我正在开发一个 angular firebase 应用，但我不知道如何解决问题。 我的 @Input 在 ngOnInit() 上始终未定义，但我必须使用此输入值初始化某些内容。

ERROR TypeError: Cannot read property 'length' of undefined

我尝试添加 *ngIf（如我在类似帖子中看到的那样）或初始化我的房间，但没有任何效果。我必须使用超时吗？还是有更好的方法？

以下是详细信息：

玩家组件：

@Input() room: Room;
    players: Player[];
    playersSubscription: Subscription;

    constructor(private playerService: PlayerService, private router: Router) { }

    ngOnInit() {
        this.playersSubscription = this.playerService.playersSubject.subscribe(
            (players: Player[]) => {
                this.players = players;
            }
        );

        this.playerService.getPlayersRoom(this.room.players);

        this.playerService.emitPlayers();

    }

html：

<div *ngIf="room">
    <ul>
        <li *ngFor="let p of room.players">{{p}}</li>
    </ul>
</div>
usernames:
<div *ngIf="players">
    <ul>
        <li *ngFor="let player of players">
            {{player.username}} // This will not works
        </li>
    </ul>
</div>

服务：

    getPlayersRoom(ids: string[]) {

        while (ids.length > 0) {
            var id = ids.splice(0, 1)[0];
            firebase.database().ref('/users/').child(id).once('value', function (snapshot) {
                this.players.push(snapshot.val());
            });
        }
        this.emitPlayers();
}

房间：

room: Room;

constructor(private router: Router,
    private route: ActivatedRoute,
    private roomService: RoomService
) { }

ngOnInit(): void {
    this.room = new Room();
    const id = this.route.snapshot.params['id'];
    this.roomService.getRoom(+id).then(
        (room: Room) => {
            this.room = room;
        }
    );
}

html：

<div *ngIf="room">
        Room n°{{room.id}} + password {{room.password}}
    <h1>PLAYERS room</h1>
    <div *ngFor="let p of room.players">
        {{p}}
    </div>
    Players:
    <app-players *ngIf="room" [room]="room"></app-players>
    end
</div>

编辑： 问题出在 ngOnInit（玩家组件）中：此时 @Input 未定义（然后它正在执行 getPlayersRoom(undefined) ）。有人知道吗？在构造函数/ngoninit 中无法使用 @Input？那我该怎么做？