Google Map API v2 （四）----- 导航路径

仍然是建议个异步小任务

 private GetPathTask mGetPathTask = null;

 private void getGuidePath(LatLng origin){
     if(mGetPathTask != null){
         mGetPathTask.cancel(true);
     }
     mGetPathTask = new GetPathTask();
     mGetPathTask.execute(origin, mMarker.getPosition());
 }

第8行的两个入参都是LatLng对象，分别为其实地点和目标地点。

GetPathTask任务定义如下：

 private class GetPathTask extends AsyncTask<LatLng, Void, List<LatLng>>{

   @Override
   protected void onPostExecute(List<LatLng> result){
     if((result == null) || result.isEmpty()){
       Toast.makeText(MapsActivity.this, R.string.GetPathErr, Toast.LENGTH_LONG).show();
       return;
     }    

   if(mGuidePath == null){
     mGuidePath = mMapView.addPolyline(new PolylineOptions()
                    .color(0xfff5cb08)
                    .width(10)
                    .geodesic(true));
   }
   mGuidePath.setPoints(result);
 }

   @Override
   protected List<LatLng> doInBackground(LatLng... params) {
     LatLng origin = params[0];
     LatLng destination = params[1];
     String responseString = null;
     String urlString = "http://maps.google.com/maps/api/directions/xml?sensor=true&mode=walking&avoid=highways";

     HttpGet httpGet = new HttpGet(urlString + "&origin=" + origin.latitude + "," + origin.longitude
                                   + "&destination=" + destination.latitude + "," + destination.longitude);

     HttpResponse mHttpResponse = null;
     HttpEntity mHttpEntity = null;
     try{
       HttpParams httpParameters = new BasicHttpParams();
       HttpConnectionParams.setConnectionTimeout(httpParameters, 3000);
       HttpClient httpClient = new DefaultHttpClient(httpParameters);   

       mHttpResponse = httpClient.execute(httpGet);

       if (mHttpResponse.getStatusLine().getStatusCode() == 200){
         mHttpEntity = mHttpResponse.getEntity();
         responseString = mHttpEntity.toString();
         responseString = EntityUtils.toString(mHttpEntity);
       }
     }
     catch (Exception e){
       log("Exception in GetPathTask doInBackground():" + e);
     }

     if(responseString == null){
         log("GetPathTask doInBackground() responseString == null");
     }else if (-1 == responseString.indexOf("<status>OK</status>")){
         log("GetPathTask doInBackground() response status error");
     }else{
         int pos1 = responseString.indexOf("<overview_polyline>");
         pos1 = responseString.indexOf("<points>", pos1 + 1);
         int pos2 = responseString.indexOf("</points>", pos1);
         responseString = responseString.substring(pos1 + 8, pos2);

         List<LatLng> points = decodePoly(responseString);
         return points;
     }
     return null;
   }

   private List<LatLng> decodePoly(String encoded) {
     List<LatLng> poly = new ArrayList<LatLng>();
     int index = 0, len = encoded.length();
     int lat = 0, lng = 0;  

     while (index < len) {
       if(isCancelled())
         break;
       int b, shift = 0, result = 0;
       do {
         b = encoded.charAt(index++) - 63;
         result |= (b & 0x1f) << shift;
         shift += 5;
       } while (b >= 0x20);
       int dlat = ((result & 1) != 0 ? ~(result >> 1) : (result >> 1));
       lat += dlat;  

       shift = 0;
       result = 0;
       do {
         b = encoded.charAt(index++) - 63;
         result |= (b & 0x1f) << shift;
         shift += 5;
       } while (b >= 0x20);
       int dlng = ((result & 1) != 0 ? ~(result >> 1) : (result >> 1));
       lng += dlng;  

       double dLat = lat / 1E5;
       double dLng = lng / 1E5;
       LatLng p = new LatLng(dLat, dLng);

       poly.add(p);
     }
     if(isCancelled())
       return null;

     return poly;
   }
 }

看doInBackgound函数

24～27行，构造一个google directions API的HTTP请求，格式类似这样：

http://maps.google.com/maps/api/directions/xml?sensor=true&mode=walking&avoid=highways&origin=纬度,经度&destination=纬度,经度
我这个设置的步行模式，避免高速，返回数据是XML格式。更多的可选项请参考官文：

https://developers.google.com/maps/documentation/directions/

48～59行，返回的response中<point></point>标签中是base64编码的数据，是一组point，即google direction 服务为我们规划出的路径途经的各个点，调用decodePoly解析成List<LatLng>。

4行，onPostExecute函数中

10～16行，调用GoogleMap.addPolyline(PolylineOptions polylineOption) 在地图上加一个折线。调用Polyline.setPoints(point)设置折线的各个点。

贴个图看看，黄色的粗线把我们导向了美丽的莫愁湖。这里我还加了一条细红线，直接指向目的地，用于在移动过程中始终标识出目的地所在方位。

google地图支持在地图上添加几种shape对象，详见官文。