include ../_util-fns

:marked
  ## Write *Component-Relative* URLs to component templates and style files

  ## 为组件模板和样式表文件提供*相对于组件的*URL

  Our components often refer to external template and style files.
  We identify those files with a URL in the `templateUrl` and `styleUrls` properties of the `@Component` metadata
  as seen here:

  我们的组件通常都是引用外部的模板和样式表文件。
  我们在`@Component`的元数据中通过`templateUrl`和`styleUrls`属性来标识出它们的位置：

+makeExample('cb-component-relative-paths/ts/app/some.component.ts','absolute-config')(format='.')
:marked
  By default, we *must* specify the full path back to the application root.
  We call this an ***absolute path*** because it is *absolute* with respect to the application root.

  默认情况下，我们*必须*指定一个一直到应用程序根目录的完整路径。
  我们称之为***绝对路径***，因为它*绝对的*以应用程序的根目录为基准。

  There are two problems with an *absolute path*:

  使用*绝对路径*有两个问题：

  1. We have to remember the full path back to the application root.

  1. 我们不得不记住到应用程序根目录的完整路径。

  1. We have to update the URL when we move the component around in the application files structure.

  1. 当我们在应用的文件结构中移动这个组件时，将不得不更新这个URL

  It would be much easier to write and maintain our application components if we could specify template and style locations
  *relative* to their component class file.

  如果我们能用*相对*于组件类文件的路径来指定模板和样式表的位置，那么编写和维护组件就会变得容易得多。

  *We can!*

  *没问题！*

.alert.is-important
  :marked
    We can if we build our application as `commonjs` modules and load those modules
    with a suitable package loader such as `systemjs` or `webpack`.
    Learn why [below](#why-default).

    如果把应用构建成`commonjs`模块，并用一个合适的包加载器(比如`systemjs`或`webpack`)加载那些模块，我们就可以用相对路径。
    [在下方](#why-default)可以学到原理。

    The Angular 2 CLI uses these technologies and defaults to the
    *component-relative path* approach described here.
    CLI users can skip this chapter or read on to understand
    how it works.

    Angular 2 CLI(命令行界面)使用这些技术，并默认采用这里所说的*组件相对路径*方法。
    用CLI用户可以跳过本章，或者继续阅读来了解它是怎么工作的。

.l-main-section
:marked
  ## _Component-Relative_ Paths
  ## _组件相对_路径

  Our goal is to specify template and style URLs *relative* to their component class files,
  hence the term ***component-relative path***.

  我们的目标把模板和样式表的URL指定为*相对*于组件类的路径，因此得名***组件相对路径***。

  The key to success is following a convention that puts related component files in well-known locations.

  成功的关键是遵循一个约定：把相对组件的文件放进众所周知的位置。

  We recommend keeping component template and component-specific style files as *siblings* of their
  companion component class files.
  Here we see the three files for `SomeComponent` sitting next to each other in the `app` folder.

  我们建议把组件的模板和组件特有的样式表文件作为组件类文件的“兄弟”。
  这里我们看到在`app`目录下依次有`SomeComponent`的三个文件。

.filetree
  .file app
    .children
      .file some.component.css
      .file some.component.html
      .file some.component.ts
    .file ...
:marked
  We'll have more files and folders &mdash; and greater folder depth &mdash; as our application grows.
  We'll be fine as long as the component files travel together as the inseparable siblings they are.

  当应用规模增长后，我们还会有更多的文件和目录，目录深度也会增加。
  如果组件的所有文件总是像形影不离的兄弟那样共进退，那该多好啊！

  ### Set the *moduleId*
  ### 设置*moduleId*

  Having adopted this file structure convention, we can specify locations of the template and style files
  relative to the component class file simply by setting the `moduleId` property of the `@Component` metadata like this

  采用这种文件结构约定，我们可以为模板和样式表文件指定相对于组件类文件的位置 —— 只要简单的在`@Component`元数据中设置`moduleId`属性就可以了，就像这样：

+makeExample('cb-component-relative-paths/ts/app/some.component.ts','module-id')(format='.')
:marked
  We strip the `app/` base path from the `templateUrl` and `styleUrls`. The result looks like this:

  我们从`templateUrl`和`styleUrls`中把基准路径`app/`去掉了。结果是这样的：

+makeExample('cb-component-relative-paths/ts/app/some.component.ts','relative-config')(format='.')

.alert.is-helpful
  :marked
    Webpack users may prefer [an alternative approach](#webpack) that uses `require`.

    Webpack用户可能更喜欢[一个替代方案](#webpack)：使用`require`。

.l-main-section
:marked
  ## Source

  ## 源码

  **We can see the [live example](/resources/live-examples/cb-component-relative-paths/ts/plnkr.html)**
  and download the source code from there
  or simply read the pertinent source here.

  **参见[在线范例](/resources/live-examples/cb-component-relative-paths/ts/plnkr.html)**，并从中下载源码或只在这里阅读相关源码。

+makeTabs(
  `cb-component-relative-paths/ts/app/some.component.ts,
  cb-component-relative-paths/ts/app/some.component.html,
  cb-component-relative-paths/ts/app/some.component.css,
  cb-component-relative-paths/ts/app/app.component.ts`,
  null,
  `app/some.component.ts, app/some.component.html, app/some.component.css, app/app.component.ts`)

a#why-default
.l-main-section
:marked
  ## Appendix: why *component-relative* is not the default
  ## 附录：为什么*组件相对路径*不是默认方式

  A *component-relative* path is obviously superior to an *absolute* path.
  Why did Angular default to the *absolute* path?
  Why do *we* have to set the `moduleId`? Why can't Angular set it?

  *组件相对路径*明显比*绝对路径*高级一点。
  为什么Angular默认采用了*绝对路径*呢？
  为什么*我们*不得不设置`moduleId`呢？Angular为什么不能自己设置它？

  First, let's look at what happens if we use a relative path and omit the `moduleId`.

  首先，如果只使用相对路径而省略掉`moduleId`，我们来看看会发生什么。

  `EXCEPTION: Failed to load some.component.html`

  `EXCEPTION: Failed to load some.component.html`

  Angular can't find the file so it throws an error.

  Angular找不到这个文件，所以它抛出一个错误。

  Why can't Angular calculate the template and style URLs from the component file's location?

  为什么Angular不能相对于组件类文件的路径来自动计算模板和样式表的URL呢？

  Because the location of the component can't be determined without the developer's help.
  Angular apps can be loaded in many ways: from individual files, from SystemJS packages, or
  from CommonJS packages, to name a few.
  We might generate modules in any of several formats.
  We might not be writing modular code at all!

  因为如果没有开发人员的帮助，组件的位置是检测不到的。
  Angular应用可能被用多种方式加载：SystemJS包、CommonJS包等等。
  我们可能会用几种格式之一来生成模块。
  我们甚至可能完全没有写成模块化代码。

  With this diversity of packaging and module load strategies,
  it's not possible for Angular to know with certainty where these files reside at runtime.

  由于存在这么多打包和模块加载策略，所以Angular不可能知道在运行期这些文件的正确位置。

  The only location Angular can be sure of is the URL of the `index.html` home page, the application root.
  So by default it resolves template and style paths relative to the URL of `index.html`.
  That's why we previously wrote our file URLs with an `app/` base path prefix.

  Angular能够确定的唯一的位置是首页`index.html`的URL，也就是应用的根目录。
  所以，默认情况下，它只能计算相对于`index.html`的模板和样式表路径。
  这就是为什么我们以前用`app/`基准路径的前缀来写文件的URL。

  But *if* we follow the recommended guidelines and we write modules in `commonjs` format
  and we use a module loader that *plays nice*,
  *then* we &mdash; the developers of the application &mdash;
  know that the semi-global `module.id` variable is available and contains
  the absolute URL of the component class module file.

  但是，*如果*我们遵循了建议的指导原则，用`commonjs`格式编写模块，并使用一个*不错的*模块加载器，
  然后，我们就知道，有一个可用的半全局变量`module.id`，它包含组件类模块文件的绝对URL。

  That knowledge enables us to tell Angular where the *component* file is
  by setting the `moduleId`:

  这种认知让我们得以通过设置`moduleId`来告诉Angular*组件类*文件在哪里：

+makeExample('cb-component-relative-paths/ts/app/some.component.ts','module-id')(format='.')

a#webpack
.l-main-section
:marked
  ## Webpack: load templates and styles with *require*

  ## Webpack: 使用*require*加载模板和样式表

  Webpack developers have an alternative to `moduleId`.

  Webpack开发者可以采用`moduleId`的另一个替代方案。

  They can load templates and styles at runtime by setting the component metadata `template` and `style` properties
  with `require` statements that reference *component-relative* URLS.

  通过把组件元数据中的`template`和`style`属性设置为`require`语句，并引用一个*相对于组件的*URL，他们可以在运行期间加载模板和样式表。

+makeExample('webpack/ts/src/app/app.component.ts')(format='.')
:marked
  See the [Introduction to Webpack](../guide/webpack.html).

  参见[Webpack简介](../guide/webpack.html)。
