular picks a simpler, faster change detection algorithm when you use a pipe.
### No pipe
The component in our next example uses the default, aggressive change detection strategy to monitor and update
In the next example, the component uses the default, aggressive change detection strategy to monitor and update
its display of every hero in the `heroes` #{_array}. Here's the template:
+makeExample('pipes/ts/src/app/flying-heroes.component.html', 'template-1', 'src/app/flying-heroes.component.html (v1)')(format='.')
:marked
The companion component class provides heroes, adds new heroes into the #{_array}, and can reset the #{_array}.
The companion component class provides heroes, adds heroes into the #{_array}, and can reset the #{_array}.
+makeExample('pipes/ts/src/app/flying-heroes.component.ts', 'v1', 'src/app/flying-heroes.component.ts (v1)')(format='.')
:marked
We can add a new hero and Angular updates the display when we do.
The `reset` button replaces `heroes` with a new #{_array} of the original heroes and Angular updates the display when we do.
If we added the ability to remove or change a hero, Angular would detect those changes too and update the display as well.
You can add heroes and Angular updates the display when you do.
If you click the `reset` button, Angular replaces `heroes` with a new #{_array} of the original heroes and updates the display.
If you added the ability to remove or change a hero, Angular would detect those changes and update the display as well.
### Flying Heroes pipe
### Flying-heroes pipe
Let's add a `FlyingHeroesPipe` to the `*ngFor` repeater that filters the list of heroes to just those heroes who can fly.
Add a `FlyingHeroesPipe` to the `*ngFor` repeater that filters the list of heroes to just those heroes who can fly.
+makeExample('pipes/ts/src/app/flying-heroes.component.html', 'template-flying-heroes', 'src/app/flying-heroes.component.html (flyers)')(format='.')
:marked
Here's the `FlyingHeroesPipe` implementation which follows the pattern for custom pipes we saw earlier.
Here's the `FlyingHeroesPipe` implementation, which follows the pattern for custom pipes described earlier.
+makeExample('pipes/ts/src/app/flying-heroes.pipe.ts', 'pure', 'src/app/flying-heroes.pipe.ts')(format='.')
:marked
When we run the sample now we see odd behavior (try it in the <live-example></live-example>).
Every hero we add is a flying hero but none of them are displayed.
Notice the odd behavior in the <live-example></live-example>:
when you add flying heroes, none of them are displayed under "Heroes who fly."
Although we're not getting the behavior we want, Angular isn't broken.
It's just using a different change detection algorithm &mdash; one that ignores changes to the list or any of its items.
Although you're not getting the behavior you want, Angular isn't broken.
It's just using a different change-detection algorithm that ignores changes to the list or any of its items.
Look at how we're adding a new hero:
Notice how a hero is added:
+makeExample('pipes/ts/src/app/flying-heroes.component.ts', 'push')(format='.')
:marked
We're adding the new hero into the `heroes` #{_array}. The reference to the #{_array} hasn't changed.
You add the hero into the `heroes` #{_array}. The reference to the #{_array} hasn't changed.
It's the same #{_array}. That's all Angular cares about. From its perspective, *same #{_array}, no change, no display update*.
We can fix that. Let's create a new #{_array} with the new hero appended and assign that to `heroes`.
To fix that, create an #{_array} with the new hero appended and assign that to `heroes`.
This time Angular detects that the #{_array} reference has changed.
It executes the pipe and updates the display with the new #{_array} which includes the new flying hero.
It executes the pipe and updates the display with the new #{_array}, which includes the new flying hero.
*If we **mutate** the #{_array}, no pipe is invoked and no display updated;
if we **replace** the #{_array}, then the pipe executes and the display is updated*.
The *Flying Heroes* extends the
code with checkbox switches and additional displays to help us experience these effects.
If you *mutate* the #{_array}, no pipe is invoked and the display isn't updated;
if you *replace* the #{_array}, the pipe executes and the display is updated.
The Flying Heroes application extends the
code with checkbox switches and additional displays to help you experience these effects.
figure.image-display
img(src='/resources/images/devguide/pipes/flying-heroes-anim.gif' alt="Flying Heroes")
:marked
Replacing the #{_array} is an efficient way to signal to Angular that it should update the display.
When do we replace the #{_array}? When the data change.
That's an easy rule to follow in *this toy* example
where the only way to change the data is by adding a new hero.
Replacing the #{_array} is an efficient way to signal Angular to update the display.
When do you replace the #{_array}? When the data change.
That's an easy rule to follow in *this* example
where the only way to change the data is by adding a hero.
More often we don't know when the data have changed,
More often, you don't know when the data have changed,
especially in applications that mutate data in many ways,
perhaps in application locations far away.
A component in such an application usually can't know about those changes.
Moreover, it's unwise to distort our component design to accommodate a pipe.
We strive as much as possible to keep the component class independent of the HTML.
Moreover, it's unwise to distort the component design to accommodate a pipe.
Strive to keep the component class independent of the HTML.
The component should be unaware of pipes.
Perhaps we should consider a different kind of pipe for filtering flying heroes, an *impure pipe*.
For filtering flying heroes, consider an *impure pipe*.
.l-main-section
:marked
## Pure and Impure Pipes
## Pure and impure pipes
There are two categories of pipes: **pure** and **impure**.
Pipes are pure by default. Every pipe we've seen so far has been pure.
We make a pipe impure by setting its pure flag to false. We could make the `FlyingHeroesPipe`
There are two categories of pipes: *pure* and *impure*.
Pipes are pure by default. Every pipe you've seen so far has been pure.
You make a pipe impure by setting its pure flag to false. You could make the `FlyingHeroesPipe`
impure like this:
+makeExample('pipes/ts/src/app/flying-heroes.pipe.ts', 'pipe-decorator')(format='.')
:marked
Before we do that, let's understand the difference between *pure* and *impure*, starting with a *pure* pipe.
Before doing that, understand the difference between pure and impure, starting with a pure pipe.
### Pure pipes
block pure-change
:marked
Angular executes a *pure pipe* only when it detects a *pure change* to the input value.
A ***pure change*** is *either* a change to a primitive input value (`String`, `Number`, `Boolean`, `Symbol`)
*or* a changed object reference (`Date`, `Array`, `Function`, `Object`).
A pure change is either a change to a primitive input value (`String`, `Number`, `Boolean`, `Symbol`)
or a changed object reference (`Date`, `Array`, `Function`, `Object`).
:marked
Angular ignores changes *within* (composite) objects.
It won't call a pure pipe if we change an input month, add to an input #{_array}, or update an input object property.
Angular ignores changes within (composite) objects.
It won't call a pure pipe if you change an input month, add to an input #{_array}, or update an input object property.
This may seem restrictive but it is also fast.
This may seem restrictive but it's also fast.
An object reference check is fast&mdash;much faster than a deep check for
differences&mdash;so Angular can quickly determine if it can skip both the
pipe execution and a view update.
For this reason, we prefer a pure pipe if we can live with the change detection strategy.
When we can't, we *may* turn to the impure pipe.
For this reason, a pure pipe is preferable when you can live with the change detection strategy.
When you can't, you *can* use the impure pipe.
.l-sub-section
:marked
Or we might not use a pipe at all.
Or you might not use a pipe at all.
It may be better to pursue the pipe's purpose with a property of the component,
a point we take up later.
a point that's discussed later in this page.
:marked
### Impure pipes
Angular executes an *impure pipe* during *every* component change detection cycle.
An impure pipe will be called a lot, as often as every keystroke or mouse-move.
Angular executes an *impure pipe* during every component change detection cycle.
An impure pipe is called often, as often as every keystroke or mouse-move.
With that concern in mind, we must implement an impure pipe with great care.
With that concern in mind, implement an impure pipe with great care.
An expensive, long-running pipe could destroy the user experience.
<a id="impure-flying-heroes"></a>
### An impure *FlyingHeroesPipe*
A flip of the switch turns our `FlyingHeroesPipe` into a `FlyingHeroesImpurePipe`.
Here's the complete implementation:
A flip of the switch turns the `FlyingHeroesPipe` into a `FlyingHeroesImpurePipe`.
The complete implementation is as follows:
+makeTabs(
'pipes/ts/src/app/flying-heroes.pipe.ts, pipes/ts/src/app/flying-heroes.pipe.ts',
'impure, pure',
'FlyingHeroesImpurePipe, FlyingHeroesPipe')(format='.')
:marked
We inherit from `FlyingHeroesPipe` to prove the point that nothing changed internally.
You inherit from `FlyingHeroesPipe` to prove the point that nothing changed internally.
The only difference is the `pure` flag in the pipe metadata.
This is a good candidate for an impure pipe because the `transform` function is trivial and fast.
@ -337,7 +336,7 @@ block pure-change
+makeExcerpt('src/app/flying-heroes.pipe.ts','filter', '')
:marked
We can derive a `FlyingHeroesImpureComponent` from `FlyingHeroesComponent`.
You can derive a `FlyingHeroesImpureComponent` from `FlyingHeroesComponent`.
- var _fnSuffix = _docsFor == 'dart' ? '.component.ts' : '-impure.component.html';
- var _region = _docsFor == 'dart' ? 'impure-component' : 'template-flying-heroes';
@ -345,21 +344,21 @@ block pure-change
:marked
The only substantive change is the pipe in the template.
We can confirm in the <live-example></live-example> that the _flying heroes_
display updates as we enter new heroes even when we mutate the `heroes` #{_array}.
You can confirm in the <live-example></live-example> that the _flying heroes_
display updates as you add heroes, even when you mutate the `heroes` #{_array}.
- var _dollar = _docsFor === 'ts' ? '$' : '';
h3#async-pipe The impure #[i AsyncPipe]
:marked
The Angular `AsyncPipe` is an interesting example of an impure pipe.
The `AsyncPipe` accepts a `#{_Promise}` or `#{_Observable}` as input
and subscribes to the input automatically, eventually returning the emitted value(s).
and subscribes to the input automatically, eventually returning the emitted values.
It is also stateful.
The `AsyncPipe` is also stateful.
The pipe maintains a subscription to the input `#{_Observable}` and
keeps delivering values from that `#{_Observable}` as they arrive.
In this next example, we bind an `#{_Observable}` of message strings
This next example binds an `#{_Observable}` of message strings
(`message#{_dollar}`) to a view with the `async` pipe.
+makeExample('pipes/ts/src/app/hero-async-message.component.ts', null, 'src/app/hero-async-message.component.ts')
@ -367,42 +366,41 @@ h3#async-pipe The impure #[i AsyncPipe]
:marked
The Async pipe saves boilerplate in the component code.
The component doesn't have to subscribe to the async data source,
it doesn't extract the resolved values and expose them for binding,
and the component doesn't have to unsubscribe when it is destroyed
extract the resolved values and expose them for binding,
and have to unsubscribe when it's destroyed
(a potent source of memory leaks).
### An impure caching pipe
Let's write one more impure pipe, a pipe that makes an HTTP request.
Write one more impure pipe, a pipe that makes an HTTP request.
Remember that impure pipes are called every few milliseconds.
If we're not careful, this pipe will punish the server with requests.
If you're not careful, this pipe will punish the server with requests.
We are careful.
The pipe only calls the server when the request URL changes and it caches the server response.
Here's the code<span if-docs="ts">, which uses the [Angular http](server-communication.html) client to retrieve data</span>:
In the following code, the pipe only calls the server when the request URL changes and it caches the server response.
The code<span if-docs="ts"> uses the [Angular http](server-communication.html) client to retrieve data</span>:
+makeExample('src/app/fetch-json.pipe.ts')
:marked
Then we demonstrate it in a harness component whose template defines two bindings to this pipe,
Now demonstrate it in a harness component whose template defines two bindings to this pipe,
both requesting the heroes from the `heroes.json` file.
+makeExample('src/app/hero-list.component.ts')
:marked
The component renders like this:
The component renders as the following:
figure.image-display
img(src='/resources/images/devguide/pipes/hero-list.png' alt="Hero List")
:marked
A breakpoint on the pipe's request for data shows that
* each binding gets its own pipe instance
* each pipe instance caches its own url and data
* each pipe instance only calls the server once
A breakpoint on the pipe's request for data shows the following:
* Each binding gets its own pipe instance.
* Each pipe instance caches its own URL and data.
* Each pipe instance only calls the server once.
### *JsonPipe*
The second `fetch` pipe binding above demonstrates more pipe chaining.
In the previous code sample, the second `fetch` pipe binding demonstrates more pipe chaining.
It displays the same hero data in JSON format by chaining through to the built-in `JsonPipe`.
.callout.is-helpful
@ -417,25 +415,25 @@ a#pure-pipe-pure-fn
### Pure pipes and pure functions
A pure pipe uses pure functions.
Pure functions process inputs and return values without detectable side-effects.
Given the same input they should always return the same output.
Pure functions process inputs and return values without detectable side effects.
Given the same input, they should always return the same output.
The pipes we saw earlier in this chapter were implemented with pure functions.
The pipes discussed earlier in this page are implemented with pure functions.
The built-in `DatePipe` is a pure pipe with a pure function implementation.
So is our `ExponentialStrengthPipe`.
So is our `FlyingHeroesPipe`.
A few steps back we reviewed the `FlyingHeroesImpurePipe` &mdash; *an impure pipe with a pure function*.
So are the `ExponentialStrengthPipe` and `FlyingHeroesPipe`.
A few steps back, you reviewed the `FlyingHeroesImpurePipe`&mdash;an impure pipe with a pure function.
But a *pure pipe* must always be implemented with a *pure function*. Failure to heed this warning will bring about many a console errors regarding expressions that have changed after they were checked.
But always implement a *pure pipe* with a *pure function*.
Otherwise, you'll see many console errors regarding expressions that changed after they were checked.
.l-main-section
:marked
## Next Steps
## Next steps
Pipes are a great way to encapsulate and share common display-value
transformations. We use them like styles, dropping them
into our templates expressions to enrich the appeal and usability
of our views.
transformations. Use them like styles, dropping them
into your template's expressions to enrich the appeal and usability
of your views.
Explore Angular's inventory of built-in pipes in the [API Reference](../api/#!?query=pipe).
Try writing a custom pipe and perhaps contributing it to the community.
@ -443,45 +441,45 @@ a#pure-pipe-pure-fn
a(id="no-filter-pipe")
.l-main-section
:marked
## No *FilterPipe* or *OrderByPipe*
## Appendix: No *FilterPipe* or *OrderByPipe*
Angular does not ship with pipes for filtering or sorting lists.
Angular doesn't provide pipes for filtering or sorting lists.
Developers familiar with AngularJS know these as `filter` and `orderBy`.
There are no equivalents in Angular.
This is not an oversight. Angular is unlikely to offer such pipes because
(a) they perform poorly and (b) they prevent aggressive minification.
This isn't an oversight. Angular doesn't offer such pipes because
they perform poorly and prevent aggressive minification.
Both `filter` and `orderBy` require parameters that reference object properties.
We learned earlier that such pipes must be [*impure*](#pure-and-impure-pipes) and that
Angular calls impure pipes in almost every change detection cycle.
Earlier in this page, you learned that such pipes must be [impure](#pure-and-impure-pipes) and that
Angular calls impure pipes in almost every change-detection cycle.
Filtering and especially sorting are expensive operations.
The user experience can degrade severely for even moderate sized lists when Angular calls these pipe methods many times per second.
The `filter` and `orderBy` have often been abused in AngularJS apps, leading to complaints that Angular itself is slow.
The user experience can degrade severely for even moderate-sized lists when Angular calls these pipe methods many times per second.
`filter` and `orderBy` have often been abused in AngularJS apps, leading to complaints that Angular itself is slow.
That charge is fair in the indirect sense that AngularJS prepared this performance trap
by offering `filter` and `orderBy` in the first place.
The minification hazard is also compelling if less obvious. Imagine a sorting pipe applied to a list of heroes.
We might sort the list by hero `name` and `planet` of origin properties something like this:
The minification hazard is also compelling, if less obvious. Imagine a sorting pipe applied to a list of heroes.
The list might be sorted by hero `name` and `planet` of origin properties in the following way:
code-example(language="html")
&lt;!-- NOT REAL CODE! -->
&lt;div *ngFor="let hero of heroes | orderBy:'name,planet'">&lt;/div>
:marked
We identify the sort fields by text strings, expecting the pipe to reference a property value by indexing
(e.g., `hero['name']`).
Unfortunately, aggressive minification *munges* the `Hero` property names so that `Hero.name` and `Hero.planet`
becomes something like `Hero.a` and `Hero.b`. Clearly `hero['name']` is not going to work.
You identify the sort fields by text strings, expecting the pipe to reference a property value by indexing
(such as `hero['name']`).
Unfortunately, aggressive minification manipulates the `Hero` property names so that `Hero.name` and `Hero.planet`
become something like `Hero.a` and `Hero.b`. Clearly `hero['name']` doesn't work.
Some of us may not care to minify this aggressively. That's *our* choice.
But the Angular product should not prevent someone else from minifying aggressively.
Therefore, the Angular team decided that everything shipped in Angular will minify safely.
While some may not care to minify this aggressively,
the Angular product shouldn't prevent anyone from minifying aggressively.
Therefore, the Angular team decided that everything Angular provides will minify safely.
The Angular team and many experienced Angular developers strongly recommend that you move
The Angular team and many experienced Angular developers strongly recommend moving
filtering and sorting logic into the component itself.
The component can expose a `filteredHeroes` or `sortedHeroes` property and take control
over when and how often to execute the supporting logic.
Any capabilities that you would have put in a pipe and shared across the app can be
written in a filtering/sorting service and injected into the component.
If these performance and minification considerations do not apply to you, you can always create your own such pipes
(along the lines of the [FlyingHeroesPipe](#impure-flying-heroes)) or find them in the community.
If these performance and minification considerations don't apply to you, you can always create your own such pipes
(similar to the [FlyingHeroesPipe](#impure-flying-heroes)) or find them in the community.