Typescript Angular：生命周期钩子是组件可用的输入数据

我有一个组件，它接收image个对象的数组作为Input个数据.

export class ImageGalleryComponent {
  @Input() images: Image[];
  selectedImage: Image;
}

我想在组件加载时，将selectedImage值设置为images数组的第一个对象.我试着在OnInit生命周期钩子中这样做:

export class ImageGalleryComponent implements OnInit {
  @Input() images: Image[];
  selectedImage: Image;
  ngOnInit() {
    this.selectedImage = this.images[0];
  }
}

这给了我一个错误Cannot read property '0' of undefined，这意味着images值没有在这个阶段设置.我也try 过OnChanges钩子，但我被卡住了，因为我无法获得有关如何观察数组变化的信息.我怎样才能达到预期的结果？

父组件如下所示:

@Component({
  selector: 'profile-detail',
  templateUrl: '...',
  styleUrls: [...],
  directives: [ImageGalleryComponent]
})

export class ProfileDetailComponent implements OnInit {
  profile: Profile;
  errorMessage: string;
  images: Image[];
  constructor(private profileService: ProfileService, private   routeParams: RouteParams){}

  ngOnInit() {
    this.getProfile();
  }

  getProfile() {
    let profileId = this.routeParams.get('id');
    this.profileService.getProfile(profileId).subscribe(
    profile => {
     this.profile = profile;
     this.images = profile.images;
     for (var album of profile.albums) {
       this.images = this.images.concat(album.images);
     }
    }, error => this.errorMessage = <any>error
   );
 }
}

父组件的模板包含以下内容:

...
<image-gallery [images]="images"></image-gallery>
...