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2018 iT 邦幫忙鐵人賽
DAY 11
2

本文同步發佈於:Claire's BLOG

通過輸入綁定數據從父項傳遞給子項

這是接受數據的子元件的內容hero-child.component.ts
子元件輸要被輸入的屬性有兩個,以@Input開頭來做宣告:

import { Component, Input } from '@angular/core';
 
import { Hero } from './hero';
 
@Component({
  selector: 'app-hero-child',
  template: '
    <h3>{{hero.name}} says:</h3>
    <p>I, {{hero.name}}, am at your service, {{masterName}}.</p>
  '
})
export class HeroChildComponent {
  @Input() hero: Hero;
  @Input('master') masterName: string;
}

而這是父元件的內容,透過屬性綁定將值塞進子元件中

父元件的完整程式碼如下:

import { Component } from '@angular/core';
 
import { HEROES } from './hero';
 
@Component({
  selector: 'app-hero-parent',
  template: '
    <h2>{{master}} controls {{heroes.length}} heroes</h2>
    <app-hero-child *ngFor="let hero of heroes"
      [hero]="hero"
      [master]="master">
    </app-hero-child>
  '
})
export class HeroParentComponent {
  heroes = HEROES;
  master = 'Master';
}

最後結果呈現:

攔截屬性的更改與設置

可將子元件的input宣告改為gettersetter,如下面範例

import { Component, Input } from '@angular/core';
 
@Component({
  selector: 'app-name-child',
  template: '<h3>"{{name}}"</h3>'
})
export class NameChildComponent {
  private _name = '';
 
  @Input()
  set name(name: string) {
    this._name = (name && name.trim()) || '<no name set>';
  }
 
  get name(): string { return this._name; }
}

這樣當父元件傳空字串來時,子元件會自動輸出字樣

Intercept input property changes with ngOnChanges()

在子組件裡,除了上述使用getter及setter外,也可以利用ngOnChanges()來取得改變的變數並做出回應
ngOnChanges()會傳入所有被改變的值changes: {[propKey: string]: SimpleChange}

下面是一個使用範例

子元件內容

import { Component, Input, OnChanges, SimpleChange } from '@angular/core';
 
@Component({
  selector: 'app-version-child',
  template: '
    <h3>Version {{major}}.{{minor}}</h3>
    <h4>Change log:</h4>
    <ul>
      <li *ngFor="let change of changeLog">{{change}}</li>
    </ul>
  '
})
export class VersionChildComponent implements OnChanges {
  @Input() major: number;
  @Input() minor: number;
  changeLog: string[] = [];
 
  ngOnChanges(changes: {[propKey: string]: SimpleChange}) {
    let log: string[] = [];
    for (let propName in changes) {
      let changedProp = changes[propName];
      let to = JSON.stringify(changedProp.currentValue);
      if (changedProp.isFirstChange()) {
        log.push(`Initial value of ${propName} set to ${to}`);
      } else {
        let from = JSON.stringify(changedProp.previousValue);
        log.push(`${propName} changed from ${from} to ${to}`);
      }
    }
    this.changeLog.push(log.join(', '));
  }
}

父元件內容

import { Component } from '@angular/core';
 
@Component({
  selector: 'app-version-parent',
  template: '
    <h2>Source code version</h2>
    <button (click)="newMinor()">New minor version</button>
    <button (click)="newMajor()">New major version</button>
    <app-version-child [major]="major" [minor]="minor"></app-version-child>
  '
})
export class VersionParentComponent {
  major = 1;
  minor = 23;
 
  newMinor() {
    this.minor++;
  }
 
  newMajor() {
    this.major++;
    this.minor = 0;
  }
}

最後成果如下圖

Parent listens for child event

子元件的EventEmitter財產是一個輸出的屬性,通常用@Output來宣告,如下:

import { Component, EventEmitter, Input, Output } from '@angular/core';
 
@Component({
  selector: 'app-voter',
  template: '
    <h4>{{name}}</h4>
    <button (click)="vote(true)"  [disabled]="voted">Agree</button>
    <button (click)="vote(false)" [disabled]="voted">Disagree</button>
  '
})
export class VoterComponent {
  @Input()  name: string;
  @Output() onVoted = new EventEmitter<boolean>();
  voted = false;
 
  vote(agreed: boolean) {
    this.onVoted.emit(agreed);
    this.voted = true;
  }
}

而父元件的內容如下:

import { Component }      from '@angular/core';
 
@Component({
  selector: 'app-vote-taker',
  template: '
    <h2>Should mankind colonize the Universe?</h2>
    <h3>Agree: {{agreed}}, Disagree: {{disagreed}}</h3>
    <app-voter *ngFor="let voter of voters"
      [name]="voter"
      (onVoted)="onVoted($event)">
    </app-voter>
  '
})
export class VoteTakerComponent {
  agreed = 0;
  disagreed = 0;
  voters = ['Mr. IQ', 'Ms. Universe', 'Bombasto'];
 
  onVoted(agreed: boolean) {
    agreed ? this.agreed++ : this.disagreed++;
  }
}

結果如圖:

Parent interacts with child via local variable

下面是範例,將子元件使用#來宣告為template變數,就可以在父元件內自由使用子元件變數:

<h3>Countdown to Liftoff (via local variable)</h3>
  <button (click)="timer.start()">Start</button>
  <button (click)="timer.stop()">Stop</button>
  <div class="seconds">{{timer.seconds}}</div>
  <app-countdown-timer #timer></app-countdown-timer>

Parent and children communicate via a service

服務內容:

import { Injectable } from '@angular/core';
import { Subject }    from 'rxjs/Subject';
 
@Injectable()
export class MissionService {
 
  // Observable string sources
  private missionAnnouncedSource = new Subject<string>();
  private missionConfirmedSource = new Subject<string>();
 
  // Observable string streams
  missionAnnounced$ = this.missionAnnouncedSource.asObservable();
  missionConfirmed$ = this.missionConfirmedSource.asObservable();
 
  // Service message commands
  announceMission(mission: string) {
    this.missionAnnouncedSource.next(mission);
  }
 
  confirmMission(astronaut: string) {
    this.missionConfirmedSource.next(astronaut);
  }
}

父元件內容

import { Component }          from '@angular/core';
 
import { MissionService }     from './mission.service';
 
@Component({
  selector: 'app-mission-control',
  template: '
    <h2>Mission Control</h2>
    <button (click)="announce()">Announce mission</button>
    <app-astronaut *ngFor="let astronaut of astronauts"
      [astronaut]="astronaut">
    </app-astronaut>
    <h3>History</h3>
    <ul>
      <li *ngFor="let event of history">{{event}}</li>
    </ul>
  ',
  providers: [MissionService]
})
export class MissionControlComponent {
  astronauts = ['Lovell', 'Swigert', 'Haise'];
  history: string[] = [];
  missions = ['Fly to the moon!',
              'Fly to mars!',
              'Fly to Vegas!'];
  nextMission = 0;
 
  constructor(private missionService: MissionService) {
    missionService.missionConfirmed$.subscribe(
      astronaut => {
        this.history.push(`${astronaut} confirmed the mission`);
      });
  }
 
  announce() {
    let mission = this.missions[this.nextMission++];
    this.missionService.announceMission(mission);
    this.history.push(`Mission "${mission}" announced`);
    if (this.nextMission >= this.missions.length) { this.nextMission = 0; }
  }
}

子元件內容

import { Component, Input, OnDestroy } from '@angular/core';
 
import { MissionService } from './mission.service';
import { Subscription }   from 'rxjs/Subscription';
 
@Component({
  selector: 'app-astronaut',
  template: '
    <p>
      {{astronaut}}: <strong>{{mission}}</strong>
      <button
        (click)="confirm()"
        [disabled]="!announced || confirmed">
        Confirm
      </button>
    </p>
  '
})
export class AstronautComponent implements OnDestroy {
  @Input() astronaut: string;
  mission = '<no mission announced>';
  confirmed = false;
  announced = false;
  subscription: Subscription;
 
  constructor(private missionService: MissionService) {
    this.subscription = missionService.missionAnnounced$.subscribe(
      mission => {
        this.mission = mission;
        this.announced = true;
        this.confirmed = false;
    });
  }
 
  confirm() {
    this.confirmed = true;
    this.missionService.confirmMission(this.astronaut);
  }
 
  ngOnDestroy() {
    // prevent memory leak when component destroyed
    this.subscription.unsubscribe();
  }
}

結果如下圖:

更多本篇完整範例請見: live example / download example


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[功能介紹-3] Hooks的生命週期
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[功能介紹-5] 動態載入元件
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