include ../_util-fns

:marked
  We’ve all used a form to log in, submit a help request, place an order, book a flight,
  schedule a meeting and perform countless other data entry tasks.
  Forms are the mainstay of business applications.

  表单是商业应用的支柱，我们用它来执行登录、求助、下单、预订机票、安排会议，以及不计其数的其它数据录入任务。

  Any seasoned web developer can slap together an HTML form with all the right tags.
  It's more challenging to create a cohesive data entry experience that guides the
  user efficiently and effectively through the workflow behind the form.

  任何经验丰富的Web开发人员都能使用适当的标签拼凑出HTML表单。
  但是，要想做出具有贴心的数据输入体验的表单，
  引导用户明晰、高效地完成表单背后的工作流程，挑战就大多了。

  *That* takes design skills that are, to be frank, well out of scope for this chapter.

  坦白地讲，*这当中*所需要的设计技能超出了本章的范围。

  It also takes framework support for
  **two-way data binding, change tracking, validation, and error handling**
  ... which we shall cover in this chapter on Angular forms.

  **双向数据绑定、变更跟踪、有效性验证和错误处理**等功能离不开框架的支持。
  本章将介绍Angular表单相关的内容。

  We will build a simple form from scratch, one step at a time. Along the way we'll learn how to

  下面，从零开始，一步一步构建出一个简单的表单。在这个过程中，我们将学会如何：

  - build an Angular form with a component and template

    使用组件和模板构建Angular表单

  - two-way data bind with `[(ngModel)]` syntax for reading and writing values to input controls

    `[(ngModel)]`语法实现双向数据绑定，用于读取和写入输入控件的值

  - track the change state and validity of form controls using `ngModel` in combination with a form

    结合表单来使用`ngModel`，可以跟踪表单控件的状态变化和有效性

  - provide strong visual feedback using special CSS classes that track the state of the controls

    使用特殊的 CSS 类来跟踪控件状态，并提供强烈的视觉反馈

  - display validation errors to users and enable/disable form controls

    向用户显示验证错误提示，以及启用/禁用表单控件

  - use [template reference variables](./template-syntax.html#ref-vars) for sharing information among HTML elements

    利用[模板引用变量](./template-syntax.html#ref-vars)在 HTML 元素之间共享信息

  Run the <live-example></live-example>.

  运行<live-example>在线例子</live-example>

.l-main-section
:marked
  ## Template-Driven Forms

  ## 模板驱动的表单

  Many of us will build forms by writing templates in the Angular [template syntax](./template-syntax.html) with
  the form-specific directives and techniques described in this chapter.

  通常，使用Angular[模板语法](./template-syntax.html)编写模板，结合本章所描述的表单专用指令和技术来构建表单。

.l-sub-section
  :marked
    That's not the only way to create a form but it's the way we'll cover in this chapter.

    这不是创建表单的唯一方式，本章中只介绍模板驱动的表单。(译注：Angular支持的另一种方式叫做模型驱动表单Model-Driven Forms)

:marked
  We can build almost any form we need with an Angular template &mdash; login forms, contact forms ... pretty much any business forms.
  We can lay out the controls creatively, bind them to data, specify validation rules and display validation errors,
  conditionally enable or disable specific controls, trigger built-in visual feedback, and much more.

  利用Angular模板，可以构建几乎所有表单 —— 登录表单、联系人表单…… 以及任何的商务表单。
  可以创造性的摆放各种控件、把它们绑定到数据、指定校验规则、显示校验错误、有条件的禁用或
  启用特定的控件、触发内置的视觉反馈等等，不胜枚举。

  It will be pretty easy because Angular handles many of the repetitive, boiler plate tasks we'd
  otherwise wrestle with ourselves.

  它用起来很简单，这是因为Angular处理了大多数重复、单调的任务，这让我们可以不必亲自操刀、身陷其中。

  We'll discuss and learn to build the following template-driven form:

  我们将讨论和学习构建如下的“模板驱动”表单：

figure.image-display
  img(src="/resources/images/devguide/forms/hero-form-1.png" width="400px" alt="Clean Form")

:marked
  Here at the *Hero Employment Agency* we use this form to maintain personal information about the
  heroes in our stable. Every hero needs a job. It's our company mission to match the right hero with the right crisis!

  这里是*英雄职业介绍所*，使用这个表单来维护候选英雄们的个人信息。每个英雄都需要一份工作。
  公司的任务就是让适当的英雄去解决它/她所擅长应对的危机！

  Two of the three fields on this form are required. Required fields have a green bar on the left to make them easy to spot.

  表单中的三个字段，其中两个是必填的。必填的字段在左侧有个绿色的竖条，方便用户分辨哪些是必填项。

  If we delete the hero name, the form displays a validation error in an attention grabbing style:

  如果删除了英雄的名字，表单就会用醒目的样式把验证错误显示出来。

figure.image-display
  img(src="/resources/images/devguide/forms/hero-form-2.png" width="400px" alt="无效！名字是必填项")

:marked
  Note that the submit button is disabled and the "required" bar to the left of the input control changed from green to red.

  注意，提交按钮被禁用了，而且输入控件左侧的“必填”条从绿色变为了红色。

.l-sub-section
  p We'll customize the colors and location of the "required" bar with standard CSS.

  p 稍后，会使用标准 CSS 来定制“必填”条的颜色和位置。

:marked
  We will build this form in the following sequence of small steps

  按下面的步骤，一点一点构建此表单：

  1. Create the `Hero` model class

     创建`Hero`模型类

  1. Create the component that controls the form

     创建控制此表单的组件

  1. Create a template with the initial form layout

     创建具有初始表单布局的模板

  1. Bind data properties to each form input control with the `ngModel` two-way data binding syntax

     使用`ngModel`双向数据绑定语法把数据属性绑定到每个表单输入控件

  1. Add the `name` attribute to each form input control

     往每个表单输入控件上添加`name`属性 (attribute)

  1. Add custom CSS to provide visual feedback

     添加自定义 CSS 来提供视觉反馈

  1. Show and hide validation error messages

     显示和隐藏有效性验证的错误信息

  1. Handle form submission with **ngSubmit**

     使用**ngSubmit**处理表单提交

  1. Disable the form’s submit button until the form is valid

     禁用此表单的提交按钮，直到表单变为有效

:marked
  ## Setup

  ## 搭建

  Create a new project folder (`angular-forms`) and follow the steps in the [QuickStart](../quickstart.html).

  创建新的项目文件夹 (`angular-forms`)，并且完成[“快速起步”](../quickstart.html)中的步骤。

include ../_quickstart_repo
:marked
  ## Create the Hero Model Class

  ## 创建Hero模型类

  As users enter form data, we capture their changes and update an instance of a model.
  We can't layout the form until we know what the model looks like.

  当用户输入表单数据时，需要捕获它们的变化，并更新到模型的实例中。
  除非知道模型里有什么，否则无法设计表单的布局。

  A model can be as simple as a "property bag" that holds facts about a thing of application importance.
  That describes well our `Hero` class with its three required fields (`id`, `name`, `power`)
  and one optional field (`alterEgo`).

  最简单的模型是个“属性包”，用来存放应用中一件事物的事实。
  这里使用三个必备字段 (`id`、`name`、`power`)，和一个可选字段 (`alterEgo`，译注：中文含义：第二人格，比如X战警中的Jean/黑凤凰)。

  Create a new file in the app folder called `hero.ts` and give it the following class definition:

  在应用文件夹中创建`hero.ts`文件，并且写入下列类定义内容：

+makeExample('forms/ts/app/hero.ts', null, 'app/hero.ts')

:marked
  It's an anemic model with few requirements and no behavior. Perfect for our demo.

  这是一个少量需求和零行为的贫血模型。对演示来说很完美。

  The TypeScript compiler generates a public field for each `public` constructor parameter and
  assigns the parameter’s value to that field automatically when we create new heroes.

  TypeScript 编译器为每个`public`构造函数参数生成一个公共字段，在创建新的英雄实例时，自动把参数值赋给这些公共字段。

  The `alterEgo` is optional and the constructor lets us omit it; note the (?) in `alterEgo?`.

  `alterEgo`是可选的，调用构造函数时可省略，注意`alterEgo?`中的问号 (?)。

  We can create a new hero like this:

  可以像这样创建新英雄：
code-example(format="").
  let myHero =  new Hero(42, 'SkyDog',
                         'Fetch any object at any distance',
                         'Leslie Rollover');
  console.log('My hero is called ' + myHero.name); // "My hero is called SkyDog"
:marked

.l-main-section
:marked
  ## Create a Form component

  ## 创建表单组件

  An Angular form has two parts: an HTML-based template and a code-based Component to handle data and user interactions.

  Angular 表单分为两部分：基于 HTML 的模板和基于代码的组件。组件用来处理数据和用户交互。

  We begin with the Component because it states, in brief, what the Hero editor can do.

  先从组件开始，是因为它可以简要说明英雄编辑器能做什么。

  Create a new file called `hero-form.component.ts` and give it the following definition:

  创建名叫`hero-form.component.ts`的文件，放进下列定义：

+makeExample('forms/ts/app/hero-form.component.ts', 'first', 'app/hero-form.component.ts')

:marked
  There’s nothing special about this component, nothing form-specific, nothing to distinguish it from any component we've written before.

  这个组件没有什么特别的地方，没有表单相关的东西，与之前写过的组件没什么不同。

  Understanding this component requires only the Angular concepts we’ve learned in previous chapters

  只需要前面章节中学过的概念，就可以完全理解这个组件：

  1. We import the `Component` decorator from the Angular library as we usually do.

     像往常一样，从 Angular 库中导入`Component`装饰器。

  1. We import the `Hero` model we just created.

     导入刚刚创建的`Hero`模型。

  1. The `@Component` selector value of "hero-form" means we can drop this form in a parent template with a `<hero-form>` tag.

     `@Component`选择器"hero-form"表示可以用`<hero-form>`标签把这个表单放进父模板。

  1. The `moduleId: module.id` property sets the base for module-relative loading of the `templateUrl`.

     `moduleId: module.id`属性设置了基地址，用于从相对模块路径加载`templateUrl`。

  1. The `templateUrl` property points to a separate file for the template HTML called `hero-form.component.html`.

     `templateUrl`属性指向一个独立的 HTML 模板文件，名叫`hero-form.component.html`。

  1. We defined dummy data for `model` and `powers` as befits a demo.
  Down the road, we can inject a data service to get and save real data
  or perhaps expose these properties as [inputs and outputs](./template-syntax.html#inputs-outputs) for binding to a
  parent component. None of this concerns us now and these future changes won't affect our form.

     为`model`和`powers`定义了供演示用的假数据。
  将来，可以注入服务来获取和保存真实数据，
  或者暴露这些属性为[输入与输出属性](./template-syntax.html#inputs-outputs)，绑定到父组件上。
  先不关心这些，因为这些未来的变化不会影响到表单。

  1. We threw in a `diagnostic` property at the end to return a JSON representation of our model.
  It'll help us see what we're doing during our development; we've left ourselves a cleanup note to discard it later.

     在最后增加`diagnostic`属性，它返回这个模型的JSON形式。
  在开发过程中，它用于调试，最后清理时会丢弃它。

  Why don't we write the template inline in the component file as we often do
  elsewhere in the Developer Guide?

  这次为什么不像在开发指南其它地方那样，以内联的方式把模板写在组件文件中呢？

  There is no “right” answer for all occasions. We like inline templates when they are short.
  Most form templates won't be short. TypeScript and JavaScript files generally aren't the best place to
  write (or read) large stretches of HTML and few editors are much help with files that have a mix of HTML and code.
  We also like short files with a clear and obvious purpose like this one.

  没有什么答案在所有场合都总是“正确”的。当模板足够短的时候，内联形式更招人喜欢。
  但大多数的表单模板都不短。通常，TypeScript 和 JavaScript 文件不是写（读）大型 HTML 的好地方，
  而且没有几个编辑器能对混写的 HTML 和代码提供足够的帮助。
  我们还是喜欢内容清晰、目标明确的短文件，像这个一样。

  We made a good choice to put the HTML template elsewhere.
  We'll write that template in a moment. Before we do, we'll take a step back
  and revise the `app.module.ts` and `app.component.ts` to make use of our new `HeroFormComponent`.

  把 HTML 模板放在其它地方是个不错的选择。等会儿就去写那个模板。
  在这之前，先回来修改`app.module.ts`和`app.component.ts`文件，用上新写的`HeroFormComponent`组件。

.l-main-section
:marked
  ## Revise the *app.module.ts*

  ## 修改 *app.module.ts*

  `app.module.ts` defines the application's root module. In it we identify the external modules we'll use in our application
  and declare the components that belong to this module, such as our `HeroFormComponent`.

  `app.module.ts`定义了应用的根模块。其中标识即将用到的外部模块，以及声明属于本模块中的组件，如`HeroFormComponent`。

  Because template-driven forms are in their own module, we need to add the `FormsModule` to the array of
  `imports` for our application module before we can use forms.

  因为模板驱动的表单位于它们自己的模块，所以在使用表单之前，需要将`FormsModule`添加到应用模块的`imports`数组中。

  Replace the contents of the "QuickStart" version with the following:

  把“快速起步”版的文件替换为如下内容：
+makeExample('forms/ts/app/app.module.ts', null, 'app/app.module.ts')

:marked
.l-sub-section
  :marked
    There are three changes:

    有三处更改：

    1. We import `FormsModule` and our new `HeroFormComponent`.

       导入`FormsModule`和新组件`HeroFormComponent`。

    1. We add the `FormsModule` to the list of `imports` defined in the `ngModule` decorator. This gives our application
    access to all of the template-driven forms features, including `ngModel`.

       把`FormsModule`添加到`ngModule`装饰器的`imports`列表中，这样应用就能访问模板驱动表单的所有特性，包括`ngModel`。

    1. We add the `HeroFormComponent` to the list of `declarations` defined in the `ngModule` decorator. This makes
    the `HeroFormComponent` component visible throughout this module.

       把`HeroFormComponent`添加到`ngModule`装饰器的`declarations`列表中，使`HeroFormComponent`组件在整个模块中可见。

.alert.is-important
  :marked
    If a component, directive, or pipe belongs to a module in the `imports` array, ​_DON'T_​ declare it in the `declarations` array.
    If you wrote it and it should belong to this module, ​_DO_​ declare it in the `declarations` array.

    如果组件、指令或管道出现在模块的`imports`数组中，_不要_把它声明在`declarations`数组中。
    如果它是你自己写的，并且属于当前模块，_就要_把它声明在`declarations`数组中。

.l-main-section
:marked
  ## Revise the *app.component.ts*

  ## 修改 *app.component.ts*

  `app.component.ts` is the application's root component. It will host our new `HeroFormComponent`.

  `app.component.ts`是应用的根组件，`HeroFormComponent`将被放在其中。

  Replace the contents of the "QuickStart" version with the following:

  把"快速起步"的版本内容替换成下列代码：
+makeExample('forms/ts/app/app.component.ts', null, 'app/app.component.ts')

:marked
.l-sub-section
  :marked
    There is only one change:

    仅有的一处修改是：

    1. The `template` is simply the new element tag identified by the component's `selector` property.
    This will display the hero form when the application component is loaded.

    1. `template`中只有新元素标签，即组件的`selector`属性。当应用组件被加载时，将显示这个英雄表单。

.l-main-section
:marked
  ## Create an initial HTML Form Template

  ## 创建初始 HTML 表单模板

  Create a new template file called `hero-form.component.html` and give it the following definition:

  新建模板文件，命名为`hero-form.component.html`，并且填写如下内容：

+makeExample('forms/ts/app/hero-form.component.html', 'start', 'app/hero-form.component.html')

:marked
  That is plain old HTML 5. We're presenting two of the `Hero` fields, `name` and `alterEgo`, and
  opening them up for user input in input boxes.

  这只是一段普通的旧式 HTML 5 代码。这里有两个`Hero`字段，`name`和`alterEgo`，供用户输入。

  The *Name* `<input>` control has the HTML5 `required` attribute;
  the *Alter Ego* `<input>` control does not because `alterEgo` is optional.

  *Name* `<input>`控件具有 HTML5 的`required`属性；但 *Alter Ego* `<input>`控件没有，因为`alterEgo`字段是可选的。

  We've got a *Submit* button at the bottom with some classes on it for styling.

  底部有个 *Submit* 按钮，具有一些 CSS 样式类。

  **We are not using Angular yet**. There are no bindings. No extra directives. Just layout.

  **还没有真正用到Angular**。没有绑定，没有额外的指令，只有布局。

  The `container`, `form-group`, `form-control`, and `btn` classes
  come from [Twitter Bootstrap](http://getbootstrap.com/css/). Purely cosmetic.
  We're using Bootstrap to gussy up our form.
  Hey, what's a form without a little style!

  `container`、`form-group`、`form-control`和`btn`类来自 [Twitter Bootstrap](http://getbootstrap.com/css/)。纯粹是装饰。
  我们使用 Bootstrap 来美化表单。嘿，一点样式都没有的表单算个啥！

.callout.is-important
  header Angular Forms Do Not Require A Style Library

  header Angular 表单不需要任何样式库

  :marked
    Angular makes no use of the `container`, `form-group`, `form-control`, and `btn` classes or
    the styles of any external library. Angular apps can use any CSS library
    ... or none at all.

    Angular 不需要`container`、`form-group`、`form-control`和`btn`类，
    或者外部库的任何样式。Angular 应用可以使用任何 CSS 库…… ，或者啥都不用。

:marked
  Let's add the stylesheet.

  添加样式表。

ol
  li
    p Open a terminal window in the application root folder and enter the command:

    p 在应用的根目录下打开终端窗口，输入如下命令：

    code-example(language="html" escape="html").
      npm install bootstrap --save

  li
    p Open <code>index.html</code> and add the following link to the <code>&lt;head></code>.

    p 打开<code>index.html</code>文件并且把下列链接添加到<code>&lt;head></code>中。

    +makeExample('forms/ts/index.html', 'bootstrap')(format=".")
:marked
.l-main-section
:marked
  ## Add Powers with ***ngFor**

  ## 用 ***ngFor*** 添加超能力

  Our hero may choose one super power from a fixed list of Agency-approved powers.
  We maintain that list internally (in `HeroFormComponent`).

  英雄可以从认证过的固定列表中选择一项超能力。
  这个列表位于`HeroFormComponent`中。

  We'll add a `select` to our
  form and bind the options to the `powers` list using `ngFor`,
  a technique we might have seen before in the [Displaying Data](./displaying-data.html) chapter.

  在表单中添加`select`，用`ngFor`把`powers`列表绑定到列表选项。
  我们在之前的[显示数据](./displaying-data.html)一章中见过`ngFor`。

  Add the following HTML *immediately below* the *Alter Ego* group.

  在 *Alter Ego* 的紧下方添加如下 HTML：
+makeExample('forms/ts/app/hero-form.component.html', 'powers', 'app/hero-form.component.html (excerpt)')(format=".")

:marked
  We are repeating the `<options>` tag for each power in the list of Powers.
  The `p` template input variable is a different power in each iteration;
  we display its name using the interpolation syntax with the double-curly-braces.

  列表中的每一项超能力都会渲染成`<option>`标签。
  模板输入变量`p`在每个迭代指向不同的超能力，使用双花括号插值表达式语法来显示它的名称。

<a id="ngModel"></a>
.l-main-section
:marked
  ## Two-way data binding with **ngModel**

  ## 使用 **ngModel** 进行双向数据绑定

  Running the app right now would be disappointing.

  如果立即运行此应用，你将会失望。

figure.image-display
  img(src="/resources/images/devguide/forms/hero-form-3.png" width="400px" alt="没有数据绑定的早期表单")
:marked
  We don't see hero data because we are not binding to the `Hero` yet.
  We know how to do that from earlier chapters.
  [Displaying Data](./displaying-data.html) taught us Property Binding.
  [User Input](./user-input.html) showed us how to listen for DOM events with an
  Event Binding and how to update a component property with the displayed value.

  因为还没有绑定到某个英雄，所以看不到任何数据。
  解决方案见前面的章节。
  [显示数据](./displaying-data.html)介绍了属性绑定。
  [用户输入](./user-input.html)介绍了如何通过事件绑定来监听DOM事件，以及如何用显示值更新组件的属性。

  Now we need to display, listen, and extract at the same time.

  现在，需要同时进行显示、监听和提取。

  We could use those techniques again in our form.
  Instead we'll introduce something new, the `[(ngModel)]` syntax, that
  makes binding our form to the model super-easy.

  虽然可以在表单中再次使用这些技术。
  但是，这里将介绍个新东西 —— `[(ngModel)]`语法，使表单绑定到模型的工作变得超级简单。

  Find the `<input>` tag for the "Name" and update it like this

  找到“Name”对应的`<input>`标签，并且像这样修改它：

+makeExample('forms/ts/app/hero-form.component.html', 'ngModel-1','app/hero-form.component.html (节选)')(format=".")

.l-sub-section
  :marked
    We appended a diagnostic interpolation after the input tag
    so we can see what we're doing.
    We left ourselves a note to throw it away when we're done.

    在 input 标签后添加用于诊断的插值表达式，以看清正在发生什么事。
    给自己留个备注，提醒我们完成后移除它。

:marked
  Focus on the binding syntax: `[(ngModel)]="..."`.

  聚焦到绑定语法`[(ngModel)]="..."`上。

  If we ran the app right now and started typing in the *Name* input box,
  adding and deleting characters, we'd see them appearing and disappearing
  from the interpolated text.
  At some point it might look like this.

  如果现在运行这个应用，开始在*姓名*输入框中键入，添加和删除字符，将看到它们从插值结果中显示和消失。
  某一瞬间，它看起来可能是这样：
figure.image-display
  img(src="/resources/images/devguide/forms/ng-model-in-action.png" width="400px" alt="操作中的ngModel")
:marked
  The diagnostic is evidence that we really are flowing values from the input box to the model and
  back again. **That's two-way data binding!**

  诊断信息可以证明，数据确实从输入框流动到模型，再反向流动回来。**这就是双向数据绑定！**

  Notice that we also added a `name` attribute to our `<input>` tag and set it to "name"
  which makes sense for the hero's name. Any unique value will do, but using a descriptive name is helpful.
  Defining a `name` attribute is a requirement when using `[(ngModel)]` in combination with a form.

  注意，`<input>`标签还添加了`name`属性 (attribute)，并设置为"name"，表示英雄的名字。
  使用任何唯一的值都可以，但使用具有描述性的名字会更有帮助。
  当在表单中使用`[(ngModel)]`时，必须要定义`name`属性。

.l-sub-section
  :marked
    Internally Angular creates `FormControls` and registers them with an `NgForm` directive that Angular
    attached to the `<form>` tag. Each `FormControl` is registered under the name we assigned to the `name` attribute.
    We'll talk about `NgForm` [later in this chapter](#ngForm).

    在内部，Angular 创建了一些`FormControl`，并把它们注册到`NgForm`指令，再将该指令附加到`<form>`标签。
    注册每个`FormControl`时，使用`name`属性值作为键值。[本章后面](#ngForm)会讨论`NgForm`。

:marked
  Let's add similar `[(ngModel)]` bindings and `name` attributes to *Alter Ego* and *Hero Power*.
  We'll ditch the input box binding message
  and add a new binding at the top to the component's `diagnostic` property.
  Then we can confirm that two-way data binding works *for the entire Hero model*.

  为*第二人格*和*超能力*属性添加类似的`[(ngModel)]`绑定和`name`属性。
  抛弃输入框的绑定消息，在组件顶部添加到`diagnostic`属性的新绑定。
  这样就能确认双向数据绑定*在整个Hero模型上*都能正常工作了。

  After revision the core of our form should have three `[(ngModel)]` bindings and `name` attributes that
  look much like this:

  修改之后的表单，其核心是三个`[(ngModel)]`绑定，看起来像这样：

+makeExample('forms/ts/app/hero-form.component.html', 'ngModel-2', 'app/hero-form.component.html (节选)')

.l-sub-section
  :marked
    - Each input element has an `id` property that is used by the `label` element's `for` attribute
    to match the label to its input control.

      每个 input 元素都有`id`属性，`label`元素的`for`属性用它来匹配到对应的输入控件。

    - Each input element has a `name` property that is required by Angular Forms to register the control with the form.

      每个 input 元素都有`name`属性，Angular 表单用它注册控件。

:marked
  If we ran the app right now and changed every Hero model property, the form might display like this:

  如果现在运行本应用，修改 Hero 模型的每个属性，表单看起来像这样：
figure.image-display
  img(src="/resources/images/devguide/forms/ng-model-in-action-2.png" width="400px" alt="ngModel in super action")
:marked
  The diagnostic near the top of the form
  confirms that all of our changes are reflected in the model.

  表单顶部的诊断信息反映出所做的一切更改。

  **Delete** the `{{diagnostic}}` binding at the top as it has served its purpose.

  表单顶部的`{{diagnostic}}`绑定已经完成了它的使命，**删除**它。

.l-sub-section
  :marked
    ### Inside [(ngModel)]

    ### [(ngModel)]内幕

    *This section is an optional deep dive into [(ngModel)]. Not interested? Skip ahead!*

    *本节是对[(ngModel)]的深入剖析，它是可选的。不感兴趣？跳过它！*

    The punctuation in the binding syntax, <span style="font-family:courier"><b>[()]</b></span>, is a good clue to what's going on.

    绑定语法中的<span style="font-family:courier"><b>[()]</b></span>是很好的线索。

    In a Property Binding, a value flows from the model to a target property on screen.
    We identify that target property by surrounding its name in brackets, <span style="font-family:courier"><b>[]</b></span>.
    This is a one-way data binding **from the model to the view**.

    在属性绑定中，值从模型中流动到屏幕上的目标属性 (property)。
    通过把属性名括在方括号中来标记出目标属性，<span style="font-family:courier"><b>[]</b></span>。
    这是**从模型到视图**的单向数据绑定。

    In an Event Binding, we flow the value from the target property on screen to the model.
    We identify that target property by surrounding its name in parentheses, <span style="font-family:courier"><b>()</b></span>.
    This is a one-way data binding in the opposite direction **from the view to the model**.

    在事件绑定中，值从屏幕上的目标属性流动到模型。
    通过把属性名括在圆括号中来标记出目标属性，<span style="font-family:courier"><b>()</b></span>。
    这是**从视图到模型**的反向单向数据绑定。

    No wonder Angular chose to combine the punctuation as <span style="font-family:courier"><b>[()]</b></span>
    to signify a two-way data binding and a **flow of data in both directions**.

    不出所料，Angular选择了组合标点 <span style="font-family:courier"><b>[()]</b></span> 来标记出双向数据绑定和**双向数据流**。

    In fact, we can break the `NgModel` binding into its two separate modes
    as we do in this re-write of the "Name" `<input>` binding:

    事实上，可以把`NgModel`绑定拆成两个独立的绑定，就像下面重写的“Name”`<input>`绑定一样：
  +makeExample('forms/ts/app/hero-form.component.html', 'ngModel-3','app/hero-form.component.html (excerpt)')(format=".")

  :marked
    <br>The Property Binding should feel familiar. The Event Binding might seem strange.

    <br>这个属性绑定看起来很眼熟，但事件绑定看起来有点怪。

    The `ngModelChange` is not an `<input>` element event.
    It is actually an event property of the `NgModel` directive.
    When Angular sees a binding target in the form <span style="font-family:courier">[(x)]</span>,
    it expects the `x` directive to have an `x` input property and an `xChange` output property.

    `ngModelChange`并不是`<input>`元素的事件。
    它实际上是来自`NgModel`指令的事件属性。
    当 Angular 在表单中看到<span style="font-family:courier">[(x)]</span>的绑定目标时，
    它会期待这个`x`指令有一个名为`x`的输入属性，和一个名为`xChange`的输出属性。

    The other oddity is the template expression, `model.name = $event`.
    We're used to seeing an `$event` object coming from a DOM event.
    The `ngModelChange` property doesn't produce a DOM event; it's an Angular `EventEmitter`
    property that returns the input box value when it fires &mdash; which is precisely what
    we should assign to the model's `name` property.

    模板表达式中的另一个古怪之处是`model.name = $event`。
    之前看到的`$event`对象来自DOM事件。
    但`ngModelChange`属性不会生成DOM事件 —— 它是Angular `EventEmitter`类型的属性，当它触发时，
    它返回的是输入框的值 —— 也正是希望赋给模型`name`属性的值。

    Nice to know but is it practical? We almost always prefer `[(ngModel)]`.
    We might split the binding if we had to do something special in
    the event handling such as debounce or throttle the key strokes.

    很高兴知道这些，但是这样现实吗？实践上中，几乎总是优先使用`[(ngModel)]`形式的双向绑定。
    只有当需要在事件处理函数中做一些特别的事情（比如合并或限制按键频率）时，才会拆分出独立的事件处理函数。

    Learn more about `NgModel` and other template syntax in the
    [Template Syntax](./template-syntax.html) chapter.

    要学习`ngModel`和其它模板语法的更多知识，见[模板语法](./template-syntax.html)。

.l-main-section
:marked
  ## Track change-state and validity with **ngModel**

  ## 通过 **ngModel** 跟踪修改状态与有效性验证

  A form isn't just about data binding. We'd also like to know the state of the controls on our form.

  表单不仅是关于数据绑定的。我们还想知道表单中各个控件的状态。

  Using `ngModel` in a form gives us more than just two way data binding. It also tells us if the user touched the control, if the value changed, or if the value became invalid.

  在表单中使用`ngModel`可以获得比仅使用双向数据绑定更多的控制权。它还会告诉我们很多信息：用户碰过此控件吗？它的值变化了吗？数据变得无效了吗？

  The *NgModel* directive doesn't just track state; it updates the control with special Angular CSS classes that reflect the state.
  We can leverage those class names to change the appearance of the
  control and make messages appear or disappear.

  *NgModel* 指令不仅仅跟踪状态。它还使用特定的 Angular CSS 类来更新控件，以反映当前状态。
  可以利用这些 CSS 类来修改控件的外观，显示或隐藏消息。

table
  tr
    th
      p State
      p 状态
    th
      p Class if true
      p 为真时的 CSS 类
    th
      p Class if false
      p 为假时的 CSS 类
  tr
    td
      p Control has been visited
      p 控件已经被访问过
    td
      p <code>ng-touched</code>
      p <code>ng-touched</code>
    td
      p <code>ng-untouched</code>
      p <code>ng-untouched</code>
  tr
    td
      p Control's value has changed
      p 控件值已经变化
    td
      p <code>ng-dirty</code>
      p <code>ng-dirty</code>
    td
      p <code>ng-pristine</code>
      p <code>ng-pristine</code>
  tr
    td
      p Control's value is valid
      p 控件值是有效的
    td
      p <code>ng-valid</code>
      p <code>ng-valid</code>
    td
      p <code>ng-invalid</code>
      p <code>ng-invalid</code>
:marked
  Let's add a temporary [template reference variable](./template-syntax.html#ref-vars) named **spy**
  to the "Name" `<input>` tag and use the spy to display those classes.

  往姓名`<input>`标签上添加名叫 **spy** 的临时[模板引用变量](./template-syntax.html#local-vars)，
  然后用这个 spy 来显示它上面的所有 CSS 类。

+makeExample('forms/ts/app/hero-form.component.html', 'ngModelName-2','app/hero-form.component.html (excerpt)')(format=".")

:marked
  Now run the app and focus on the *Name* input box.
  Follow the next four steps *precisely*

  现在，运行本应用，并让*姓名*输入框获得焦点。
  然后严格按照下面四个步骤来做：

  1. Look but don't touch

     查看输入框，但别碰它

  1. Click in the input box, then click outside the text input box

     点击输入框，然后点击输入框外面

  1. Add slashes to the end of the name

     在名字的末尾添加些斜杠

  1. Erase the name

     删除名字

  The actions and effects are as follows:

  动作和它对应的效果如下：

figure.image-display
  img(src="/resources/images/devguide/forms/control-state-transitions-anim.gif"  alt="控件状态转换")
:marked
  We should be able to see the following four sets of class names and their transitions:

  应该能看到下列四组类名以及它们的变迁：
figure.image-display
  img(src="/resources/images/devguide/forms/ng-control-class-changes.png" width="400px" alt="控件状态转换")

:marked
  The (`ng-valid` | `ng-invalid`) pair are most interesting to us. We want to send a
  strong visual signal when the data are invalid and we want to mark required fields.
  So we add custom CSS for visual feedback.

  (`ng-valid` | `ng-invalid`)这一对是我们最感兴趣的。当数据变得无效时，我们希望发出强力的视觉信号，
  还想要标记出必填字段。可以通过加入自定义 CSS 来提供视觉反馈。

  **Delete** the `#spy` template reference variable and `TODO` as they have served their purpose.

  **删除**模板引用变量`#spy`和`TODO`，因为它们已经完成了使命。

.l-main-section
:marked
  ## Add Custom CSS for Visual Feedback

  ## 添加用于视觉反馈的自定义 CSS

  We realize we can mark required fields and invalid data at the same time with a colored bar
  on the left of the input box:

  可以在输入框的左侧添加带颜色的竖条，用于标记必填字段和无效输入：

figure.image-display
  img(src="/resources/images/devguide/forms/validity-required-indicator.png" width="400px" alt="无效表单")

:marked
  We achieve this effect by adding two styles to a new `forms.css` file
  that we add to our project as a sibling to `index.html`.

  在新建的`forms.css`文件中，添加两个样式来实现这一效果。把这个文件添加到项目中，与`index.html`相邻。

+makeExample('forms/ts/forms.css',null,'forms.css')(format=".")
:marked
  These styles select for the two Angular validity classes and the HTML 5 "required" attribute.

  这些样式的作用于两个 Angular 有效性类和 HTML 5 的“required”属性。

  We update the `<head>` of the `index.html` to include this style sheet.

  更新`index.html`中的`<head>`来包含这个样式表。
+makeExample('forms/ts/index.html', 'styles', 'index.html (节选)')(format=".")
:marked
  ## Show and Hide Validation Error messages

  ## 显示和隐藏验证错误信息

  We can do better.

  我们能做的更好。

  The "Name" input box is required. Clearing it turns the bar red. That says *something* is wrong but we
  don't know *what* is wrong or what to do about it.
  We can leverage the `ng-invalid` class to reveal a helpful message.

  “Name”输入框是必填的，清空它会让左侧的条变红。这表示*某些东西*是错的，但我们不知道错在哪里，或者如何纠正。
  可以借助`ng-invalid`类来给出有用的提示。

  Here's the way it should look when the user deletes the name:

  当用户删除姓名时，看起来应该是这样的：

figure.image-display
  img(src="/resources/images/devguide/forms/name-required-error.png" width="400px" alt="必须填写姓名")

:marked
  To achieve this effect we extend the `<input>` tag with

  要达到这个效果，在`<input>`标签中添加：

  1. a [template reference variable](./template-syntax.html#ref-vars)

     [模板引用变量](./template-syntax.html#ref-vars)

  1. the "*is required*" message in a nearby `<div>` which we'll display only if the control is invalid.

     “is required”消息，放在邻近的`<div>`元素中，只有当控件无效时，才显示它。

  Here's how we do it for the *name* input box:

  对*Name*输入框做如下修改：
+makeExample('forms/ts/app/hero-form.component.html',
  'name-with-error-msg',
  'app/hero-form.component.html (节选)')(format=".")
:marked
  We need a template reference variable to access the input box's Angular control from within the template.
  Here we created a variable called `name` and gave it the value "ngModel".

  模板引用变量可以访问模板中输入框的 Angular 控件。
  这里，创建了名叫`name`的变量，并且赋值为"ngModel"。

.l-sub-section
  :marked
    Why "ngModel"?
    A directive's [exportAs](../api/core/index/Directive-decorator.html) property
    tells Angular how to link the reference variable to the directive.
    We set `name` to `ngModel` because the `ngModel` directive's `exportAs` property happens to be "ngModel".

    为什么是“ngModel”？
    指令的 [exportAs](../api/core/index/DirectiveMetadata-class.html#!#exportAs) 属性告诉 Angular 如何链接模板引用变量到指令。
    这里把`name`设置为`ngModel`是因为`ngModel`指令的`exportAs`属性设置成了“ngModel”。

    Now we can control visibility of the "name" error message by binding properties of the `name` control to the message `<div>` element's `hidden` property.

    现在，把`div`元素的`hidden`属性绑定到`name`控件的属性，这样就可以控制“姓名”字段错误信息的可见性了。
+makeExample('forms/ts/app/hero-form.component.html',
  'hidden-error-msg',
  'app/hero-form.component.html (节选)')
:marked
  In this example, we hide the message when the control is valid or pristine;
  pristine means the user hasn't changed the value since it was displayed in this form.

  上例中，当控件是有效的 (valid) 或全新的 (pristine) 时，隐藏消息。
  “全新的”意味着从它被显示在表单中开始，用户还从未修改过它的值。

  This user experience is the developer's choice. Some folks want to see the message at all times.
  If we ignore the `pristine` state, we would hide the message only when the value is valid.
  If we arrive in this component with a new (blank) hero or an invalid hero,
  we'll see the error message immediately, before we've done anything.

  这种用户体验取决于开发人员的选择。有些人会希望任何时候都显示这条消息。
  如果忽略了`pristine`状态，就会只在值有效时隐藏此消息。
  如果往这个组件中传入全新（空）的英雄，或者无效的英雄，将立刻看到错误信息 —— 虽然我们还啥都没做。

  Some folks find that behavior disconcerting. They only want to see the message when the user makes an invalid change.
  Hiding the message while the control is "pristine" achieves that goal.
  We'll see the significance of this choice when we [add a new hero](#new-hero) to the form.

  有些人会为这种行为感到不安。它们希望只有在用户做出无效的更改时才显示这个消息。
  如果当控件是“全新”状态时也隐藏消息，就能达到这个目的。
  在往表单中[添加新英雄](#new-hero)时，将看到这种选择的重要性。

  The Hero *Alter Ego* is optional so we can leave that be.

  英雄的*第二人格*是可选项，所以不用改它。

  Hero *Power* selection is required.
  We can add the same kind of error handling to the `<select>` if we  want
  but it's not imperative because the selection box already constrains the
  power to valid value.

  英雄的*超能力*选项是必填的。
  只要愿意，可以往`<select>`上添加相同的错误处理。
  但没有必要，这个选择框已经限制了“超能力”只能选有效值。

<a id="new-hero"></a>
<a id="reset"></a>
.l-main-section
:marked
  ## Add a hero and reset the form

  ## 添加英雄及重置表单

  We'd like to add a new hero in this form.
  We place a "New Hero" button at the bottom of the form and bind its click event to a component method.

  我们希望在这个表单中添加新的英雄。
  在表单的底部放置“New Hero（新增英雄）”按钮，并把它的点击事件绑定到组件方法。
+makeExample('forms/ts/app/hero-form.component.html',
  'new-hero-button',
  'app/hero-form.component.html (新增英雄按钮)')
:marked
+makeExample('forms/ts/app/hero-form.component.ts',
  'new-hero-v1',
  'app/hero-form.component.ts (新增英雄按钮 - v1)')(format=".")
:marked
  Run the application again, click the *New Hero* button, and the form clears.
  The *required* bars to the left of the input box are red, indicating invalid `name` and `power` properties.
  That's understandable as these are required fields.
  The error messages are hidden because the form is pristine; we haven't changed anything yet.

  再次运行应用，点击 *New Hero* 按钮，表单被清空了。
  输入框左侧的*必填项*竖条是红色的，表示`name`和`power`属性是无效的。
  这可以理解，因为有一些必填字段。
  错误信息是隐藏的，因为表单还是全新的，还没有修改任何东西。

  Enter a name and click *New Hero* again.
  This time we see an error message! Why? We don't want that when we display a new (empty) hero.

  输入名字，再次点击 *New Hero* 按钮。
  这次，出现了错误信息！为什么？我们不希望显示新（空）的英雄时，出现错误信息。

  Inspecting the element in the browser tools reveals that the *name* input box is no longer pristine.
  Replacing the hero *did not restore the pristine state* of the control.

  使用浏览器工具审查这个元素就会发现，这个*name*输入框并不是全新的。
  更换了英雄*并不会重置控件的“全新”状态*。
.l-sub-section
  :marked
    Upon reflection, we realize that Angular cannot distinguish between
    replacing the entire hero and clearing the `name` property programmatically.
    Angular makes no assumptions and leaves the control in its current, dirty state.

    沉思一番你会发现，Angular 没办法区分是替换了整个英雄数据还是用程序清除了`name`属性。
    Angular 不能作出假设，只好让控件保留当前状态 —— 脏状态。
:marked
  We'll have to reset the form controls manually with a small trick.
  We add an `active` flag to the component, initialized to `true`. When we add a new hero,
  we toggle `active` false and then immediately back to true with a quick `setTimeout`.

  可以用个小花招来重置表单控件。
  给组件添加`active`标记，初始化为`true`。当添加新的英雄时，把`active`标记设置为`false`，
  再通过快速的`setTimeout`函数迅速把它设置回`true`。
+makeExample('forms/ts/app/hero-form.component.ts',
  'new-hero',
  'app/hero-form.component.ts (新增英雄 - 最终版)')(format=".")
:marked
  Then we bind the form element to this `active` flag.

  然后，把 form 元素绑定到这个`active`标志。
+makeExample('forms/ts/app/hero-form.component.html',
  'form-active',
  'app/hero-form.component.html (Form标签)')
:marked
  With `NgIf` bound to the `active` flag,
  clicking "New Hero" removes the form from the DOM and recreates it in a blink of an eye.
  The re-created form is in a pristine state. The error message is hidden.

  因为`NgIf`绑定到`active`标志，点击 "New Hero" 将从DOM中移除这个表单，并在一眨眼的功夫重建它。
  重新创建的表单处于“全新”状态。错误信息被隐藏了。
.l-sub-section
  :marked
    This is a temporary workaround while we await a proper form reset feature.

    这只是临时的变通方案，将来会用更好的方式来重置表单。
:marked

.l-main-section
:marked
  ## Submit the form with **ngSubmit**

  ## 使用 **ngSubmit** 提交表单

  The user should be able to submit this form after filling it in.
  The Submit button at the bottom of the form
  does nothing on its own but it will
  trigger a form submit because of its type (`type="submit"`).

  在填表完成之后，用户还应该能提交这个表单。
  “Submit（提交）”按钮位于表单的底部，它自己不做任何事，但因为有特殊的 type 值 (`type="submit"`)，所以会触发表单提交。

  A "form submit" is useless at the moment.
  To make it useful, we'll update the `<form>` tag with another Angular directive, `NgSubmit`,
  and bind it to the `HeroFormComponent.submit()` method with an  event binding

  仅仅触发“表单提交”在目前是没用的。
  要让它有用，还要用另外的 Angular 指令更新`<form>`标签 —— `NgSubmit`，
  并且通过事件绑定把它绑定到`HeroFormComponent.submit()`方法。
+makeExample('forms/ts/app/hero-form.component.html', 'ngSubmit')(format=".")

:marked
  We slipped in something extra there at the end!  We defined a
  template reference variable, **`#heroForm`**, and initialized it with the value, "ngForm".

  上面代码的最后出现一些额外的东西！定义了模板引用变量**`#heroForm`**，并初始化为"ngForm"。

  The variable `heroForm` is now a reference to the `NgForm` directive that governs the form as a whole.

  现在`heroForm`变量引用的是`NgForm`指令，它代表的是表单的整体。
<a id="ngForm"></a>
.l-sub-section
  :marked
    ### The NgForm directive

    ### NgForm指令

    What `NgForm` directive? We didn't add an [NgForm](../api/forms/index/NgForm-directive.html) directive!

    什么`NgForm`指令？之前没有添加过[NgForm](../api/common/index/NgForm-directive.html)指令啊！

    Angular did. Angular creates and attaches an `NgForm` directive to the `<form>` tag automatically.

    是 Angular 干的。Angular 自动创建了`NgForm`指令，并把它附加到`<form>`标签。

    The `NgForm` directive supplements the `form` element with additional features.
    It holds the controls we created for the elements with `ngModel` directive and `name` attribute
    and monitors their properties including their validity.
    It also has its own `valid` property which is true only *if every contained
    control* is valid.

    `NgForm`指令为`form`元素扩充了额外的特性。
    它持有通过`ngModel`指令和`name`属性为各个元素创建的那些控件，并且监视它们的属性变化，包括有效性。
    它还有自己的`valid`属性，只有当*其中所有控件*都有效时，它才有效。

:marked
  Later in the template we bind the button's `disabled` property to the form's over-all validity via
  the `heroForm` variable. Here's that bit of markup:

  模板中稍后的部分，通过`heroForm`变量把按钮的`disabled`属性绑定到表单的整体有效性。这里是那点HTML：

+makeExample('forms/ts/app/hero-form.component.html', 'submit-button')
:marked
  Re-run the application. The form opens in a valid state and the button is enabled.

  重新运行应用。表单打开时，状态是有效的，按钮是可用的。

  Now delete the *Name*. We violate the "name required" rule which
  is duly noted in our error message as before. And now the Submit button is also disabled.

  现在，删除*姓名*。我们违反了“必填姓名”规则，它还是像以前那样显示出错误信息。同时，Submit 按钮也被禁用了。

  Not impressed?  Think about it for a moment. What would we have to do to
  wire the button's enable/disabled state to the form's validity without Angular's help?

  没感动吗？再想一会儿。如果没有 Angular `NgForm`的帮助，又该怎么让按钮的禁用/启用状态和表单的有效性关联起来呢？

  For us, it was as simple as

  有了Angular，它就是这么简单：

  1. Define a template reference variable on the (enhanced) form element

     定义模板引用变量，放在（强化过的）form 元素上

  2. Reference that variable in a button some 50 lines away.

     从50行之外的按钮上引用这个变量。

.l-main-section
:marked
  ## Toggle two form regions (extra credit)

  ## 切换两个表单区域（额外的奖励）

  Submitting the form isn't terribly dramatic at the moment.

  提交表单还是不够激动人心。

.l-sub-section
  :marked
    An unsurprising observation for a demo. To be honest,
    jazzing it up won't teach us anything new about forms.
    But this is an opportunity to exercise some of our newly won
    binding skills.
    If you're not interested, you can skip to the chapter's conclusion
    and not miss a thing.

    对演示来说，这个收场很平淡的。老实说，即使让它更出彩，也无法教给我们任何关于表单的新知识。
    但这是练习新学到的绑定技能的好机会。
    如果你不感兴趣，可以跳到本章的总结部分，不用担心错失任何东西。

:marked
  Let's do something more strikingly visual.
  Let's hide the data entry area and display something else.

  来实现一些更炫的视觉效果吧。
  隐藏掉数据输入框，显示一些其它东西。

  Start by wrapping the form in a `<div>` and bind
  its `hidden` property to the `HeroFormComponent.submitted` property.

  先把表单包裹进`<div>`中，再把它的`hidden`属性绑定到`HeroFormComponent.submitted`属性。

+makeExample('forms/ts/app/hero-form.component.html', 'edit-div', 'app/hero-form.component.html (节选)')(format=".")

:marked
  The main form is visible from the start because the
  the `submitted` property is false until we submit the form,
  as this fragment from the `HeroFormComponent` reminds us:

  主表单从一开始就是可见的，因为`submitted`属性是false，直到提交了这个表单。
  来自`HeroFormComponent`的代码片段告诉了我们这一点：

+makeExample('forms/ts/app/hero-form.component.ts', 'submitted')(format=".")

:marked
  When we click the Submit button, the `submitted` flag becomes true and the form disappears
  as planned.

  当点击 Submit 按钮时，`submitted`标志会变成 true，并且表单像预想中一样消失了。

  Now we need to show something else while the form is in the submitted state.
  Add the following block of HTML below the `<div>` wrapper we just wrote:

  现在，当表单处于已提交状态时，需要显示一些别的东西。
  在刚刚写的`<div>`包装下方，添加下列HTML块：
+makeExample('forms/ts/app/hero-form.component.html', 'submitted', 'app/hero-form.component.html (节选)')

:marked
  There's our hero again, displayed read-only with interpolation bindings.
  This slug of HTML only appears while the component is in the submitted state.

  英雄又出现了，它通过插值表达式绑定显示为只读内容。
  这一小段 HTML 只在组件处于已提交状态时才会显示。

  We added an Edit button whose click event is bound to an expression
  that clears the `submitted` flag.

  添加了“Edit（编辑）”按钮，将click事件绑定到表达式，用于清除`submitted`标志。

  When we click it, this block disappears and the editable form reappears.

  当点它时，这个只读块消失了，可编辑的表单重新出现了。

  That's as much drama as we can muster for now.

  够炫吗？好吧，我们已经尽力了！

.l-main-section
:marked
  ## Conclusion

  ## 结论

  The Angular form techniques discussed in this chapter take
  advantage of the following framework features to provide support for data modification, validation and more:

  本章讨论的 Angular 表单技术利用了下列框架特性来支持数据修改、验证和更多操作：

  - An Angular HTML form template.

    Angular HTML 表单模板。

  - A form component class with a `Component` decorator.

    带有`Component`装饰器的表单组件类。

  - The `ngSubmit` directive for handling the form submission.

    用来处理表单提交的`ngSubmit`指令。

  - Template reference variables such as `#heroForm`, `#name` and `#power`.

    模板引用变量，如`#heroForm`、`#name`和`#power`。

  - The `[(ngModel)]` syntax and a `name` attribute for two-way data binding, validation and change tracking.

    用于双向数据绑定、数据验证和变化追踪的`[(ngModel)]`语法和`name`属性。

  - The reference variable’s `valid` property on input controls to check if a control is valid and show/hide error messages.

    指向 input 控件的引用变量上的`valid`属性，可用于检查控件是否有效、是否显示/隐藏错误信息。

  - Controlling the submit button's enabled state by binding to `NgForm` validity.

    通过绑定到`NgForm`的有效性状态，控制提交按钮的禁用状态。

  - Custom CSS classes that provide visual feedback to users about invalid controls.

    定制 CSS 类来给用户提供无效控件的视觉反馈。

  Our final project folder structure should look like this:

  最终的项目目录结构看起来是这样：

.filetree
  .file angular-forms
  .children
    .file app
    .children
      .file app.component.ts
      .file app.module.ts
      .file hero.ts
      .file hero-form.component.html
      .file hero-form.component.ts
      .file main.ts
    .file node_modules ...  
    .file index.html
    .file package.json
    .file tsconfig.json
:marked
  Here’s the final version of the source:

  这里是源码的最终版本：

+makeTabs(
  `forms/ts/app/hero-form.component.ts,
   forms/ts/app/hero-form.component.html,
   forms/ts/app/hero.ts,
   forms/ts/app/app.module.ts,
   forms/ts/app/app.component.ts,
   forms/ts/app/main.ts,
   forms/ts/index.html,
   forms/ts/forms.css`,
  'final, final,,,,,',
  `hero-form.component.ts,
   hero-form.component.html,
   hero.ts,
   app.module.ts,
   app.component.ts,
   main.ts,
   index.html,
   forms.css`)
:marked
