angular-docs-cn/aio/content/guide/displaying-data.md

Displaying data in views

Angular components form the data structure of your application. The HTML template associated with a component provides the means to display that data in the context of a web page. Together, a component's class and template form a view of your application data.

The process of combining data values with their representation on the page is called data binding. You display your data to a user (and collect data from the user) by binding controls in the HTML template to the data properties of the component class.

In addition, you can add logic to the template by including directives, which tell Angular how to modify the page as it is rendered.

Angular defines a template language that expands HTML notation with syntax that allows you to define various kinds of data binding and logical directives. When the page is rendered, Angular interprets the template syntax to update the HTML according to your logic and current data state. Before you read the complete template syntax guide, the exercises on this page give you a quick demonstration of how template syntax works.

In this demo, 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. The final UI looks like this:

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

{@a interpolation}

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}}.

Use the CLI command ng new displaying-data to create a workspace and app named displaying-data.

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.

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

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

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

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 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 <app-root>. That element is a placeholder in the body of your index.html file:

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.

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.

Choosing the template source

The @Component metadata tells Angular where to find the component's template. You can store your component's template in one of two places.

• You can define the template inline using the template property of the @Component decorator. An inline template is useful for a small demo or test.
• Alternatively, you can define the template in a separate HTML file and link to that file in the templateUrl property of the @Component decorator. This configuration is typical for anything more complex than a small test or demo, and is the default when you generate a new component.

In either style, the template data bindings have the same access to the component's properties. Here the app uses inline HTML because the template is small and the demo is simpler without the additional HTML file.

By default, the Angular CLI command ng generate component generates components with a template file. You can override that by adding the "-t" (short for inlineTemplate=true) option:

ng generate component hero -t

Initialization

The following example uses variable assignment to initialize the components.

You could instead declare and initialize the properties using a constructor. This app uses more terse "variable assignment" style simply for brevity.

{@a ngFor}

Add logic to loop through data

The *ngFor directive (predefined by Angular) lets you loop through data. The following example uses the directive to show all of the values in an array property.

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.

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

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":

Don't forget the leading asterisk (*) in *ngFor. It is an essential part of the syntax. Read more about ngFor and * in the ngFor section of the Built-in directives 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 section of the Built-in directives page.

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.

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

Now the heroes 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 Hero class:

ng generate class hero

This command creates the following code.

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:

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:

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.

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

{@a ngIf}

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:

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 Built-in directives 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 template expression operators.

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.

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 heroes 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: