回调在Angular2 / Firebase中生成“ TypeError：这是未定义的”

我试图了解这里的问题以及为什么如果我以某种方式调用函数会收到错误消息，而以另一种方式调用函数却没有收到错误消息，为什么呢？这是首先产生错误的方法：

player.service.ts文件

在@Injectable我有

private roomsRef: Firebase;

constructor() {
    this.roomsRef   = new Firebase("https://xxx.firebaseio.com/rooms");
}

postGameActions(roomId: string) {
        console.log("post game actions start on " + roomId);

//error on below 'this'        
        this.roomsRef.child(roomId).child("board").once("value", snap => {
           let board = snap.val();
           //do stuff with 'board' object
        });
}

room.component.ts文件

activateTimer(roomId: string, roomName: string, callback) {
    var timer = setInterval(redirectTimer, 1000);
    var seconds: number = 5;
    var timerHtml = document.getElementById("divRedirectTimer" + roomName);
    timerHtml.innerHTML = "[" + seconds + "]";

    function redirectTimer() {
        timerHtml.innerHTML = "[" + (seconds--) + "]";
        if(seconds === 0) {
            clearInterval(timer);
            callback(roomId);
        }
    };
}

像这样调用非工作版本

activateTimer(room.id, room.name, _playerService.postGameActions)

错误：

EXCEPTION: TypeError: this is undefined

工作版本

像这样工作正常，但不使用，setInterval()因为activateTimer仅调用service方法

room.component.ts文件

activateTimer(roomId: string, roomName: string) {
    var timer = setInterval(redirectTimer, 1000);
    var seconds: number = 5;
    var timerHtml = document.getElementById("divRedirectTimer" + roomName);
    timerHtml.innerHTML = "[" + seconds + "]";

    function redirectTimer() {
        timerHtml.innerHTML = "[" + (seconds--) + "]";
        if(seconds === 0) {
            clearInterval(timer);
        }
    }

    this._playerService.postGameActions(roomId);

调用这样的工作版本

activateTimer(room.id, room.name)

当我postGameActions作为回调调用时，为什么“ this”未定义？我确定我在这里缺少一些简单的东西