JavaScript 对象 - 构造后对象值未定义

Question

我正在尝试创建一个处理 Google Maps Api 的对象，如下所示:

function GoogleMap(container, mapOptions) {
    this.Container = container;
    this.Options = mapOptions;
    this.Map = new google.maps.Map(document.getElementById(this.Container), this.Options);

    // Direction
    this.DirectionService = new google.maps.DirectionsService();
    this.DirectionRenderer = new google.maps.DirectionsRenderer();
    this.DirectionRenderer.setMap(this.Map);
    this.DirectionId = 0;
    this.DirectionResponse = new Array();
    this.DrawDirectionDriving = drawDirectionDriving;
}

drawDirectionDriving函数是这样的:

function drawDirectionDriving(start, end) {
  var request = {
    origin: start,
    destination: end,
    travelMode: google.maps.TravelMode.DRIVING
  };

  this.DirectionService.route(request,
    function (response, status) {
      if (status == google.maps.DirectionsStatus.OK) {
        this.DirectionRenderer.setDirections(response);
        this.DirectionResponse[this.DirectionId] = response;
        this.DirectionId++;
      }
      else {
        alert("Error during drawing direction, Google is not responding...");
      }
    }
  );
}

在某个地方，我正在使用这样的对象:

var myGoogleMap;

function MapInit() {
    myGoogleMap = new GoogleMap("divMap", myMapOptions);
    myGoogleMap.DrawDirectionDriving("İstanbul", "Ankara");
}

Google map 显示在我的浏览器上，构造对象没有问题，但 DrawDirectionDriving 函数出错。

当我在此行上创建断点时:“myGoogleMap.DrawDirectionDriving(" Istanbul 尔", "安卡拉");"“DirectionRenderer”似乎已构建，但在这一行之后(在“Draw”方法之后)，DirectionRenderer 对象似乎为 null(未定义)，因此它会出现这样的错误“无法获取 setDirections 属性，它为 null bla bla ...”

你能帮我一下吗？

提前致谢...

Answer 1

this keyword确实指向 route 回调函数中的其他内容。它的 DirectionRenderer 属性解析为 null/undefined，并且从中获取 setDirections 属性将导致异常。

使用解引用变量:

function drawDirectionDriving(start, end) {
  var request = {
    origin: start,
    destination: end,
    travelMode: google.maps.TravelMode.DRIVING
  };
  var that = this;

  this.DirectionService.route(request,
    function (response, status) {
      if (status == google.maps.DirectionsStatus.OK) {
        that.DirectionRenderer.setDirections(response);
        that.DirectionResponse[this.DirectionId] = response;
        that.DirectionId++;
//      ^^^^ points to the GoogleMap instance
      }
      else {
        alert("Error during drawing direction, Google is not responding...");
      }
    }
  );
}