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。