257 lines
9.3 KiB
Markdown
257 lines
9.3 KiB
Markdown
# The Hero Editor
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The application now has a basic title.
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Next you will create a new component to display hero information
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and place that component in the application shell.
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## Create the heroes component
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Using the Angular CLI, generate a new component named `heroes`.
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<code-example language="sh" class="code-shell">
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ng generate component heroes
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</code-example>
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The CLI creates a new folder, `src/app/heroes/`, and generates
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the three files of the `HeroesComponent`.
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The `HeroesComponent` class file is as follows:
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<code-example
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path="toh-pt1/src/app/heroes/heroes.component.ts" region="v1"
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title="app/heroes/heroes.component.ts (initial version)" linenums="false">
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</code-example>
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You always import the `Component` symbol from the Angular core library
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and annotate the component class with `@Component`.
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`@Component` is a decorator function that specifies the Angular metadata for the component.
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The CLI generated three metadata properties:
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1. `selector`— the component's CSS element selector
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1. `templateUrl`— the location of the component's template file.
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1. `styleUrls`— the location of the component's private CSS styles.
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{@a selector}
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The [CSS element selector](https://developer.mozilla.org/en-US/docs/Web/CSS/Type_selectors),
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`'app-heroes'`, matches the name of the HTML element that identifies this component within a parent component's template.
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The `ngOnInit` is a [lifecycle hook](guide/lifecycle-hooks#oninit).
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Angular calls `ngOnInit` shortly after creating a component.
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It's a good place to put initialization logic.
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Always `export` the component class so you can `import` it elsewhere ... like in the `AppModule`.
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### Add a _hero_ property
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Add a `hero` property to the `HeroesComponent` for a hero named "Windstorm."
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<code-example path="toh-pt1/src/app/heroes/heroes.component.ts" region="add-hero" title="heroes.component.ts (hero property)" linenums="false">
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</code-example>
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### Show the hero
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Open the `heroes.component.html` template file.
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Delete the default text generated by the Angular CLI and
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replace it with a data binding to the new `hero` property.
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<code-example path="toh-pt1/src/app/heroes/heroes.component.1.html" title="heroes.component.html" region="show-hero-1" linenums="false">
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</code-example>
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## Show the _HeroesComponent_ view
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To display the `HeroesComponent`, you must add it to the template of the shell `AppComponent`.
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Remember that `app-heroes` is the [element selector](#selector) for the `HeroesComponent`.
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So add an `<app-heroes>` element to the `AppComponent` template file, just below the title.
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<code-example path="toh-pt1/src/app/app.component.html" title="src/app/app.component.html" linenums="false">
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</code-example>
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Assuming that the CLI `ng serve` command is still running,
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the browser should refresh and display both the application title and the hero name.
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## Create a Hero class
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A real hero is more than a name.
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Create a `Hero` class in its own file in the `src/app` folder.
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Give it `id` and `name` properties.
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<code-example path="toh-pt1/src/app/hero.ts" title="src/app/hero.ts" linenums="false">
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</code-example>
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Return to the `HeroesComponent` class and import the `Hero` class.
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Refactor the component's `hero` property to be of type `Hero`.
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Initialize it with an `id` of `1` and the name `Windstorm`.
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The revised `HeroesComponent` class file should look like this:
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<code-example path="toh-pt1/src/app/heroes/heroes.component.ts" linenums="false"
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title= "src/app/heroes/heroes.component.ts">
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</code-example>
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The page no longer displays properly because you changed the hero from a string to an object.
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## Show the hero object
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Update the binding in the template to announce the hero's name
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and show both `id` and `name` in a details layout like this:
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<code-example
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path="toh-pt1/src/app/heroes/heroes.component.1.html"
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region="show-hero-2"
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title="heroes.component.html (HeroesComponent's template)" linenums="false">
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</code-example>
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The browser refreshes and displays the hero's information.
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## Format with the _UppercasePipe_
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Modify the `hero.name` binding like this.
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<code-example
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path="toh-pt1/src/app/heroes/heroes.component.html"
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region="pipe">
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</code-example>
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The browser refreshes and now the hero's name is displayed in capital letters.
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The word `uppercase` in the interpolation binding,
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right after the pipe operator ( | ),
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activates the built-in `UppercasePipe`.
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[Pipes](guide/pipes) are a good way to format strings, currency amounts, dates and other display data.
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Angular ships with several built-in pipes and you can create your own.
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## Edit the hero
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Users should be able to edit the hero name in an `<input>` textbox.
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The textbox should both _display_ the hero's `name` property
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and _update_ that property as the user types.
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That means data flow from the component class _out to the screen_ and
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from the screen _back to the class_.
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To automate that data flow, setup a two-way data binding between the `<input>` form element and the `hero.name` property.
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### Two-way binding
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Refactor the details area in the `HeroesComponent` template so it looks like this:
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<code-example path="toh-pt1/src/app/heroes/heroes.component.1.html" region="name-input" title="src/app/heroes/heroes.component.html (HeroesComponent's template)" linenums="false">
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</code-example>
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**[(ngModel)]** is Angular's two-way data binding syntax.
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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`.
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### The missing _FormsModule_
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Notice that the app stopped working when you added `[(ngModel)]`.
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To see the error, open the browser development tools and look in the console
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for a message like
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<code-example language="sh" class="code-shell">
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Template parse errors:
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Can't bind to 'ngModel' since it isn't a known property of 'input'.
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</code-example>
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Although `ngModel` is a valid Angular directive, it isn't available by default.
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It belongs to the optional `FormsModule` and you must _opt-in_ to using it.
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## _AppModule_
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Angular needs to know how the pieces of your application fit together
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and what other files and libraries the app requires.
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This information is called _metadata_
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Some of the metadata is in the `@Component` decorators that you added to your component classes.
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Other critical metadata is in [`@NgModule`](guide/ngmodules) decorators.
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The most important `@NgModule` decorator annotates the top-level **AppModule** class.
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The Angular CLI generated an `AppModule` class in `src/app/app.module.ts` when it created the project.
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This is where you _opt-in_ to the `FormsModule`.
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### Import _FormsModule_
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Open `AppModule` (`app.module.ts`) and import the `FormsModule` symbol from the `@angular/forms` library.
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<code-example path="toh-pt1/src/app/app.module.ts" title="app.module.ts (FormsModule symbol import)"
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region="formsmodule-js-import">
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</code-example>
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Then add `FormsModule` to the `@NgModule` metadata's `imports` array, which contains a list of external modules that the app needs.
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<code-example path="toh-pt1/src/app/app.module.ts" title="app.module.ts ( @NgModule imports)"
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region="ng-imports">
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</code-example>
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When the browser refreshes, the app should work again. You can edit the hero's name and see the changes reflected immediately in the `<h2>` above the textbox.
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### Declare _HeroesComponent_
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Every component must be declared in _exactly one_ [NgModule](guide/ngmodules).
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_You_ didn't declare the `HeroesComponent`.
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So why did the application work?
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It worked because the Angular CLI declared `HeroesComponent` in the `AppModule` when it generated that component.
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Open `src/app/app.module.ts` and find `HeroesComponent` imported near the top.
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<code-example path="toh-pt1/src/app/app.module.ts" region="heroes-import" >
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</code-example>
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The `HeroesComponent` is declared in the `@NgModule.declarations` array.
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<code-example path="toh-pt1/src/app/app.module.ts" region="declarations">
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</code-example>
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Note that `AppModule` declares both application components, `AppComponent` and `HeroesComponent`.
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## Final code review
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Your app should look like this <live-example></live-example>. Here are the code files discussed on this page.
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<code-tabs>
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<code-pane title="src/app/heroes/heroes.component.ts" path="toh-pt1/src/app/heroes/heroes.component.ts">
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</code-pane>
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<code-pane title="src/app/heroes/heroes.component.html" path="toh-pt1/src/app/heroes/heroes.component.html">
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</code-pane>
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<code-pane title="src/app/app.module.ts"
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path="toh-pt1/src/app/app.module.ts">
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</code-pane>
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<code-pane title="src/app/app.component.ts" path="toh-pt1/src/app/app.component.ts">
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</code-pane>
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<code-pane title="src/app/app.component.html" path="toh-pt1/src/app/app.component.html">
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</code-pane>
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<code-pane title="src/app/hero.ts"
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path="toh-pt1/src/app/hero.ts">
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</code-pane>
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</code-tabs>
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## Summary
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* You used the CLI to create a second `HeroesComponent`.
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* You displayed the `HeroesComponent` by adding it to the `AppComponent` shell.
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* You applied the `UppercasePipe` to format the name.
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* You used two-way data binding with the `ngModel` directive.
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* You learned about the `AppModule`.
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* You imported the `FormsModule` in the `AppModule` so that Angular would recognize and apply the `ngModel` directive.
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* You learned the importance of declaring components in the `AppModule`
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and appreciated that the CLI declared it for you.
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