从Distance Matrix Javascript API的回调函数访问类属性

Question

这是我的代码：

declare var google;
var service = new google.maps.DistanceMatrixService();
class BranchesPage {

   public branches: any = [];
   public allBranches: any = [];

   public travelDetails: any = [];

   constructor(){
       this.getBranches();
   }

   getBranches(){
       this.travelDetails = [];
       this.branchProvider.getBranches()
         .subscribe(data => {
             this.allBranches = data;
             this.getBranchDistanceAndTime();
             loadingBranches.dismiss();
         }, err => {
             console.log(err);
         });
    }

    getBranchDistanceAndTime(){
       for(let branch of this.allBranches){
           this.getTravelDetails(branch);
       }
    }

    getTravelDetails(branch){
        service.getDistanceMatrix(
        {
            origins: [new google.maps.LatLng(6.870128,79.880340)],
            destinations: [new google.maps.LatLng(branch.lat, branch.lng)],
            travelMode: 'DRIVING'
         }, this.callback);
     }

     callback(response, status) {
        let travelDetailsObject;
        if (status == 'OK') {
           var origins = response.originAddresses;
           var destinations = response.destinationAddresses;

           for (var i = 0; i < origins.length; i++) {
              var results = response.rows[i].elements;
              for (var j = 0; j < results.length; j++) {
                  var element = results[j];
                  var distance = element.distance.text;
                  var duration = element.duration.text;
                  var from = origins[i];
                  var to = destinations[j];
                  travelDetailsObject = {
                      distance: distance,
                      duration: duration
                  }
             }
          }
          console.log(travelDetailsObject)//this works perfectly retrieving the correct distances and durations
          this.travelDetails.push(travelDetailsObject); //error is on this line
      }
 }

我无法从回调函数访问travelDetails属性，因为它会引发以下异常Uncaught TypeError: Cannot read property 'travelDetails' of null

由于某种原因，this关键字在回调函数中不可用。任何解决方法？我已经被困在这个问题上超过一天了。任何帮助将不胜感激。

谢谢你们

PS：在有人要求之前，branch.lat和branch.lng会按预期返回值。