Displaying Data


You can display data by binding controls in an HTML template to properties of an Angular component.

在 Angular 中最典型的数据显示方式,就是把 HTML 模板中的控件绑定到 Angular 组件的属性。

In this page, you'll create a component with a list of heroes. You'll display the list of hero names and conditionally show a message below the list.

本章中,你将创建一个带英雄列表的组件。 你将显示英雄名字的列表,并根据条件在列表下方显示一条消息。

The final UI looks like this:


Final UI

Thedemonstrates all of the syntax and code snippets described in this page.


Showing component properties with interpolation


The easiest way to display a component property is to bind the property name through interpolation. With interpolation, you put the property name in the view template, enclosed in double curly braces: {{myHero}}.

要显示组件的属性,最简单的方式就是通过插值表达式 (interpolation) 来绑定属性名。 要使用插值表达式,就把属性名包裹在双花括号里放进视图模板,如 {{myHero}}

Follow the Getting Started instructions for creating a new project named displaying-data.


Delete the app.component.html file. It is not needed for this example.

删除 app.component.html 文件,这个范例中不再需要它了。

Then modify the app.component.ts file by changing the template and the body of the component.

然后,到 app.component.ts 文件中修改组件的模板和代码。

When you're done, it should look like this:


import { Component } from '@angular/core'; @Component({ selector: 'app-root', template: ` <h1>{{title}}</h1> <h2>My favorite hero is: {{myHero}}</h2> ` }) export class AppComponent { title = 'Tour of Heroes'; myHero = 'Windstorm'; }

You added two properties to the formerly empty component: title and myHero.

再把两个属性 titlemyHero 添加到之前空白的组件中。

The template displays the two component properties using double curly brace interpolation:


template: ` <h1>{{title}}</h1> <h2>My favorite hero is: {{myHero}}</h2> `

The template is a multi-line string within ECMAScript 2015 backticks (`). The backtick (`)—which is not the same character as a single quote (')—allows you to compose a string over several lines, which makes the HTML more readable.

模板是包在 ECMAScript 2015 反引号 (`) 中的一个多行字符串。 反引号 (`) — 注意,不是单引号 (') — 允许把一个字符串写在多行上, 使 HTML 模板更容易阅读。

Angular automatically pulls the value of the title and myHero properties from the component and inserts those values into the browser. Angular updates the display when these properties change.

Angular 自动从组件中提取 titlemyHero 属性的值,并且把这些值插入浏览器中。当这些属性发生变化时,Angular 就会自动刷新显示。

More precisely, the redisplay occurs after some kind of asynchronous event related to the view, such as a keystroke, a timer completion, or a response to an HTTP request.

严格来说,“重新显示”是在某些与视图有关的异步事件之后发生的,例如,按键、定时器完成或对 HTTP 请求的响应。

Notice that you don't call new to create an instance of the AppComponent class. Angular is creating an instance for you. How?

注意,你没有调用 new 来创建 AppComponent 类的实例,是 Angular 替你创建了它。那么它是如何创建的呢?

The CSS selector in the @Component decorator specifies an element named <app-root>. That element is a placeholder in the body of your index.html file:

注意 @Component 装饰器中指定的 CSS 选择器 selector,它指定了一个叫 <app-root> 的元素。 该元素是 index.html 文件里的一个占位符。

<body> <app-root></app-root> </body>

When you bootstrap with the AppComponent class (in main.ts), Angular looks for a <app-root> in the index.html, finds it, instantiates an instance of AppComponent, and renders it inside the <app-root> tag.

当你通过 main.ts 中的 AppComponent 类启动时,Angular 在 index.html 中查找一个 <app-root> 元素, 然后实例化一个 AppComponent,并将其渲染到 <app-root> 标签中。

Now run the app. It should display the title and hero name:


Title and Hero

The next few sections review some of the coding choices in the app.


Template inline or template file?

内联 (inline) 模板还是模板文件?

You can store your component's template in one of two places. You can define it inline using the template property, or you can define the template in a separate HTML file and link to it in the component metadata using the @Component decorator's templateUrl property.

你可以在两种地方存放组件模板。 你可以使用 template 属性把它定义为内联的,或者把模板定义在一个独立的 HTML 文件中, 再通过 @Component 装饰器中的 templateUrl 属性, 在组件元数据中把它链接到组件。

The choice between inline and separate HTML is a matter of taste, circumstances, and organization policy. Here the app uses inline HTML because the template is small and the demo is simpler without the additional HTML file.

到底选择内联 HTML 还是独立 HTML 取决于个人喜好、具体状况和组织级策略。 上面的应用选择内联 HTML ,是因为模板很小,而且没有额外的 HTML 文件显得这个演示简单些。

In either style, the template data bindings have the same access to the component's properties.


By default, the Angular CLI command ng generate componentgenerates components with a template file. You can override that with:

默认情况下,Angular CLI 命令 ng generate component在生成组件时会带有模板文件,你可以通过参数来覆盖它:

ng generate component hero -it

Constructor or variable initialization?


Although this example uses variable assignment to initialize the components, you could instead declare and initialize the properties using a constructor:


export class AppCtorComponent { title: string; myHero: string; constructor() { this.title = 'Tour of Heroes'; this.myHero = 'Windstorm'; } }

This app uses more terse "variable assignment" style simply for brevity.


Showing an array property with *ngFor

使用 ngFor 显示数组属性

To display a list of heroes, begin by adding an array of hero names to the component and redefine myHero to be the first name in the array.

要显示一个英雄列表,先向组件中添加一个英雄名字数组,然后把 myHero 重定义为数组中的第一个名字。

export class AppComponent { title = 'Tour of Heroes'; heroes = ['Windstorm', 'Bombasto', 'Magneta', 'Tornado']; myHero = this.heroes[0]; }

Now use the Angular ngFor directive in the template to display each item in the heroes list.

接着,在模板中使用 Angular 的 ngFor 指令来显示 heroes 列表中的每一项。

template: ` <h1>{{title}}</h1> <h2>My favorite hero is: {{myHero}}</h2> <p>Heroes:</p> <ul> <li *ngFor="let hero of heroes"> {{ hero }} </li> </ul> `

This UI uses the HTML unordered list with <ul> and <li> tags. The *ngFor in the <li> element is the Angular "repeater" directive. It marks that <li> element (and its children) as the "repeater template":

这个界面使用了由 <ul><li> 标签组成的无序列表。<li> 元素里的 *ngFor 是 Angular 的“迭代”指令。 它将 <li> 元素及其子级标记为“迭代模板”:

<li *ngFor="let hero of heroes"> {{ hero }} </li>

Don't forget the leading asterisk (*) in *ngFor. It is an essential part of the syntax. For more information, see the Template Syntax page.

不要忘记 *ngFor 中的前导星号 (*)。它是语法中不可或缺的一部分。 更多信息,见模板语法

Notice the hero in the ngFor double-quoted instruction; it is an example of a template input variable. Read more about template input variables in the microsyntax section of the Template Syntax page.

注意看 ngFor 双引号表达式中的 hero,它是一个模板输入变量。 更多模板输入变量的信息,见模板语法中的 微语法 (microsyntax)

Angular duplicates the <li> for each item in the list, setting the hero variable to the item (the hero) in the current iteration. Angular uses that variable as the context for the interpolation in the double curly braces.

Angular 为列表中的每个条目复制一个 <li> 元素,在每个迭代中,把 hero 变量设置为当前条目(英雄)。 Angular 把 hero 变量作为双花括号插值表达式的上下文。

In this case, ngFor is displaying an array, but ngFor can repeat items for any iterable object.

本例中,ngFor 用于显示一个“数组”, 但 ngFor 可以为任何可迭代的 (iterable) 对象重复渲染条目。

Now the heroes appear in an unordered list.


After ngfor

Creating a class for the data


The app's code defines the data directly inside the component, which isn't best practice. In a simple demo, however, it's fine.

应用代码直接在组件内部直接定义了数据。 作为演示还可以,但它显然不是最佳实践。

At the moment, the binding is to an array of strings. In real applications, most bindings are to more specialized objects.


To convert this binding to use specialized objects, turn the array of hero names into an array of Hero objects. For that you'll need a Hero class:

要将此绑定转换成使用对象,需要把这个英雄名字数组变成 Hero 对象数组。但首先得有一个 Hero 类。

ng generate class hero

With the following code:


export class Hero { constructor( public id: number, public name: string) { } }

You've defined a class with a constructor and two properties: id and name.


It might not look like the class has properties, but it does. The declaration of the constructor parameters takes advantage of a TypeScript shortcut.

它可能看上去不像是有属性的类,但它确实有,利用的是 TypeScript 提供的简写形式 —— 用构造函数的参数直接定义属性。

Consider the first parameter:


public id: number,

That brief syntax does a lot:


  • Declares a constructor parameter and its type.


  • Declares a public property of the same name.


  • Initializes that property with the corresponding argument when creating an instance of the class.


Using the Hero class

使用 Hero 类

After importing the Hero class, the AppComponent.heroes property can return a typed array of Hero objects:

导入了 Hero 类之后,组件的 heroes 属性就可以返回一个类型化的Hero 对象数组了。

heroes = [ new Hero(1, 'Windstorm'), new Hero(13, 'Bombasto'), new Hero(15, 'Magneta'), new Hero(20, 'Tornado') ]; myHero = this.heroes[0];

Next, update the template. At the moment it displays the hero's id and name. Fix that to display only the hero's name property.

接着,修改模板。 现在它显示的是英雄的 idname。 要修复它,只显示英雄的 name 属性就行了。

template: ` <h1>{{title}}</h1> <h2>My favorite hero is: {{myHero.name}}</h2> <p>Heroes:</p> <ul> <li *ngFor="let hero of heroes"> {{ hero.name }} </li> </ul> `

The display looks the same, but the code is clearer.


Conditional display with NgIf

通过 NgIf 进行条件显示

Sometimes an app needs to display a view or a portion of a view only under specific circumstances.


Let's change the example to display a message if there are more than three heroes.


The Angular ngIf directive inserts or removes an element based on a truthy/falsy condition. To see it in action, add the following paragraph at the bottom of the template:

Angular 的 ngIf 指令会根据一个布尔条件来显示或移除一个元素。 来看看实际效果,把下列语句加到模板的底部:

<p *ngIf="heroes.length > 3">There are many heroes!</p>

Don't forget the leading asterisk (*) in *ngIf. It is an essential part of the syntax. Read more about ngIf and * in the ngIf section of the Template Syntax page.

不要忘了 *ngIf 中的前导星号 (*)。它是本语法中不可或缺的一部分。 更多 ngIf* 的内容,见模板语法中的ngIf

The template expression inside the double quotes, *ngIf="heroes.length > 3", looks and behaves much like TypeScript. When the component's list of heroes has more than three items, Angular adds the paragraph to the DOM and the message appears. If there are three or fewer items, Angular omits the paragraph, so no message appears. For more information, see the template expressions section of the Template Syntax page.

双引号中的模板表达式 *ngIf="heros.length > 3",外观和行为很象 TypeScript 。 当组件中的英雄列表有三个以上的条目时,Angular 把这个段落添加到 DOM 中,于是消息显示了出来。 更多信息,见模板语法中的模板表达式

Angular isn't showing and hiding the message. It is adding and removing the paragraph element from the DOM. That improves performance, especially in larger projects when conditionally including or excluding big chunks of HTML with many data bindings.

Angular 并不是在显示和隐藏这条消息,它是在从 DOM 中添加和移除这个段落元素。 这会提高性能,特别是在一些大的项目中有条件地包含或排除一大堆带着很多数据绑定的 HTML 时。

Try it out. Because the array has four items, the message should appear. Go back into app.component.ts and delete or comment out one of the elements from the hero array. The browser should refresh automatically and the message should disappear.

试一下。因为这个数组中有四个条目,所以消息应该显示出来。 回到 app.component.ts,从英雄数组中删除或注释掉一个元素。 浏览器应该自动刷新,消息应该会消失。



Now you know how to use:


  • Interpolation with double curly braces to display a component property.

    带有双花括号的插值表达式 (interpolation) 来显示一个组件属性。

  • ngFor to display an array of items.

    ngFor 显示数组。

  • A TypeScript class to shape the model data for your component and display properties of that model.

    用一个 TypeScript 类来为你的组件描述模型数据并显示模型的属性。

  • ngIf to conditionally display a chunk of HTML based on a boolean expression.

    ngIf 根据一个布尔表达式有条件地显示一段 HTML。

Here's the final code:


import { Component } from '@angular/core'; import { Hero } from './hero'; @Component({ selector: 'app-root', template: ` <h1>{{title}}</h1> <h2>My favorite hero is: {{myHero.name}}</h2> <p>Heroes:</p> <ul> <li *ngFor="let hero of heroes"> {{ hero.name }} </li> </ul> <p *ngIf="heroes.length > 3">There are many heroes!</p> ` }) export class AppComponent { title = 'Tour of Heroes'; heroes = [ new Hero(1, 'Windstorm'), new Hero(13, 'Bombasto'), new Hero(15, 'Magneta'), new Hero(20, 'Tornado') ]; myHero = this.heroes[0]; }export class Hero { constructor( public id: number, public name: string) { } }import { NgModule } from '@angular/core'; import { BrowserModule } from '@angular/platform-browser'; import { AppComponent } from './app.component'; @NgModule({ imports: [ BrowserModule ], declarations: [ AppComponent ], bootstrap: [ AppComponent ] }) export class AppModule { }import { enableProdMode } from '@angular/core'; import { platformBrowserDynamic } from '@angular/platform-browser-dynamic'; import { AppModule } from './app/app.module'; import { environment } from './environments/environment'; if (environment.production) { enableProdMode(); } platformBrowserDynamic().bootstrapModule(AppModule);