angular-cn/aio/content/tutorial/toh-pt1.md

The hero editor

The application now has a basic title. Next you will create a new component to display hero information and place that component in the application shell.

For the sample application that this page describes, see the .

Create the heroes component

Using the Angular CLI, generate a new component named heroes.

ng generate component heroes

The CLI creates a new folder, src/app/heroes/, and generates the three files of the HeroesComponent along with a test file.

The HeroesComponent class file is as follows:

You always import the Component symbol from the Angular core library and annotate the component class with @Component.

@Component is a decorator function that specifies the Angular metadata for the component.

The CLI generated three metadata properties:

1. selector— the component's CSS element selector
2. templateUrl— the location of the component's template file.
3. styleUrls— the location of the component's private CSS styles.

{@a selector}

The CSS element selector, 'app-heroes', matches the name of the HTML element that identifies this component within a parent component's template.

The ngOnInit() is a lifecycle hook. Angular calls ngOnInit() shortly after creating a component. It's a good place to put initialization logic.

Always export the component class so you can import it elsewhere ... like in the AppModule.

Add a hero property

Add a hero property to the HeroesComponent for a hero named "Windstorm."

Show the hero

Open the heroes.component.html template file. Delete the default text generated by the Angular CLI and replace it with a data binding to the new hero property.

Show the HeroesComponent view

To display the HeroesComponent, you must add it to the template of the shell AppComponent.

Remember that app-heroes is the element selector for the HeroesComponent. So add an <app-heroes> element to the AppComponent template file, just below the title.

Assuming that the CLI ng serve command is still running, the browser should refresh and display both the application title and the hero name.

Create a Hero interface

A real hero is more than a name.

Create a Hero interface in its own file in the src/app folder. Give it id and name properties.

Return to the HeroesComponent class and import the Hero interface.

Refactor the component's hero property to be of type Hero. Initialize it with an id of 1 and the name Windstorm.

The revised HeroesComponent class file should look like this:

The page no longer displays properly because you changed the hero from a string to an object.

Show the hero object

Update the binding in the template to announce the hero's name and show both id and name in a details layout like this:

The browser refreshes and displays the hero's information.

Format with the UppercasePipe

Modify the hero.name binding like this.

The browser refreshes and now the hero's name is displayed in capital letters.

The word uppercase in the interpolation binding, right after the pipe operator ( | ), activates the built-in UppercasePipe.

Pipes are a good way to format strings, currency amounts, dates and other display data. Angular ships with several built-in pipes and you can create your own.

Edit the hero

Users should be able to edit the hero name in an <input> textbox.

The textbox should both display the hero's name property and update that property as the user types. That means data flows from the component class out to the screen and from the screen back to the class.

To automate that data flow, setup a two-way data binding between the <input> form element and the hero.name property.

Two-way binding

Refactor the details area in the HeroesComponent template so it looks like this:

[(ngModel)] is Angular's two-way data binding syntax.

Here it binds the hero.name property to the HTML textbox so that data can flow in both directions: from the hero.name property to the textbox, and from the textbox back to the hero.name.

The missing FormsModule

Notice that the application stopped working when you added [(ngModel)].

To see the error, open the browser development tools and look in the console for a message like

Template parse errors: Can't bind to 'ngModel' since it isn't a known property of 'input'.

Although ngModel is a valid Angular directive, it isn't available by default.

It belongs to the optional FormsModule and you must opt-in to using it.

AppModule

Angular needs to know how the pieces of your application fit together and what other files and libraries the application requires. This information is called metadata.

Some of the metadata is in the @Component decorators that you added to your component classes. Other critical metadata is in @NgModule decorators.

The most important @NgModule decorator annotates the top-level AppModule class.

The Angular CLI generated an AppModule class in src/app/app.module.ts when it created the project. This is where you opt-in to the FormsModule.

Import FormsModule

Open AppModule (app.module.ts) and import the FormsModule symbol from the @angular/forms library.

Then add FormsModule to the @NgModule metadata's imports array, which contains a list of external modules that the application needs.

When the browser refreshes, the application should work again. You can edit the hero's name and see the changes reflected immediately in the <h2> above the textbox.

Declare HeroesComponent

Every component must be declared in exactly one NgModule.

You didn't declare the HeroesComponent. So why did the application work?

It worked because the Angular CLI declared HeroesComponent in the AppModule when it generated that component.

Open src/app/app.module.ts and find HeroesComponent imported near the top.

The HeroesComponent is declared in the @NgModule.declarations array.

Note that AppModule declares both application components, AppComponent and HeroesComponent.

Final code review

Here are the code files discussed on this page.

Summary

• You used the CLI to create a second HeroesComponent.
• You displayed the HeroesComponent by adding it to the AppComponent shell.
• You applied the UppercasePipe to format the name.
• You used two-way data binding with the ngModel directive.
• You learned about the AppModule.
• You imported the FormsModule in the AppModule so that Angular would recognize and apply the ngModel directive.
• You learned the importance of declaring components in the AppModule and appreciated that the CLI declared it for you.