Multiple Components

2.2

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Our app is growing. Use cases are flowing in for reusing components, passing data to components, and creating more reusable assets. Let's separate the heroes list from the hero details and make the details component reusable.

Run the for this part.

Where we left off

Before we continue with our Tour of Heroes, let’s verify we have the following structure. If not, we’ll need to go back and follow the previous pages.

angular_tour_of_heroes
lib
app_component.dart
web
index.html
main.dart
styles.css
pubspec.yaml

Keep the app compiling and running

We want to start the Dart compiler, have it watch for changes, and start our server. We'll do this by typing

pub serve

This will keep the application running while we continue to build the Tour of Heroes.

Making a Hero Detail Component

Our heroes list and our hero details are in the same component in the same file. They're small now but each could grow. We are sure to receive new requirements for one and not the other. Yet every change puts both components at risk and doubles the testing burden without benefit. If we had to reuse the hero details elsewhere in our app, the heroes list would tag along for the ride.

Our current component violates the Single Responsibility Principle. It's only a tutorial but we can still do things right — especially if doing them right is easy and we learn how to build Angular apps in the process.

Let’s break the hero details out into its own component.

Separating the Hero Detail Component

Add a new file named hero_detail_component.dart to the lib folder and create HeroDetailComponent as follows.

lib/hero_detail_component.dart (initial version)

import 'package:angular2/core.dart'; @Component( selector: 'my-hero-detail', ) class HeroDetailComponent { }

Naming conventions

We like to identify at a glance which classes are components and which files contain components.

Notice that we have an AppComponent in a file named app_component.dart and our new HeroDetailComponent is in a file named hero_detail_component.dart.

All of our component names end in "Component". All of our component file names end in "_component".

We spell our filenames in lower underscore case (AKA snake_case) so we don't worry about case sensitivity on the server or in source control.

We begin by importing the Angular core.dart file, so that we can use common types like @Component when we create our component.

We create metadata with the @Component annotation where we specify the selector name that identifies this component's element.

When we finish here, we'll import it into AppComponent and create a corresponding <my-hero-detail> element.

Hero Detail Template

At the moment, the Heroes and Hero Detail views are combined in one template in AppComponent. Let’s cut the Hero Detail content from AppComponent and paste it into the new template property of HeroDetailComponent.

We previously bound to the selectedHero.name property of the AppComponent. Our HeroDetailComponent will have a hero property, not a selectedHero property. So we replace selectedHero with hero everywhere in our new template. That's our only change. The result looks like this:

lib/hero_detail_component.dart (template)

template: ''' <div *ngIf="hero != null"> <h2>{{hero.name}} details!</h2> <div><label>id: </label>{{hero.id}}</div> <div> <label>name: </label> <input [(ngModel)]="hero.name" placeholder="name"> </div> </div>'''

Now our hero detail layout exists only in the HeroDetailComponent.

Add the hero property

Let’s add that hero property we were talking about to the component class.

Hero hero;

Uh oh. We declared the hero property as type Hero but our Hero class is over in the app_component.dart file. We have two components, each in their own file, that need to reference the Hero class.

We solve the problem by relocating the Hero class from app_component.dart to its own hero.dart file.

lib/hero.dart

class Hero { final int id; String name; Hero(this.id, this.name); }

Add the following import statement near the top of both app_component.dart and hero_detail_component.dart.

import 'hero.dart';

The hero property is an input

The HeroDetailComponent must be told what hero to display. Who will tell it? The parent AppComponent!

The AppComponent knows which hero to show: the hero that the user selected from the list. The user's selection is in its selectedHero property.

We will soon update the AppComponent template so that it binds its selectedHero property to the hero property of our HeroDetailComponent. The binding might look like this:

<my-hero-detail [hero]="selectedHero"></my-hero-detail>

Notice that the hero property is the target of a property binding — it's in square brackets to the left of the (=).

Angular insists that we declare a target property to be an input property. If we don't, Angular rejects the binding and throws an error.

We explain input properties in more detail here where we also explain why target properties require this special treatment and source properties do not.

There are a couple of ways we can declare that hero is an input. We'll do it the way we prefer, by annotating the hero property with @Input().

@Input() Hero hero;

Learn more about @Input() in the Attribute Directives page.

Refresh the AppComponent

We return to the AppComponent and teach it to use the HeroDetailComponent.

We begin by importing the HeroDetailComponent so we can refer to it.

import 'hero_detail_component.dart';

Find the location in the template where we removed the Hero Detail content and add an element tag that represents the HeroDetailComponent.

<my-hero-detail></my-hero-detail>

my-hero-detail is the name we set as the selector in the HeroDetailComponent metadata.

The two components won't coordinate until we bind the selectedHero property of the AppComponent to the HeroDetailComponent element's hero property like this:

<my-hero-detail [hero]="selectedHero"></my-hero-detail>

The AppComponent’s template should now look like this

app_component.dart (template)

template: ''' <h1>{{title}}</h1> <h2>My Heroes</h2> <ul class="heroes"> <li *ngFor="let hero of heroes" [class.selected]="hero == selectedHero" (click)="onSelect(hero)"> <span class="badge">{{hero.id}}</span> {{hero.name}} </li> </ul> <my-hero-detail [hero]="selectedHero"></my-hero-detail> ''',

Thanks to the binding, the HeroDetailComponent should receive the hero from the AppComponent and display that hero's detail beneath the list. The detail should update every time the user picks a new hero.

It's not happening yet!

We click among the heroes. No details. We look for an error in the console of the browser development tools. No error.

It is as if Angular were ignoring the new tag. That's because it is ignoring the new tag.

The directives list

A browser ignores HTML tags and attributes that it doesn't recognize. So does Angular.

We've imported HeroDetailComponent, we've used it in the template, but we haven't told Angular about it.

We tell Angular about it by listing it in the metadata directives list. Let's add that list property to the bottom of the @Component configuration object, immediately after the template and styles properties.

directives: const [HeroDetailComponent]

It works!

When we view our app in the browser we see the list of heroes. When we select a hero we can see the selected hero’s details.

What's fundamentally new is that we can use this HeroDetailComponent to show hero details anywhere in the app.

We’ve created our first reusable component!

Reviewing the App Structure

Let’s verify that we have the following structure after all of our good refactoring in this page:

angular_tour_of_heroes
lib
app_component.dart
hero.dart
hero_detail_component.dart
web
index.html
main.dart
styles.css
pubspec.yaml

Here are the code files we discussed in this page.

import 'package:angular2/core.dart'; import 'hero.dart'; @Component( selector: 'my-hero-detail', template: ''' <div *ngIf="hero != null"> <h2>{{hero.name}} details!</h2> <div><label>id: </label>{{hero.id}}</div> <div> <label>name: </label> <input [(ngModel)]="hero.name" placeholder="name"> </div> </div>''' ) class HeroDetailComponent { @Input() Hero hero; } import 'package:angular2/core.dart'; import 'hero.dart'; import 'hero_detail_component.dart'; final List<Hero> mockHeroes = [ new Hero(11, 'Mr. Nice'), new Hero(12, 'Narco'), new Hero(13, 'Bombasto'), new Hero(14, 'Celeritas'), new Hero(15, 'Magneta'), new Hero(16, 'RubberMan'), new Hero(17, 'Dynama'), new Hero(18, 'Dr IQ'), new Hero(19, 'Magma'), new Hero(20, 'Tornado') ]; @Component( selector: 'my-app', template: ''' <h1>{{title}}</h1> <h2>My Heroes</h2> <ul class="heroes"> <li *ngFor="let hero of heroes" [class.selected]="hero == selectedHero" (click)="onSelect(hero)"> <span class="badge">{{hero.id}}</span> {{hero.name}} </li> </ul> <my-hero-detail [hero]="selectedHero"></my-hero-detail> ''', styles: const [ ''' .selected { background-color: #CFD8DC !important; color: white; } .heroes { margin: 0 0 2em 0; list-style-type: none; padding: 0; width: 10em; } .heroes li { cursor: pointer; position: relative; left: 0; background-color: #EEE; margin: .5em; padding: .3em 0em; height: 1.6em; border-radius: 4px; } .heroes li.selected:hover { color: white; } .heroes li:hover { color: #607D8B; background-color: #EEE; left: .1em; } .heroes .text { position: relative; top: -3px; } .heroes .badge { display: inline-block; font-size: small; color: white; padding: 0.8em 0.7em 0em 0.7em; background-color: #607D8B; line-height: 1em; position: relative; left: -1px; top: -4px; height: 1.8em; margin-right: .8em; border-radius: 4px 0px 0px 4px; } ''' ], directives: const [HeroDetailComponent] ) class AppComponent { final String title = 'Tour of Heroes'; final List<Hero> heroes = mockHeroes; Hero selectedHero; void onSelect(Hero hero) { selectedHero = hero; } } class Hero { final int id; String name; Hero(this.id, this.name); }

Recap

Let’s take stock of what we’ve built.

  • We created a reusable component
  • We learned how to make a component accept input
  • We learned to bind a parent component to a child component.
  • We learned to declare the application directives we need in a directives list.

Run the for this part.

What's next

Our Tour of Heroes has become more reusable with shared components.

We're still getting our (mock) data within the AppComponent. That's not sustainable. We should refactor data access to a separate service and share it among the components that need data.

We’ll learn to create services in the next tutorial page.