Angular 元件的生命週期鉤子讓你在關鍵時機執行初始化、清理等操作。

鉤子執行順序

ngOnChanges → ngOnInit → ngDoCheck → 
ngAfterContentInit → ngAfterContentChecked →
ngAfterViewInit → ngAfterViewChecked → ngOnDestroy

常用鉤子

ngOnInit

export class UserListComponent implements OnInit {
  users: User[] = [];

  ngOnInit() {
    this.userService.getUsers().subscribe(u => this.users = u);
  }
}

ngOnChanges

export class ChildComponent implements OnChanges {
  @Input() data: string;

  ngOnChanges(changes: SimpleChanges) {
    if (changes['data']) {
      console.log('data changed:', changes['data'].currentValue);
    }
  }
}

ngOnDestroy

export class DataComponent implements OnDestroy {
  private sub = Subscription.EMPTY;

  ngOnInit() {
    this.sub = this.dataService.stream$.subscribe(...);
  }

  ngOnDestroy() {
    this.sub.unsubscribe();
  }
}

ngAfterViewInit

export class ChartComponent implements AfterViewInit {
  @ViewChild('canvas') canvas: ElementRef;

  ngAfterViewInit() {
    // DOM 渲染完成後初始化圖表
    this.initChart(this.canvas.nativeElement);
  }
}

正確使用生命週期鉤子是避免記憶體洩漏和錯誤初始化的基礎。