include ../../../../_includes/_util-fns
:markdown
  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. 
  
  Once data arrive, we could push their raw `toString` values directly to screen. 
  That rarely makes for a good user experience. 
  Almost everyone prefers a simple birthday date 
  (<span style="font-family:courier">April 15, 1988</span>) to the original raw string format 
  ( <span style="font-family:courier">Fri Apr 15 1988 00:00:00 GMT-0700 (Pacific Daylight Time)</span> ). 
  
  Clearly some values benefit from a bit of massage. We soon discover that we
  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.

  Welcome, Angular pipes, the simple display-value transformations that we can declare in our HTML! 

.l-main-section
:markdown
  ## 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 
  a human-friendly date. 
  
  Here's a complete mini-app with a `DatePipe`:
  <!--
    All date samples are in my plunker
    http://plnkr.co/edit/RDlOma?p=preview
  -->
  
+makeExample('pipes/ts/src/app/app.ts', 'hero-birthday')

:markdown
  Focus on the component's template to see how we applied the built-in `DatePipe` 
  while binding the `birthday` property. 
+makeExample('pipes/ts/src/app/app.html', 'hero-birthday-template')(format=".")

:markdown
  Angular [template syntax](./template-syntax.html#pipe) includes a pipe operator ( | ) which we're
  using to flow the birthday value on the left through to the `Date` pipe function on the right.
  All pipes work this way.

.l-main-section
:markdown
  ## Built-in pipes
  Angular comes with a stock set of pipes such as 
  `DatePipe`, `UpperCasePipe`, `LowerCasePipe`, `CurrencyPipe`, and `PercentPipe`.
  They are all immediately available for use in any template.
.l-sub-section
  :markdown
    Learn more about these and many other built-in pipes in the  the [API Reference](../api/); 
    filter for entries that include the word "pipe". 

.l-main-section
:markdown
  ## 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`)
  
  We'll modify our birthday example to give the date pipe a format parameter.
  The formatted date should display as **<span style="font-family:courier">04/15/88</span>**.

+makeExample('pipes/ts/src/app/app.html', 'format-birthday')(format=".")

:markdown
  The parameter value can be any valid
  [template expression](./template-expression.html#template-expressions) 
  such as a string literal or a component property.

  Let's revise our example to bind the pipe's format parameter
  to the component's `format` property.
+makeExample('pipes/ts/src/app/hero-birthday.2.ts', 'hero-birthday2')  

:markdown
  We 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').
  
  As we click the button, the displayed date alternates between
  "**<span style="font-family:courier">04/15/1988</span>**" and
  "**<span style="font-family:courier">Friday, April 15, 1988</span>**".

figure.image-display
    img(src='/resources/images/devguide/pipes/date-format-toggle-anim.gif' alt="Date Format Toggle")
:markdown
.l-sub-section
  :markdown
    Learn more about the `DatePipes` format options in the [API Docs](../api/core/DatePipe-class.html).
:markdown
  ## 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 
  **<span style="font-family:courier">APR 15, 1988</span>**

+makeExample('pipes/ts/src/app/app.html', 'chained-birthday')

:markdown
  If we pass a parameter to a filter, we have to add parentheses
  to help the template compiler with the evaluation order.
  The following example displays
  **<span style="font-family:courier">FRIDAY, APRIL 15, 1988</span>** 

+makeExample('pipes/ts/src/app/app.html', 'chained-parameter-birthday')

.l-sub-section
  p Future improvements in the template compiler may eliminate the need for parentheses.

.l-main-section
:markdown
  ## Custom Pipes

  We can write our own custom pipes.
  
  Let's make a custom pipe named `ExponentialStrengthPipe`
  that can boost a hero's powers.
  
  Create a new file, `exponential-strength-pipe.ts`, and enter the following:
  <!--
    The exponential pipe samples are in my plunker
    http://plnkr.co/edit/8Nnnwf?p=preview
  -->
+makeExample('pipes/ts/src/app/exponential-strength-pipe.ts')

:markdown
  This pipe definition reveals several few key points
  * We import the `Pipe` decorator from the Angular library (while getting the usual symbols)
  * A pipe is a class
  * We decorate the class with the `@Pipe` decorator function. 
  * The `@Pipe` decorator takes an object with a name property whose value is the
     pipe name that we'll use within a template expression. It must be a valid JavaScript identier.
     Our pipe's name is `exponenentialStrength`. 
  * The pipe class implements a `transform` method 
  * `transform` takes a value and an optional array of strings. 
     The value can be of any type but the arguments array must be an array of strings.
  * There will be one item in the array for each parameter passed to the pipe
  * `transform` returns a modified value that Angular converts to a string.

  Now let's create a component to demonstrate our pipe.
+makeExample('pipes/ts/src/app/power-booster.ts')
figure.image-display
    img(src='/resources/images/devguide/pipes/power-booster.png' alt="Power Booster")

:markdown
  Two things to note:
  1. We use the pipe in the template expression exactly as we described in the pipe's comments. 
  We pass the value to transform from the left and give our pipe an exponent parameter of `10`.
  
  1. We must list our pipe in the @Component decorator's `pipes` array.
  
.callout.is-critical
  header Remember the pipes array!
  :markdown
    Angular reports an error if we neglect to list our custom pipe. 
    We didn't list the `DatePipe` in our previous example because all
    Angular built-in pipes are pre-registered. 
    Custom pipes must be registered manually.
:markdown
  If we are inclined to try this in a live-coding tool (such a [plunker](http://plnkr.co/)),
  we can probe its behavior by changing the value and the optional exponent in the template.
  
  ## Power Boost Calculator (extra-credit)
  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 `ng-model`.

+makeExample('pipes/ts/src/app/power-boost-calculator.ts')

figure.image-display
    img(src='/resources/images/devguide/pipes/power-boost-calculator.png' alt="Power Boost Calculator")
:markdown 

.l-main-section
:markdown
  ## Stateful Pipes

  There are two categories of pipes, stateless and stateful. 
  
  Stateless pipes are pure functions that flow input data 
  through without remembering anything or causing detectable side-effects.
  
  Most pipes are stateless. The `DatePipe` in our first example is a stateless pipe. So is our custom `ExponentialStrengthPipe`.
  
  Stateful pipes are conceptually similar to classes in object-oriented programming. They can manage the data they transform. A pipe that creates an HTTP request, stores the response and displays the output, is a stateful pipe. 
  Pipes that retrieve or request data should be used cautiously, since working with network data tends to introduce error conditions that are better handled in JavaScript than in a template. 
  We can mitigate this risk by creating a custom pipe for a particular backend and bake-in the essential error-handling.

  ## The stateful `AsyncPipe`
  The Angular Async pipe is a remarkable example of a stateful pipe. 
  The Async pipe can receive a Promise or Observable as input 
  and subscribe to the input automatically, eventually returning the emitted value(s). 
  
  It is stateful because the pipe maintains a subscription to the input and its returned values depend on that subscription.

  In the next example, we bind a simple promise to a view with the async pipe.
+makeExample('pipes/ts/src/app/app.ts', 'async-message')

:markdown
  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 (in the case of Obsevable stream sources like `EventEmitter`)
  the component doesn't have to unsubscribe when it is destroyed
  (a potent source of memory leaks).
 
  ### Implementing a Stateful Pipe
  Pipes are stateless by default. 
  We must declare a pipe to be stateful 
  by setting the `pure` property of the `@Pipe` decorator to `false`.
  This setting tells Angular’s change detection system to 
  check the output of this pipe each cycle, whether its input has changed or not.

  Here's how we'll decorate our new stateful `FetchJsonPipe` that 
  makes an HTTP `fetch` request and (eventually) displays the data in the server's response:
+makeExample('pipes/ts/src/app/fetch-json-pipe.ts', 'pipe-metadata')
:markdown
  Immediately below we have the finished pipe. Its input value is an url to an endpoint that returns a JSON file. 
  The pipe makes a one-time async request to the server and eventually receives the JSON response.
+makeExample('pipes/ts/src/app/fetch-json-pipe.ts')
:markdown
  Next we use this pipe in two template bindings where we
  1. display hero names in an `ng-for` repeater
  1. chain the fetched results to the built-in `JsonPipe` that renders
  the data in JSON format

+makeExample('pipes/ts/src/app/hero-list-component.ts')

figure.image-display
    img(src='/resources/images/devguide/pipes/hero-list.png' alt="Hero List")

.l-main-section
:markdown
  ## 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.
  
  Explore Angular's inventory of built-in pipes in the [API Reference](../api/). 
  Try writing a custom pipe and perhaps contributing it to the community.