[Angular 2] More on *ngFor, @ContentChildren & QueryList<>

In previous artical, we introduce the how to use *ngFor. The limitation for previous solution to display all the heros is we hard cord all heros in our heroes component. First, in real world app, we cannot hard cord;  Seond, even we don't hard code, do http instead, it is still not good enough. We should leave Heroes component as dump component, just rendering the ui, no logic should be involved. 

So instead of doing this in app.ts:

@Component({ 
    selector: 'app',
    template: `
            <heroes>
            </heroes>
        `
})

We try another way like this:

@Component({ 
    selector: 'app',
    template: `

            <heroes>
                <hero name="Superman" id="1"></hero>
                <hero name="Batman" id="2"></hero>
                <hero name="BatGirl" id="3"></hero>
                <hero name="Robin" id="4"></hero>
                <hero name="Flash" id="5"></hero>
                <hero name="Zhentian" id="6"></hero>
            </heroes>

        `
})

Well, I know, still hard code, but just show how ngFor can be used.

Now, inside 'heroes' tag, we add now 'hero' tag. And we want to display those inside 'heroes' component:

import {Component, ContentChildren, QueryList} from "@angular/core";
import {Hero} from './hero';
/*
const HEROES = [
    {id: 1, name:'Superman'},
    {id: 2, name:'Batman'},
    {id: 5, name:'BatGirl'},
    {id: 3, name:'Robin'},
    {id: 4, name:'Flash'}
];*/

@Component({
    selector:'heroes',
    styleUrls: [
        'heroes.component.css'
    ],
    template: `
    <table>
        <thead>
            <th>Name</th>
            <th>Index</th>
        </thead>
        <tbody>
            <tr *ngFor="let hero of heroes; let i = index; trackBy: trackBy(hero);
             let isEven=even; let isFirst=first; let isLast=last;"
             [ngClass]="{'even': isEven, 'first': isFirst, 'last': isLast}">
                <td>{{hero.name}}</td>
                <td>{{i}}</td>
            </tr>
        </tbody>
    </table>
`
})
export class Heroes {
    //heroes = HEROES;
    @ContentChildren(Hero)
    heroes: QueryList<Hero>

    trackBy(hero){
        return hero ? hero.id: undefined;
    }
}

You can see, we have commented out the hard code array. Instead, we use:

    @ContentChildren(Hero)
    heroes: QueryList<Hero>

'Hero' here, is a element directive:

import {Directive, Input} from "@angular/core";


@Directive({
    selector: 'hero',
})
export class Hero {

    @Input()
    id: number;

    @Input()
    name:string;
    
}

@ContentChildren will check the Children in HTML DOM tree, which will get:

                <hero name="Superman" id="1"></hero>
                <hero name="Batman" id="2"></hero>
                <hero name="BatGirl" id="3"></hero>
                <hero name="Robin" id="4"></hero>
                <hero name="Flash" id="5"></hero>
                <hero name="Zhentian" id="6"></hero>

QueryList<Hero>: Only get 'Hero' directive.

QueryList is a class provided by Angular and when we use QueryList with a ContentChildren Angular populate this with the components that match the query and then keeps the items up to date if the state of the application changes .

However, QueryList requires a ContentChildren to populate it, so let’s take a look at that now. 

What's cool about *ngFor, it not only accpets Array, but also any iterable type, we have list of DOM element 'hero', which are iterable, so ngFor will able to display those also.