NgModule 简介link

Introduction to moduleslink

Angular 应用是模块化的，它拥有自己的模块化系统，称作 NgModule。 一个 NgModule 就是一个容器，用于存放一些内聚的代码块，这些代码块专注于某个应用领域、某个工作流或一组紧密相关的功能。 它可以包含一些组件、服务提供者或其它代码文件，其作用域由包含它们的 NgModule 定义。 它还可以导入一些由其它模块中导出的功能，并导出一些指定的功能供其它 NgModule 使用。

Angular apps are modular and Angular has its own modularity system called NgModules. NgModules are containers for a cohesive block of code dedicated to an application domain, a workflow, or a closely related set of capabilities. They can contain components, service providers, and other code files whose scope is defined by the containing NgModule. They can import functionality that is exported from other NgModules, and export selected functionality for use by other NgModules.

每个 Angular 应用都至少有一个 NgModule 类，也就是根模块，它习惯上命名为 AppModule，并位于一个名叫 app.module.ts 的文件中。引导这个根模块就可以启动你的应用。

Every Angular app has at least one NgModule class, the root module, which is conventionally named AppModule and resides in a file named app.module.ts. You launch your app by bootstrapping the root NgModule.

虽然小型的应用可能只有一个 NgModule，不过大多数应用都会有很多特性模块。应用的根模块之所以叫根模块，是因为它可以包含任意深度的层次化子模块。

While a small application might have only one NgModule, most apps have many more feature modules. The root NgModule for an app is so named because it can include child NgModules in a hierarchy of any depth.

@NgModule 元数据link

NgModule metadatalink

NgModule 是一个带有 @NgModule() 装饰器的类。@NgModule() 装饰器是一个函数，它接受一个元数据对象，该对象的属性用来描述这个模块。其中最重要的属性如下。

An NgModule is defined by a class decorated with @NgModule(). The @NgModule() decorator is a function that takes a single metadata object, whose properties describe the module. The most important properties are as follows.

• declarations（可声明对象表） —— 那些属于本 NgModule 的组件、指令、管道。

  declarations: The components, directives, and pipes that belong to this NgModule.

• exports（导出表） —— 那些能在其它模块的组件模板中使用的可声明对象的子集。

  exports: The subset of declarations that should be visible and usable in the component templates of other NgModules.

• imports（导入表） —— 那些导出了本模块中的组件模板所需的类的其它模块。

  imports: Other modules whose exported classes are needed by component templates declared in this NgModule.

• providers —— 本模块向全局服务中贡献的那些服务的创建器。 这些服务能被本应用中的任何部分使用。（你也可以在组件级别指定服务提供者，这通常是首选方式。）

  providers: Creators of services that this NgModule contributes to the global collection of services; they become accessible in all parts of the app. (You can also specify providers at the component level.)

• bootstrap —— 应用的主视图，称为根组件。它是应用中所有其它视图的宿主。只有根模块才应该设置这个 bootstrap 属性。

  bootstrap: The main application view, called the root component, which hosts all other app views. Only the root NgModule should set the bootstrap property.

下面是一个简单的根 NgModule 定义：

Here's a simple root NgModule definition.

      
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      import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
@NgModule({
  imports:      [ BrowserModule ],
  providers:    [ Logger ],
  declarations: [ AppComponent ],
  exports:      [ AppComponent ],
  bootstrap:    [ AppComponent ]
})
export class AppModule { }
    

NgModule 和组件link

NgModules and componentslink

NgModule 为其中的组件提供了一个编译上下文环境。根模块总会有一个根组件，并在引导期间创建它。 但是，任何模块都能包含任意数量的其它组件，这些组件可以通过路由器加载，也可以通过模板创建。那些属于这个 NgModule 的组件会共享同一个编译上下文环境。

NgModules provide a compilation context for their components. A root NgModule always has a root component that is created during bootstrap, but any NgModule can include any number of additional components, which can be loaded through the router or created through the template. The components that belong to an NgModule share a compilation context.

组件及其模板共同定义视图。组件还可以包含视图层次结构，它能让你定义任意复杂的屏幕区域，可以将其作为一个整体进行创建、修改和销毁。 一个视图层次结构中可以混合使用由不同 NgModule 中的组件定义的视图。 这种情况很常见，特别是对一些 UI 库来说。

A component and its template together define a view. A component can contain a view hierarchy, which allows you to define arbitrarily complex areas of the screen that can be created, modified, and destroyed as a unit. A view hierarchy can mix views defined in components that belong to different NgModules. This is often the case, especially for UI libraries.

当你创建一个组件时，它直接与一个叫做宿主视图的视图关联起来。 宿主视图可以是视图层次结构的根，该视图层次结构可以包含一些内嵌视图，这些内嵌视图又是其它组件的宿主视图。 这些组件可以位于相同的 NgModule 中，也可以从其它 NgModule 中导入。 树中的视图可以嵌套到任意深度。

When you create a component, it's associated directly with a single view, called the host view. The host view can be the root of a view hierarchy, which can contain embedded views, which are in turn the host views of other components. Those components can be in the same NgModule, or can be imported from other NgModules. Views in the tree can be nested to any depth.

NgModule 和 JavaScript 的模块link

NgModules and JavaScript moduleslink

NgModule 系统与 JavaScript（ES2015）用来管理 JavaScript 对象的模块系统不同，而且也没有直接关联。 这两种模块系统不同但互补。你可以使用它们来共同编写你的应用。

The NgModule system is different from and unrelated to the JavaScript (ES2015) module system for managing collections of JavaScript objects. These are complementary module systems that you can use together to write your apps.

JavaScript 中，每个文件是一个模块，文件中定义的所有对象都从属于那个模块。 通过 export 关键字，模块可以把它的某些对象声明为公共的。 其它 JavaScript 模块可以使用import 语句来访问这些公共对象。

In JavaScript each file is a module and all objects defined in the file belong to that module. The module declares some objects to be public by marking them with the export key word. Other JavaScript modules use import statements to access public objects from other modules.

      
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      import { NgModule } from '@angular/core';
import { AppComponent } from './app.component';
    

      
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      export class AppModule { }
    

Angular 自带的库link

Angular librarieslink

Angular 会作为一组 JavaScript 模块进行加载，你可以把它们看成库模块。每个 Angular 库的名称都带有 @angular 前缀。 使用 npm 包管理器安装 Angular 的库，并使用 JavaScript 的 import 语句导入其中的各个部分。

Angular loads as a collection of JavaScript modules. You can think of them as library modules. Each Angular library name begins with the @angular prefix. Install them with the node package manager npm and import parts of them with JavaScript import statements.

例如，像下面这样，从 @angular/core 库中导入 Angular 的 Component 装饰器：

For example, import Angular's Component decorator from the @angular/core library like this.

      
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      import { Component } from '@angular/core';
    

还可以使用 JavaScript 的导入语句从 Angular 库中导入 Angular 模块。 比如，下列代码从 platform-browser 库中导入了 BrowserModule 这个 NgModule。

You also import NgModules from Angular libraries using JavaScript import statements. For example, the following code imports the BrowserModule NgModule from the platform-browser library.

      
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      import { BrowserModule } from '@angular/platform-browser';
    

在上面这个简单的根模块范例中，应用的根模块需要来自 BrowserModule 中的素材。要访问这些素材，就要把它加入 @NgModule 元数据的 imports 中，代码如下：

In the example of the simple root module above, the application module needs material from within BrowserModule. To access that material, add it to the @NgModule metadata imports like this.

      
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      imports:      [ BrowserModule ],
    

通过这种方式，你可以同时使用 Angular 和 JavaScript 的这两种模块系统。 虽然这两种模块系统容易混淆（它们共享了同样的词汇 import 和 export），不过只要多用用你就会熟悉它们各自的语境了。

In this way you're using the Angular and JavaScript module systems together. Although it's easy to confuse the two systems, which share the common vocabulary of "imports" and "exports", you will become familiar with the different contexts in which they are used.