angular-cn/public/docs/ts/latest/guide/pipes.jade

block includes
  include ../_util-fns

: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.

  每个应用开始的时候差不多都是一些简单任务：获取数据、转换它们，然后把它们显示给用户。
  获取数据可能简单到创建一个局部变量就行，也可能复杂到从WebSocket中获取数据流。

  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>.

  一旦取到数据，我们可以把它们原始值的`toString`结果直接推入视图中。
  但这种做法很少能具备良好的用户体验。
  比如，几乎每个人都更喜欢简单的日期格式，例如<samp>1988-04-15</samp>，而不是服务端传过来的原始字符串格
  式 —— <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
  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.

  显然，对某些值做一点修饰会显得更友好。我们很快就会发现，相同的“数据转换”工作要重复做很多次，甚至会出现在很多应用中。
  我们甚至可以把它看做样式。
  事实上，我们确实更喜欢像样式那样把它们应用到HTML模板中。
p.
  Welcome, Angular pipes, the simple display-value transformations that we can declare in our HTML!
  Try the #[+liveExampleLink2()].

p.
  欢迎来到Angular“管道(Pipe)”的世界！我们可以把这种简单的“显示值”转换器声明在HTML中。
  试试#[+liveExampleLink2('在线例子')].

.l-main-section
:marked
  ## Using Pipes
  ## 使用管道

  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:

  管道把数据作为输入，然后转换它，给出期望的输出。
  我们将把组件的`birthday`属性转换成对人类更友好的日期格式，来说明这一点：

+makeExample('pipes/ts/app/hero-birthday1.component.ts', null, 'app/hero-birthday1.component.ts')(format='.')

:marked
  Focus on the component's template.

  重点看下组件的模板。

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

:marked
  Inside the interpolation expression we flow the component's `birthday` value through the
  [pipe operator](./template-syntax.html#pipe) ( | ) to the [Date pipe](../api/common/DatePipe-class.html)
  function on the right. All pipes work this way.

  在这个插值表达式中，我们让组件的`birthday`值通过[管道操作符](./template-syntax.html#pipe)( | )流动到
  右侧的[Date管道](../api/common/DatePipe-class.html)函数中。所有管道都会用这种方式工作。

.l-main-section
:marked
  ## Built-in pipes
  ## 内建的管道
  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.

  Angular内置了一些管道，比如`DatePipe`、`UpperCasePipe`、`LowerCasePipe`、`CurrencyPipe`和`PercentPipe`。
  它们全都可以直接用在任何模板中。

.l-sub-section
  :marked
    Learn more about these and many other built-in pipes in the  the [API Reference](../api/#!?apiFilter=pipe);
    filter for entries that include the word "pipe".

    要学习更多内建管道的知识，参见[API参考](../api/#!?apiFilter=pipe)，并用“pipe”为关键词对结果进行过滤。

    Angular 2 doesn't have a `FilterPipe` or an `OrderByPipe` for reasons explained in an [appendix below](#no-filter-pipe).

    Angular 2没有`FilterPipe`或`OrderByPipe`管道，原因在[后面的附录中](#no-filter-pipe)有解释。

.l-main-section
:marked
  ## Parameterizing a Pipe
  ## 对管道进行参数化

  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`)

  管道可能接受任何数量的可选参数来对它的输出进行微调。
  我们可以在管道名后面添加一个冒号( : )再跟一个参数值，来为管道添加参数(比如`currency:'EUR'`)。
  如果我们的管道可以接受多个参数，那么就用冒号来分隔这些参数值(比如`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>**.

  我们将通过修改生日模板来给这个日期管道提供一个格式化参数。
  当格式化完该英雄的4月15日生日之后，它应该被渲染成**<samp>04/15/88</samp>**。

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

:marked
  The parameter value can be any valid
  [template expression](./template-syntax.html#template-expressions)
  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.

  参数值可以是任何有效的[模板表达式](./template-syntax.html#template-expressions)，比如字符串字面量或组件的属性。
  换句话说，借助属性绑定，我们也可以像用绑定来控制生日的值一样，控制生日的显示格式。

  Let's write a second component that *binds* the pipe's format parameter
  to the component's `format` property. Here's the template for that component:

  我们来写第二个组件，它把管道的格式参数*绑定*到该组件的`format`属性。这里是新组件的模板：

+makeExample('pipes/ts/app/hero-birthday2.component.ts', 'template', '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.
  That method toggles the component's `format` property between a short form
  (`'shortDate'`) and a longer form (`'fullDate'`).

  我们还能把一个按钮添加到模板中，并把它的点击事件绑定到组件的`toggleFormat()`方法。
  此方法会在短日期格式(`'shortDate'`)和长日期格式(`'fullDate'`)之间切换组件的`format`属性。

+makeExample('pipes/ts/app/hero-birthday2.component.ts', 'class', 'app/hero-birthday2.component.ts (class)')(format='.')

:marked
  As we click the button, the displayed date alternates between
  "**<samp>04/15/1988</samp>**" and
  "**<samp>Friday, April 15, 1988</samp>**".

  当我们点击按钮的时候，显示的日志会在"**<samp>04/15/1988</samp>**"和"**<samp>Friday, April 15, 1988</samp>**"之间切换。

figure.image-display
    img(src='/resources/images/devguide/pipes/date-format-toggle-anim.gif' alt="Date Format Toggle")
:marked

.l-sub-section
  :marked
    Learn more about the `DatePipes` format options in the [API Docs](../api/common/DatePipe-class.html).

    要了解更多`DatePipes`的格式选项，请参阅[API文档](../api/common/DatePipe-class.html)。

:marked
  ## Chaining pipes
  ## 链式管道

  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>**

  我们可以把管道链在一起，以组合出一些潜在的有用功能。
  下面这个例子中，我们把`birthday`链到`DatePipe`管道，然后又链到`UpperCasePipe`，这样我们就可以把生日显示成大写形式了。
  比如下面的代码就会把生日显示成**<samp>APR 15, 1988</samp>**：

+makeExample('pipes/ts/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.

  下面这个显示**<samp>FRIDAY, APRIL 15, 1988</samp>**的例子用同样的方式链接了这两个管道，而且同时还给`date`管道传进去一个参数。

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

.l-main-section
:marked
  ## Custom Pipes
  ## 自定义管道

  We can write our own custom pipes.
  Here's a custom pipe named `ExponentialStrengthPipe` that can boost a hero's powers:

  我们还可以写自己的自定义管道。
  下面就是一个名叫`ExponentialStrengthPipe`的管道，它可以放大英雄的能力：

+makeExample('pipes/ts/app/exponential-strength.pipe.ts', null, 'app/exponential-strength.pipe.ts')(format=".")

:marked
  This pipe definition reveals several key points

  在这个管道的定义中体现了几个关键点：

  * A pipe is a class decorated with pipe metadata.

  * 管道是一个带有“管道元数据(pipe metadata)”装饰器的类。

  * The pipe class implements the `PipeTransform` interface's `transform` method that
  accepts an input value followed by optional parameters and returns the transformed value.

  * 这个管道类实现了`PipeTransform`接口的`transform`方法，该方法接受一个输入值和一些可选参数，并返回转换后的值。

  * 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`.

  * 当每个输入值被传给`transform`方法时，还会带有一个额外的参数，比如我们这个管道中的`exponent`(放大指数)。

  * We tell Angular that this is a pipe by applying the
  `@Pipe` #{_decorator} which we import from the core Angular library.

  * 我们通过`@Pipe`#{_decoratorCn}告诉Angular：这是一个管道。该#{_decoratorCn}是从Angular的`core`库中引入的。

  * 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`.

  * 这个`@Pipe`#{_decoratorCn}允许我们定义管道的名字，这个名字会被用在模板表达式中。它必须是一个有效的JavaScript标识符。
      比如，我们这个管道的名字是`exponentialStrength`。

.l-sub-section
  :marked
    ### The *PipeTransform* Interface
    ### *PipeTransform*接口

    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.

    `transform`方法是管道的基本要素。
    `PipeTransform`*接口*中定义了它，并用它指导各种工具和编译器。
    从技术角度看，它是可选的。Angular不会管它，而是直接查找并执行`transform`方法。

:marked
  Now we need a component to demonstrate our pipe.

现在，我们需要一个组件来演示这个管道。
+makeExample('pipes/ts/app/power-booster.component.ts',null,'app/power-booster.component.ts')(format='.')
figure.image-display
    img(src='/resources/images/devguide/pipes/power-booster.png' alt="Power Booster")

:marked
  Two things to note:

  要注意的有两点：
  1. We use our custom pipe the same way we use the built-in pipes.

  1. 我们使用自定义管道的方式和内建管道完全相同。

  1. We must include our pipe in the `pipes` #{_array} of the `@Component` #{_decorator}.

  1. 我们必须在`@Component`#{_decoratorCn}的`pipes`数组中包含这个管道。

.callout.is-helpful
  header Remember the pipes #{_array}!
  header 记住`pipes`数组
  :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
    Angular built-in pipes are pre-registered.
    Custom pipes must be registered manually.

    如果我们忘了列出自定义管道，Angular会报告一个错误。
    我们不用在以前的例子中列出`DatePipe`，是因为所有的Angular内建管道都被预先注册过了。
    而自定义管道就必须被手动注册。

p.
  If we try the #[+liveExampleLink()],
  we can probe its behavior by changing the value and the optional exponent in the template.

p.
  试试这个#[+liveExampleLink()]，然后在模板中修改能量值和可选的放大指数，就能看懂它的行为方式。

:marked
  ## Power Boost Calculator (extra-credit)
  ## 能力倍增计算器（加分项）

  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`.

  仅仅升级模板来测试这个自定义管道其实没多大意思。
  我们干脆把这个例子升级为“能力倍增计算器”，它可以把该管道和使用`ngModel`的双向数据绑定组合起来。

+makeExample('pipes/ts/app/power-boost-calculator.component.ts', null, '/app/power-boost-calculator.component.ts')(format='.')

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

.l-main-section
a#change-detection
:marked
  ## Pipes and Change Detection
  ## 管道与变更检测

  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通过*变更检测*过程来查找绑定值的更改，并在每一次JavaScript事件之后运行：每次按键、鼠标移动、定时器以及服务器的响应。
  这可能会让变更检测显得很昂贵，但是Angular会尽可能降低变更检测的成本。

  Angular picks a simpler, faster change detection algorithm when we use a pipe. Let's see how.

  当我们使用管道时，Angular选用了一种简单、快速的变更检测算法。

  ### No pipe
  ### 无管道

  The component in our next example uses the default, aggressive change detection strategy to monitor and update
  its display of every hero in the `heroes` #{_array}. Here's the template:

  我们下一个例子中的组件使用默认的、激进（昂贵）的变更检测策略来检测和更新`heroes`数组中的每个英雄。下面是它的模板：

+makeExample('pipes/ts/app/flying-heroes.component.html', 'template-1', '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}.

  和模板相伴的组件类可以提供英雄数组，能把新的英雄添加到数组中，还能重置英雄数组。

+makeExample('pipes/ts/app/flying-heroes.component.ts', 'v1', '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.

  我们可以添加新的英雄，加完之后，Angular就会更新显示。
  `reset`按钮会把`heroes`替换成一个由原来的英雄组成的新数组，重置完之后，Angular就会更新显示。
  如果我们提供了删除或修改英雄的能力，Angular也会检测到那些更改，并更新显示。

  ### Flying Heroes pipe
  ### “会飞的英雄”管道

  Let's add a `FlyingHeroesPipe` to the `*ngFor` repeater that filters the list of heroes to just those heroes who can fly.

  我们来往`*ngFor`重复器中添加一个`FlyingHeroesPipe`管道，这个管道能过滤出所有会飞的英雄。

+makeExample('pipes/ts/app/flying-heroes.component.html', 'template-flying-heroes', 'app/flying-heroes.component.html (flyers)')(format='.')
:marked
  Here's the `FlyingHeroesPipe` implementation which follows the pattern for custom pipes we saw earlier.

  下面是`FlyingHeroesPipe`的实现，它遵循了我们以前见过的那些写自定义管道的模式。

+makeExample('pipes/ts/app/flying-heroes.pipe.ts', 'pure', 'app/flying-heroes.pipe.ts')(format='.')

p.
  When we run the sample now we see odd behavior (try it in the #[+liveExampleLink()]).
  Every hero we add is a flying hero but none of them are displayed.

p.
  如果我们现在就运行这个例子，我们会看到一些古怪的行为（在#[+liveExampleLink()]中试试）。
  我们添加的每个英雄都会飞，但他们一个都没有显示出来。

:marked
  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.

  虽然我们没有得到期望的行为，但Angular也没有出错。
  这里只是用了另一种变更检测算法 —— 它会忽略对列表及其子项所做的任何更改。

  Look at how we're adding a new hero:

  来看看我们是如何添加新英雄的：

+makeExample('pipes/ts/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.
  It's the same #{_array}. That's all Angular cares about. From its perspective, *same #{_array}, no change, no display update*.

  当我们往`heroes`数组中添加一个新的英雄时，这个数组的引用并没有改变。它还是那个数组。而引用却是Angular所关心的一切。
  从Angular的角度来看，*这是同一个数组，没有变化，也就不需要更新显示*。

  We can fix that. Let's create a new #{_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.

  我们可以修复它。让我们创建一个新数组，把这个英雄追加进去，并把它赋给`heroes`。
  这次，Angular检测到数组的引用变化了。它执行了这个管道，并使用这个新数组更新显示，这次它就包括新的飞行英雄了。

  *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.

  *如果我们**修改了**这个数组，不会执行任何管道，显示也没有更新。
  如果我们**替换了**这个数组，管道就会被执行，显示也更新了。*
  这个*飞行英雄*的例子用检查框和额外的显示内容扩展了原有代码，来帮我们体验这些效果。

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.

  直接替换这个数组是通知Angular更新显示的一种高效方式。
  我们该什么时候替换这个数组呢？当数据变化的时候。
  在这个*玩具级*例子中，这是一个简单的规则，因为这里修改数据的唯一途径就是添加新英雄。

  More often we 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 is 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.
  The component should be unaware of pipes.

  更多情况下，我们不知道什么时候数据变化了，尤其是在那些有很多种途径改动数据的程序中 —— 可能在程序中很远的地方。
  组件就是一个通常无法知道那些改动的例子。此外，它会导致削足适履 —— 扭曲我们的组件设计来适应管道。
  我们要尽可能保持组件类独立于HTML。组件不应该关心管道的存在。

  Perhaps we should consider a different kind of pipe for filtering flying heroes, an *impure pipe*.

  也行我们可以考虑用另一种管道来过滤会飞的英雄了，一个*非纯(impure)管道*。

.l-main-section
:marked
  ## Pure and Impure Pipes
  ## 纯(pure)管道与非纯(impure)管道

  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`
  impure like this:

  有两类管道：**纯**的与**非纯**的。
  默认情况下，管道都是纯的。我们以前见到的每个管道都是纯的。
  通过把它的`pure`标志设置为`false`，我们可以制作一个非纯管道。我们可以像这样让`FlyingHeroesPipe`变成非纯的：

+makeExample('pipes/ts/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.

  在继续往下走之前，我们先理解一下*纯*和*非纯*之间的区别，从*纯*管道开始。

  ### Pure pipes
  ### 纯管道

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`).

    Angular只有在它检测到输入值发生了*纯变更*时才会执行*纯管道*。
    ***纯变更***是指对原始类型值(`String`、`Number`、`Boolean`、`Symbol`)的更改，
    或者对对象引用(`Date`、`Array`、`Function`、`Object`)的更改。

: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会忽略(复合)对象*内部*的更改。
  如果我们更改了输入日期(`Date`)中的月份、往一个输入数组(`Array`)中添加新值或者更新了一个输入对象(`Object`)的属性，Angular都不会调用纯管道。

  This may seem restrictive but is is 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.

  这可能看起来是一种限制，但它保证了速度。
  对象引用的检查是非常快的（比递归的深检查要快得多），所以Angular可以快速的决定是否应该在执行管道和更新视图时跳过它。

  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.

  因此，如果我们要和变更检测策略打交道，就会更喜欢用纯管道。
  如果不能，我们就*可以*转回到非纯管道。

.l-sub-section
  :marked
    Or we 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.

    或者我们也可以完全不用管道。
    有时候，使用组件的属性能比用管道更好的达到目的，这一点我们等后面会再提起。

:marked
  ### Impure pipes
  ### 非纯管道

  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会在每个组件的变更检测周期中执行*非纯管道*。
  非纯管道可能会被调用很多次，和每个按键或每次鼠标移动一样频繁。

  With that concern in mind, we must implement an impure pipe with great care.
  An expensive, long-running pipe could destroy the user experience.

  要在脑子里绷着这根弦，我们必须小心翼翼的实现非纯管道。
  一个昂贵、迟钝的管道将摧毁用户体验。

  <a id="impure-flying-heroes"></a>
  ### An impure *FlyingHeroesPipe*
  ### 非纯版本的*FlyingHeroesPipe*

  A flip of the switch turns our `FlyingHeroesPipe` into a `FlyingHeroesImpurePipe`.
  Here's the complete implementation:

  我们把`FlyingHeroesPipe`换成了`FlyingHeroesImpurePipe`。
  下面是完整的实现：
+makeTabs(
  'pipes/ts/app/flying-heroes.pipe.ts, pipes/ts/app/flying-heroes.pipe.ts',
  'impure, pure',
  'FlyingHeroesImpurePipe, FlyingHeroesPipe')(format='.')

:marked
  We inherit from `FlyingHeroesPipe` to prove the point that nothing changed internally.
  The only difference is the `pure` flag in the pipe metadata.

  我们把它从`FlyingHeroesPipe`中继承下来，以证明无需改动内部代码。
  唯一的区别是管道元数据中的`pure`标志。

  This is a good candidate for an impure pipe because the `transform` function is trivial and fast.

  这是一个很好地非纯管道候选者，因为它的`transform`函数又小又快。

+makeExample('pipes/ts/app/flying-heroes.pipe.ts','filter')(format='.')

  We can derive a `FlyingHeroesImpureComponent` that we derive from the `FlyingHeroesComponent`.

  我们可以从`FlyingHeroesComponent`派生出一个`FlyingHeroesImpureComponent`。

+makeExample('pipes/ts/app/flying-heroes.component.ts','impure-component','app/flying-heroes.component.ts (FlyingHeroesImpureComponent)')(format='.')
p.
  The only substantive change is the pipe.
  We can confirm in the #[+liveExampleLink()] that the #[i flying heroes]
  display updates as we enter new heroes even when we mutate the
  #[code heroes] #{_array}.

p.
  唯一的重大改动就是管道。
  我们可以在#[+liveExampleLink()]中确认，当我们输入新的英雄甚至修改#[code heroes]数组时，这个#[i 会飞的英雄]的显示也跟着更新了。

- var _dollar = _docsFor === 'ts' ? '$' : '';
h3#async-pipe The impure #[i AsyncPipe]
h3#async-pipe 非纯管道#[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).

  Angular的`AsyncPipe`是一个有趣的非纯管道的例子。
  `AsyncPipe`接受一个`Promise`或`Observable`作为输入，并且自动订阅这个输入，最终返回它们给出的值。

  It 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
  (`message#{_dollar}`) to a view with the `async` pipe.

+makeExample('pipes/ts/app/hero-async-message.component.ts', null, 'app/hero-async-message.component.ts')

: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
  (a potent source of memory leaks).

  ### An impure caching pipe

  Let's write one more impure pipe, a pipe that makes an HTTP request to the server.
  Normally, that's a horrible idea.
  It's probably a horrible idea no matter what we do.
  We're forging ahead anyway to make a point.
  Remember that impure pipes are called every few microseconds.
  If we're not careful, this pipe will punish the server with requests.

  We are careful. Our pipe only makes a server call if the request URL has changed.
  It caches the request URL and waits for a result which it also caches when it arrives.
  The pipe returns the cached result (which is null while a request is in flight)
  after every Angular call and only contacts the server as necessary.

  Here's the code, which uses the [Angular http](server-communication.html) facility
  to retrieve a `heroes.json` file:

+makeExample('pipes/ts/app/fetch-json.pipe.ts', null, 'app/fetch-json.pipe.ts')
:marked
  Then we demonstrate it in a harness component whose template defines two bindings to this pipe.
+makeExample('pipes/ts/app/hero-list.component.ts', 'template', 'app/hero-list.component.ts (template)')
:marked
  Despite the two bindings and what we know to be frequent pipe calls,
  the nework tab in the browser developer tools confirms that there is only one request for the file.

  The component renders like this:

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

:marked
  ### *JsonPipe*

  The second binding involving the `FetchPipe` uses more pipe chaining.
  We take the same fetched results displayed in the first binding
  and display them again, this time in JSON format by chaining through to the built-in `JsonPipe`.

.callout.is-helpful
  header Debugging with the json pipe
  :marked
    The [JsonPipe](../api/common/JsonPipe-class.html)
    provides an easy way to diagnosis a mysteriously failing data binding or
    inspect an object for future binding.

:marked
  Here's the complete component implementation:

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

a(id="pure-pipe-pure-fn")
:marked
  ### 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.

  The pipes we saw earlier in this chapter were 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*.

  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.

.l-main-section
:marked
  ## 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/#!?apiFilter=pipe).
  Try writing a custom pipe and perhaps contributing it to the community.

a(id="no-filter-pipe")
.l-main-section
:marked
  ## No *FilterPipe* or *OrderByPipe*

  Angular does not ship with pipes for filtering or sorting lists.
  Developers familiar with Angular 1 know these as `filter` and `orderBy`.
  There are no equivalents in Angular 2.

  This is not an oversight. Angular 2 is unlikely to offer such pipes because
  (a) they perform poorly and (b) they 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.

  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 Angular 1 apps, leading to complaints that Angular itself is slow.
  That charge is fair in the indirect sense that Angular 1 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:
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.

  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.

  The Angular team and many experienced Angular developers strongly recommend that you move
  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.