Angular_文档_02_数据绑定_模板语法

Angular官方文档学习笔记第2篇_数据绑定_模板语法

Displaying Data

You can display data by 

binding controls in an HTML template 

to properties of an Angular component.

In this page, you'll create a component with a list of girls. 

You'll display the list of girl names 

and conditionally show a message below the list.

The final UI looks like this:

The live example / download example demonstrates 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: {{myGirl}}.

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

ng create displaying-data

如图所示:(可能要花几分钟)

然后cd displaying-data

执行ng serve启动项目

打开浏览器http://localhost:4200可以访问项目的首页

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

Then modify the app.component.ts file 

by changing the template and the body of the component.

意思就是: 去掉templateUrl: './app.component.html'

换成

template: `

未闻花名

`

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

src/app/app.component.ts

1. import { Component } from '@angular/core';
2.  
3. @Component({
4. selector: 'app-root',
5. template: `

6. {{title}}

7. My favorite girl is: {{myGirl}}

8. `
9. })
10. export class AppComponent {
11. title = '未闻花名';
12. myGirl = '面码';
13. }

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

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

src/app/app.component.ts (template)

template: `
  
{{title}}
  
My favorite girl is: {{myGirl}}
  `

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.

Angular automatically pulls the value of the title and myHero properties from the component 

and inserts those values into the browser. 

Angular 自动 updates the display when these properties change.

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.

Notice that you don't call new to create an instance of the AppComponent class. 

Angular is creating an instance for you. How?

The CSS selector in the @Component decorator specifies an element named . 

That element is a placeholder in the body of your index.html file:

src/index.html (body)

When you bootstrap with the AppComponent class (in main.ts), 

Angular looks for a  in the index.html, 

finds it, instantiates an instance of AppComponent, 

and renders it inside the  tag.

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

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

这句话什么意思???

Template inline or template file?

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.

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.

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

By default, the Angular CLI generates components with a template file. You can override that with:

什么意思???

-it 表示的是使用内联模板

inline template的缩写

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;
  myGirl: string;

  constructor() {
    this.title = '未闻花名';
    this.myGirl = '面码';
  }
}

This app uses more terse ( 简短的 ) "variable assignment" style simply for brevity.

Showing an array property with *ngFor

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.

src/app/app.component.ts (class)

export class AppComponent {
  title = '世界萌王';
  heroes = ['面码', '逢坂大河', '艾拉', '古手梨花'];
  myHero = this.heroes[0];
}

Now use the Angular ngFor  directive in the template 

to display each item in the heroes list.

src/app/app.component.ts (template)

template: `
  
{{title}}
  
My favorite hero is: {{myHero}}
  
Heroes:
  

    
ngFor="let hero of heroes">
      {{ hero }}
    
  
`

This UI uses the HTML unordered list with 

 and 
•  tags. 

  The *ngFor in the 

•  element is the Angular "repeater" directive. 

  It marks that 

•  element (and its children) as the "repeater template":
  src/app/app.component.ts (li)

   *ngFor="let hero of heroes">
    {{ hero }}
  

Don't forget the leading asterisk (*) in *ngFor. 

It is an essential part of the syntax. 

For more information, see the Template Syntax page.

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 ( 微语法 ) section of the Template Syntax page.




Angular duplicates the 

•  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.

  In this case, ngFor  is displaying an array, 

  but ngFor  can repeat items for any iterable object.

  Now the girls appear in an unordered list.

  

  
  

  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.

  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 Girl class:

  ng generate class girl

  
  

  With the following code:

  src/app/hero.ts

  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.

  Consider the first parameter:

  src/app/hero.ts (id)

  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

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

  src/app/app.component.ts (heroes)

  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.

  src/app/app.component.ts (template)

  template: `
    
  {{title}}
    
  My favorite hero is: {{myHero.name}}
    
  Heroes:
    
  
      
  ngFor="let hero of heroes">
        {{ hero.name }}
      
    
  `

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

  
  

  Conditional display with NgIf

  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:

  src/app/app.component.ts (message)

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

  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.

  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.

  Angular isn't showing and hiding the message. 

  划重点! Vue中指令有v-show和v-if
  

  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.

  
  

  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.

  
  

  Summary

  Now you know how to use:

  • Interpolation with double curly braces to display a component property.
  • ngFor to display an array of items.
  • A TypeScript class to shape the model data for your component and display properties of that model.
  • ngIf to conditionally display a chunk of HTML based on a boolean expression.

  Here's the final code:

  Model 类 girl.ts

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

  App Module ts文件

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

  App Component ts文件

  import { Component } from '@angular/core';
  import { Girl } from './girl'
  @Component({
    selector: 'app-root',
    template: `
    	
  {{title}}
    	
  My favorite girl is: {{firstGirl.name}}
        
  
            
  
                {{ girl.name }}
            
        
        
        
  
            There are many girls!
        
    `,
    styleUrls: ['./app.component.css']
  })
  export class AppComponent {
    // title: string
    // girlName: string
  
      title = '世界萌王'
      girls = [
          new Girl(1,'面码'),
          new Girl(2,'逢坂大河'),
          new Girl(3,'艾拉'),
          new Girl(4,'古手梨花')
      ]
      // 必须使用this
      firstGirl = this.girls[0]
  
    constructor(){
        // this.title = '未闻花名';
        // this.girlName = '面码';
    }
  }

  效果如下:

  
  

  
  

  
  

  
  

  
  

  

  Template Syntax

  The Angular application manages what the user sees and can do, 

  achieving this through the interaction of a component class instance (the component) and its user-facing template.

  
  

  You may be familiar with the component/template duality 

  from your experience with model-view-controller (MVC) 

  or model-view-viewmodel (MVVM). 

  
  

  In Angular, the component plays the part of the controller/viewmodel ( ViewModel 驱动视图的数据 ), 

  and the template represents the view.

  
  

  This page is a comprehensive technical reference to the Angular template language. 

  It explains basic principles of the template language 

  and describes most of the syntax that you'll encounter elsewhere in the documentation.

  
  

  Many code snippets illustrate the points and concepts, 

  all of them available in the Template Syntax Live Code / download example.

  
  

  HTML in templates

  HTML is the language of the Angular template. 

  Almost all HTML syntax is valid template syntax. 

  
  

  The Syntax';

  Fortunately, Angular data binding is on alert for dangerous HTML. 

  It sanitizes the values before displaying them. 

  It will not allow HTML with script tags to leak into the browser, 

  neither with interpolation nor property binding.

  
  

  src/app/app.component.html

  
  "{{evilTitle}}" is the interpolated evil title.
  " [innerHTML]="evilTitle">" is the property bound evil title.

  
  

  Interpolation handles the script tags differently than property binding but 

  both approaches render the content harmlessly.

  
  

  效果如下:

  

  ---

  Attribute, class, and style bindings

  The template syntax provides specialized one-way bindings for 

  scenarios ( 场景 ) less well suited to property binding.

  
  

  Attribute binding

  You can set the value of an attribute directly with an attribute binding.

  This is the only exception to the rule that a binding sets a target property. 

  This is the only binding that creates and sets an attribute.

  This guide stresses repeatedly that setting an element property with a property binding 

  is always preferred to ( 优于 ) setting the attribute with a string. 

  Why does Angular offer attribute binding?

  
  

  You must use attribute binding when there is no element property to bind.

  Consider the ARIA, SVG, and table span attributes. 

  ARIA: Accessible Rich Internet Applications
  

  They are pure attributes. 

  They do not correspond to element properties, 

  and they do not set element properties. 

  There are no property targets to bind to.

  
  

  This fact becomes painfully obvious when you write something like this.

   colspan="{{1 + 1}}">Three-Four

  
  

  And you get this error:

  Template parse errors:
  Can't bind to 'colspan' since it isn't a known native property

  
  

  
  

  As the message says, the  element does not have a colspan property. 

  It has the "colspan" attribute, 

  but interpolation and property binding can set only properties, not attributes.

  
  

  You need attribute bindings to create and bind to such attributes.

  注意: property binding 与 attribute binding的区别

  Attribute binding syntax resembles property binding. 

  Instead of an element property between brackets, 

  1. start with the prefix attr, 

  2. followed by a dot (.) 

  3. and the name of the attribute.

  You then set the attribute value, using an expression that resolves to a string.

  Bind [attr.colspan] to a calculated value:

  src/app/app.component.html

  content_copy border=1>
    
     [attr.colspan]="1 + 1">One-Two
  
    
  
    FiveSix
  

  Here's how the table renders:

  
  

  
  

  
  

  
  

  One of the primary use cases for attribute binding 

  is to set ARIA attributes, as in this example:

  
  

  Accessible Rich Internet Applications (ARIA) 

  规定了能够让 Web 内容和 Web 应用

  （特别是那些由 Ajax 和 JavaScript 开发的）

  对于残障人士更易使用的各种机制。

  
  

  src/app/app.component.html

  
   [attr.aria-label]="actionName">{{actionName}} with Aria

  ---

  
  

  Class binding

  You can add and remove CSS class names 

  from an element's class attribute with a class binding.

  
  

  Class binding syntax resembles property binding. 

  Instead of an element property between brackets, 

  1. start with the prefix class, 

  2. optionally followed by a dot (.) 

  3. and the name of a CSS class: [class.class-name].

  2和3为可选的

  
  

  The following examples show 

  how to add and remove the application's "special" class with class bindings. 

  
  

  Here's how to set the attribute without binding:

  src/app/app.component.html

  
   class="bad curly special">Bad curly special

You can replace that with a binding to a string of the desired class names; 

this is an all-or-nothing, replacement binding.

src/app/app.component.html

 class="bad curly special"
     [class]="badCurly">Bad curly

Finally, you can bind to a specific class name. 

Angular adds the class when the template expression evaluates to truthy. 

It removes the class when the expression is falsy.

src/app/app.component.html

 [class.special]="isSpecial">The class binding is special

class="special" [class.special]="!isSpecial">This one is not so special

While 尽管 this is a fine way to toggle a single class name, 

但是, the NgClass directive is usually preferred 

when managing multiple class names at the same time.

---

Style binding

You can set inline styles with a style binding.

Style binding syntax resembles property binding.  (resemble类似)

Instead of an element property between brackets, 

start with the prefix style , followed by a dot (.) 

and the name of a CSS style property: 

例如:  [style.style-property]

src/app/app.component.html

 [style.color]="isSpecial ? 'red': 'green'">Red
 [style.background-color]="canSave ? 'cyan': 'grey'" >Save

Some style binding styles have a unit extension. (带单位)

The following example conditionally sets the font size in “em” and “%” units .

例如:  [style.font-size.px]="isTitle? 24: 16"

src/app/app.component.html

 [style.font-size.em]="isSpecial ? 3 : 1" >Big
 [style.font-size.%]="!isSpecial ? 150 : 50" >Small

While(尽管) this is a fine way to set a single style, 

(但是) the NgStyle directive is generally preferred 

when setting several inline styles at the same time.

Note that :

a style property name can be written in either dash-case, as shown above, 

or camelCase, such as fontSize.

---

Event binding ( (event) 或者 (click) )

The bindings directives you've met so far 

flow data in one direction: from a component to an element.

Users don't just stare at the screen. 

They enter text into input boxes. 

They pick items from lists. 

They click buttons. 

Such user actions may result in : 

a flow of data in the opposite direction: from an element to a component.

The only way to know about a user action is to listen for certain events 

such as keystrokes, mouse movements, clicks, and touches. 

You declare your interest in user actions through Angular event binding.

Event binding syntax consists of a target event name within parentheses (小括号) on the left of an equal sign(等号), 

and a quoted template statement on the right. 

The following event binding listens for the button's click events, 

calling the component's onSave() method whenever a click occurs:

src/app/app.component.html

 (click)="onSave()">Save

Target event

A name between parentheses — for example, (click) — 

identifies the target event. 

In the following example, the target is the button's click event.

src/app/app.component.html

 (click)="onSave()">Save

Some people prefer the on- prefix alternative, known as the canonical form (规范形式):

src/app/app.component.html

 on-click="onSave()">On Save

Element events may be the more common targets, 

but Angular looks first to see if the name matches an event property of a known directive, 

as it does in the following example:

src/app/app.component.html

 (myClick)="clickMessage=$event" clickable>click with myClick