It Takes Many Heroes
Our story needs more heroes. We’ll expand our Tour of Heroes app to display a list of heroes, allow the user to select a hero, and display the hero’s details.
Run the live example for this part.
Let’s take stock of what we’ll need to display a list of heroes. First, we need a list of heroes. We want to display those heroes in the view’s template, so we’ll need a way to do that.
Where We Left Off
Before we continue with Part 2 of the Tour of Heroes, let’s verify we have the following structure after Part 1. If not, we’ll need to go back to Part 1 and figure out what we missed.
Keep the app transpiling and running
We want to start the TypeScript compiler, have it watch for changes, and start our server. We'll do this by typing
npm start
This will keep the application running while we continue to build the Tour of Heroes.
Displaying Our Heroes
Creating heroes
Let’s create an array of ten heroes.
app.component.ts (hero array)
const HEROES: Hero[] = [ { id: 11, name: 'Mr. Nice' }, { id: 12, name: 'Narco' }, { id: 13, name: 'Bombasto' }, { id: 14, name: 'Celeritas' }, { id: 15, name: 'Magneta' }, { id: 16, name: 'RubberMan' }, { id: 17, name: 'Dynama' }, { id: 18, name: 'Dr IQ' }, { id: 19, name: 'Magma' }, { id: 20, name: 'Tornado' } ];
The HEROES
array is of type Hero
, the class defined in part one, to create an array of heroes. We aspire to fetch this list of heroes from a web service, but let’s take small steps first and display mock heroes.
Exposing heroes
Let’s create a public property in AppComponent
that exposes the heroes for binding.
app.component.ts (hero array property)
heroes = HEROES;
We did not have to define the heroes
type. TypeScript can infer it from the HEROES
array.
We could have defined the heroes list here in this component class. But we know that ultimately we’ll get the heroes from a data service. Because we know where we are heading, it makes sense to separate the hero data from the class implementation from the start.
Displaying heroes in a template
Our component has heroes
. Let’s create an unordered list in our template to display them. We’ll insert the following chunk of HTML below the title and above the hero details.
app.component.ts (heroes template)
<h2>My Heroes</h2> <ul class="heroes"> <li> <!-- each hero goes here --> </li> </ul>
Now we have a template that we can fill with our heroes.
Listing heroes with ngFor
We want to bind the array of heroes
in our component to our template, iterate over them, and display them individually. We’ll need some help from Angular to do this. Let’s do this step by step.
First modify the <li>
tag by adding the built-in directive *ngFor
.
app.component.ts (ngFor)
<li *ngFor="let hero of heroes">
The leading asterisk (*
) in front of ngFor
is a critical part of this syntax.
The (*
) prefix to ngFor
indicates that the <li>
element and its children constitute a master template.
The ngFor
directive iterates over the heroes
array returned by the AppComponent.heroes
property and stamps out instances of this template.
The quoted text assigned to ngFor
means “take each hero in the heroes
array, store it in the local hero
variable, and make it available to the corresponding template instance”.
The let
keyword before "hero" identifies hero
as a template input variable. We can reference this variable within the template to access a hero’s properties.
Learn more about ngFor
and template input variables in the Displaying Data and Template Syntax chapters.
Now we insert some content between the <li>
tags that uses the hero
template variable to display the hero’s properties.
app.component.ts (ngFor template)
<li *ngFor="let hero of heroes"> <span class="badge">{{hero.id}}</span> {{hero.name}} </li>
When the browser refreshes, we see a list of heroes!
Styling our heroes
Our list of heroes looks pretty bland. We want to make it visually obvious to a user which hero we are hovering over and which hero is selected.
Let’s add some styles to our component by setting the styles
property on the @Component
decorator to the following CSS classes:
app.component.ts (styles)
styles: [` .selected { background-color: #CFD8DC !important; color: white; } .heroes { margin: 0 0 2em 0; list-style-type: none; padding: 0; width: 15em; } .heroes li { cursor: pointer; position: relative; left: 0; background-color: #EEE; margin: .5em; padding: .3em 0; height: 1.6em; border-radius: 4px; } .heroes li.selected:hover { background-color: #BBD8DC !important; color: white; } .heroes li:hover { color: #607D8B; background-color: #DDD; left: .1em; } .heroes .text { position: relative; top: -3px; } .heroes .badge { display: inline-block; font-size: small; color: white; padding: 0.8em 0.7em 0 0.7em; background-color: #607D8B; line-height: 1em; position: relative; left: -1px; top: -4px; height: 1.8em; margin-right: .8em; border-radius: 4px 0 0 4px; } `]
Notice that we again use the back-tick notation for multi-line strings.
That's a lot of styles! We can put them inline as shown here, or we can move them out to their own file which will make it easier to code our component. We'll do this in a later chapter. For now let's keep rolling.
When we assign styles to a component they are scoped to that specific component. Our styles will only apply to our AppComponent
and won't "leak" to the outer HTML.
Our template for displaying the heroes should now look like this:
app.component.ts (styled heroes)
<h2>My Heroes</h2> <ul class="heroes"> <li *ngFor="let hero of heroes"> <span class="badge">{{hero.id}}</span> {{hero.name}} </li> </ul>
Selecting a Hero
We have a list of heroes and we have a single hero displayed in our app. The list and the single hero are not connected in any way. We want the user to select a hero from our list, and have the selected hero appear in the details view. This UI pattern is widely known as "master-detail". In our case, the master is the heroes list and the detail is the selected hero.
Let’s connect the master to the detail through a selectedHero
component property bound to a click event.
Click event
We modify the <li>
by inserting an Angular event binding to its click event.
app.component.ts (template excerpt)
<li *ngFor="let hero of heroes" (click)="onSelect(hero)"> <span class="badge">{{hero.id}}</span> {{hero.name}} </li>
Focus on the event binding
(click)="onSelect(hero)"
The parenthesis identify the <li>
element’s click
event as the target. The expression to the right of the equal sign calls the AppComponent
method, onSelect()
, passing the template input variable hero
as an argument. That’s the same hero
variable we defined previously in the ngFor
.
Learn more about Event Binding in the User Input and Templating Syntax chapters.
Add the click handler
Our event binding refers to an onSelect
method that doesn’t exist yet. We’ll add that method to our component now.
What should that method do? It should set the component’s selected hero to the hero that the user clicked.
Our component doesn’t have a “selected hero” yet either. We’ll start there.
Expose the selected hero
We no longer need the static hero
property of the AppComponent
. Replace it with this simple selectedHero
property:
app.component.ts (selectedHero)
selectedHero: Hero;
We’ve decided that none of the heroes should be selected before the user picks a hero so we won’t initialize the selectedHero
as we were doing with hero
.
Now add an onSelect
method that sets the selectedHero
property to the hero
the user clicked.
app.component.ts (onSelect)
onSelect(hero: Hero): void { this.selectedHero = hero; }
We will be showing the selected hero's details in our template. At the moment, it is still referring to the old hero
property. Let’s fix the template to bind to the new selectedHero
property.
app.component.ts (template excerpt)
<h2>{{selectedHero.name}} details!</h2> <div><label>id: </label>{{selectedHero.id}}</div> <div> <label>name: </label> <input [(ngModel)]="selectedHero.name" placeholder="name"/> </div>
Hide the empty detail with ngIf
When our app loads we see a list of heroes, but a hero is not selected. The selectedHero
is undefined
. That’s why we'll see the following error in the browser’s console:
EXCEPTION: TypeError: Cannot read property 'name' of undefined in [null]
Remember that we are displaying selectedHero.name
in the template. This name property does not exist because selectedHero
itself is undefined.
We'll address this problem by keeping the hero detail out of the DOM until there is a selected hero.
We wrap the HTML hero detail content of our template with a <div>
. Then we add the ngIf
built-in directive and set it to the selectedHero
property of our component.
app.component.ts (ngIf)
<div *ngIf="selectedHero"> <h2>{{selectedHero.name}} details!</h2> <div><label>id: </label>{{selectedHero.id}}</div> <div> <label>name: </label> <input [(ngModel)]="selectedHero.name" placeholder="name"/> </div> </div>
Remember that the leading asterisk (*
) in front of ngIf
is a critical part of this syntax.
When there is no selectedHero
, the ngIf
directive removes the hero detail HTML from the DOM. There will be no hero detail elements and no bindings to worry about.
When the user picks a hero, selectedHero
becomes "truthy" and ngIf
puts the hero detail content into the DOM and evaluates the nested bindings.
ngIf
and ngFor
are called “structural directives” because they can change the structure of portions of the DOM. In other words, they give structure to the way Angular displays content in the DOM.
Learn more about ngIf
, ngFor
and other structural directives in the Structural Directives and Template Syntax chapters.
The browser refreshes and we see the list of heroes but not the selected hero detail. The ngIf
keeps it out of the DOM as long as the selectedHero
is undefined. When we click on a hero in the list, the selected hero displays in the hero details. Everything is working as we expect.
Styling the selection
We see the selected hero in the details area below but we can’t quickly locate that hero in the list above. We can fix that by applying the selected
CSS class to the appropriate <li>
in the master list. For example, when we select Magneta from the heroes list, we can make it pop out visually by giving it a subtle background color as shown here.
We’ll add a property binding on class
for the selected
class to the template. We'll set this to an expression that compares the current selectedHero
to the hero
.
The key is the name of the CSS class (selected
). The value is true
if the two heroes match and false
otherwise. We’re saying “apply the selected
class if the heroes match, remove it if they don’t”.
app.component.ts (setting the CSS class)
[class.selected]="hero === selectedHero"
Notice in the template that the class.selected
is surrounded in square brackets ([]
). This is the syntax for a property binding, a binding in which data flows one way from the data source (the expression hero === selectedHero
) to a property of class
.
app.component.ts (styling each hero)
<li *ngFor="let hero of heroes" [class.selected]="hero === selectedHero" (click)="onSelect(hero)"> <span class="badge">{{hero.id}}</span> {{hero.name}} </li>
Learn more about property bindings in the Template Syntax chapter.
The browser reloads our app. We select the hero Magneta and the selection is clearly identified by the background color.
We select a different hero and the tell-tale color switches to that hero.
Here's the complete app.component.ts
as it stands now:
app.component.ts
import { Component } from '@angular/core'; export class Hero { id: number; name: string; } const HEROES: Hero[] = [ { id: 11, name: 'Mr. Nice' }, { id: 12, name: 'Narco' }, { id: 13, name: 'Bombasto' }, { id: 14, name: 'Celeritas' }, { id: 15, name: 'Magneta' }, { id: 16, name: 'RubberMan' }, { id: 17, name: 'Dynama' }, { id: 18, name: 'Dr IQ' }, { id: 19, name: 'Magma' }, { id: 20, name: '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> <div *ngIf="selectedHero"> <h2>{{selectedHero.name}} details!</h2> <div><label>id: </label>{{selectedHero.id}}</div> <div> <label>name: </label> <input [(ngModel)]="selectedHero.name" placeholder="name"/> </div> </div> `, styles: [` .selected { background-color: #CFD8DC !important; color: white; } .heroes { margin: 0 0 2em 0; list-style-type: none; padding: 0; width: 15em; } .heroes li { cursor: pointer; position: relative; left: 0; background-color: #EEE; margin: .5em; padding: .3em 0; height: 1.6em; border-radius: 4px; } .heroes li.selected:hover { background-color: #BBD8DC !important; color: white; } .heroes li:hover { color: #607D8B; background-color: #DDD; left: .1em; } .heroes .text { position: relative; top: -3px; } .heroes .badge { display: inline-block; font-size: small; color: white; padding: 0.8em 0.7em 0 0.7em; background-color: #607D8B; line-height: 1em; position: relative; left: -1px; top: -4px; height: 1.8em; margin-right: .8em; border-radius: 4px 0 0 4px; } `] }) export class AppComponent { title = 'Tour of Heroes'; heroes = HEROES; selectedHero: Hero; onSelect(hero: Hero): void { this.selectedHero = hero; } }
The Road We’ve Travelled
Here’s what we achieved in this chapter:
- Our Tour of Heroes now displays a list of selectable heroes
- We added the ability to select a hero and show the hero’s details
- We learned how to use the built-in directives
ngIf
andngFor
in a component’s template
Run the live example for this part.
The Road Ahead
Our Tour of Heroes has grown, but it’s far from complete. We can't put the entire app into a single component. We need to break it up into sub-components and teach them to work together as we learn in the next chapter.
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