@Input 數據從父組件傳到子組件

子組件：

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

}

第二個@Input為子組件的屬性名masterName指定一個別名master(譯者註：不推薦為起別名，請參見風格指南)

父組件：

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" // 子組件的 @Input() hero: Hero 對應這裏的 [hero]
      [master]="master" // @Input(‘master‘) masterName 對應這裏的 【master】
    >
 

    </app-hero-child>
  `
})
export class HeroParentComponent {
  heroes = HEROES;
  master = ‘Master‘;
}

通過setter截聽輸入屬性值的變化

使用一個輸入屬性的setter，以攔截父組件中值的變化，並采取行動。

子組件NameChildComponent的輸入屬性name上的這個setter，會trim掉名字裏的空格，並把空值替換成默認字符串

  @Input()
  set name(name: string) {
    this._name = (name && name.trim()) || ‘<no name set>‘;
  }
  get name(): string { return this._name; }

通過ngOnChanges()來截聽輸入屬性值的變化

使用OnChanges生命周期鉤子接口的ngOnChanges()方法來監測輸入屬性值的變化並做出回應

當需要監視多個、交互式輸入屬性的時候，本方法比用屬性的setter更合適

父組件監聽子組件的事件(EventEmitter)

子組件暴露一個EventEmitter屬性，當事件發生時，子組件利用該屬性emits(向上彈射)事件。父組件綁定到這個事件屬性，並在事件發生時作出回應。

子組件的EventEmitter屬性是一個輸出屬性，通常帶有@Output裝飾器，就像在VoterComponent中看到的

子組件：

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;
  }
}

父組件：

import { Component }      from ‘@angular/core‘;
 
@Component({
  selector: ‘app-vote-taker‘,
  template: `
    <app-voter *ngFor="let voter of voters"
      [name]="voter"
　　　　// onVoted 對應 子組件的@Output onVoted
      (onVoted)="onVoted($event)">
    </app-voter>
  `
})
export class VoteTakerComponent {
  onVoted(agreed: boolean) {
    // agreed 就是子組件傳遞的參數
  }
}

子組件：點擊按鈕會觸發true或false(布爾型有效載荷)的事件。

父組件：VoteTakerComponent綁定了一個事件處理器(onVoted())，用來響應子組件的事件($event)並更新一個計數器。

父組件調用子組件方法或者屬性

父組件不能使用數據綁定來讀取子組件的屬性或調用子組件的方法。但可以在父組件模板裏，新建一個本地變量來代表子組件，然後利用這個變量來讀取子組件的屬性和調用子組件的方法

• 父組件模板上調用子組件的方法：

子組件：

  start() { 
  }
  stop()  {
  }

父組件：

  template: `
  <button (click)="timer.start()">Start</button>
  <button (click)="timer.stop()">Stop</button>
  <app-countdown-timer #timer></app-countdown-timer>
  `,

把本地變量(#timer)放到(<countdown-timer>)標簽中，用來代表子組件。這樣父組件的模板就得到了子組件的引用，於是可以在父組件的模板中訪問子組件的所有屬性和方法

• 父組件js 調用子組件的方法@ViemChild()：

父組件：

import { AfterViewInit, ViewChild } from ‘@angular/core‘;
import { Component }                from ‘@angular/core‘;
import { CountdownTimerComponent }  from ‘./countdown-timer.component‘;
 
@Component({
  selector: ‘app-countdown-parent-vc‘,
  template: `
  <button (click)="start()">Start</button>
  <button (click)="stop()">Stop</button>
  <div class="seconds">{{ seconds() }}</div>
  <app-countdown-timer></app-countdown-timer>
  `,
  styleUrls: [‘../assets/demo.css‘]
})
export class CountdownViewChildParentComponent implements AfterViewInit {
 
  @ViewChild(CountdownTimerComponent)
  private timerComponent: CountdownTimerComponent;
 
  seconds() { return 0; }
 
  ngAfterViewInit() {
    setTimeout(() => this.seconds = () => this.timerComponent.seconds, 0);
  }
 
  start() { this.timerComponent.start(); }
  stop() { this.timerComponent.stop(); }
}

通過@ViewChild屬性裝飾器，將子組件CountdownTimerComponent註入到私有屬性timerComponent裏面

組件元數據裏就不再需要#timer本地變量了。而是把按鈕綁定到父組件自己的start和stop方法

然後Angular會調用ngAfterViewInit生命周期鉤子，但這時候再更新父組件視圖的倒計時就已經太晚了。Angular的單向數據流規則會阻止在同一個周期內更新父組件視圖。我們在顯示秒數之前會被迫再等一輪

（子組件CountdownTimerComponent和原來一樣）

父組件和子組件通過服務來通訊？？？

父組件和它的子組件共享同一個服務，利用該服務在家庭內部實現雙向通訊。

該服務實例的作用域被限制在父組件和其子組件內。這個組件子樹之外的組件將無法訪問該服務或者與它們通訊。

這個MissionService把MissionControlComponent和多個AstronautComponent子組件連接起來

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);
  }
}

MissionControlComponent提供服務的實例，並將其共享給它的子組件(通過providers元數據數組)，子組件可以通過構造函數將該實例註入到自身

父組件：MissionControlComponent

import { Component }          from ‘@angular/core‘;
import { MissionService }     from ‘./mission.service‘;
 
@Component({
  selector: ‘app-mission-control‘,
  template: `
    <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; }
  }
}

子組件：AstronautComponent

AstronautComponent也通過自己的構造函數註入該服務。由於每個AstronautComponent都是MissionControlComponent的子組件，所以它們獲取到的也是父組件的這個服務實例

mport { 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();
  }
}

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