Reading the return value of a java script function with "evaluateJavaScript" from Qt

Question

This is my route planner widget implemented with Qt5 and WebKit

How to get a JavaScript function return value in C++?

After a long time of not using JavaScript, I implemented a route planner with the Google maps API V3. I defined some functions to access the scripting part of it via Qt's WebKit function: evaluateJavaScript(). Now the hard part is, to get the position data of the markers (see picture, markers are red). In the JavaScript end, I just return an array. How can I read the "latlng"-array in the Qt end?

Illustration - This is what I want simplified:

// JavaScript
function getJSFoo() {
  return foofoo;
}
// C++
void readInQt() {
  // something like this:
  float val = evaluateJavaScript("getJSFoo()");
  // Do stuff
}

Thanks in advance!

This my code, more or less how I use it. At JavaScript's end:

// Add a marker to the map and push to the array.
function addMarker(location) {
  var marker = new google.maps.Marker({
      position: location,
      map: map,
      draggable:true
  });
  
  var LatLng = marker.getPosition();
  var sLatLng = "Latitude:    " + marker.getPosition().lat().toString() + 
                "\nLongitude: " + marker.getPosition().lng().toString();
  marker.setTitle(sLatLng);

  flightMarker.push(marker);
  return flightMarker;
}

function getMarkers() {
  var latlng = []
  for (var i = 0; i < flightMarker.length; i++) {
    latlng.push(flightMarker[i].getPosition() );
  }
  return latlng;
}

At Qt's end:

void sl_submitRoute() {
    getWaypoints();
}

QList<float> getWaypoints() {
    QList<float> lWaypoints;
    // I don't know what I do here :D
    QVariant varMarkers = mView->page()->mainFrame()->evaluateJavaScript(QString("getMarkers();") );
    QList <QVariant> allListObj = varMarkers.toList();
    qDebug() << allListObj;

    return lWaypoints;
}

I don't know much about QML and I need the output for other reasons than just QML typical rendering — dgrat
– dgrat, Commented Apr 11, 2014 at 20:32

dgrat · Accepted Answer · 2014-04-12 08:35:01Z

2

I found the solution. Easiest way is to convert data into a QStringList and back. Otherwise data type conversion could result in a leak.

JavaScript

function getMarkers() {
  var latlng = []
  for (var i = 0; i < flightMarker.length; i++) {
    var sPos = flightMarker[i].getPosition().lat().toString() + ":" + flightMarker[i].getPosition().lng().toString();
    latlng.push(sPos);
  }
  return latlng;
}

Qt

// Returns a list of points: Latitude & Longitude
RouteF getWaypoints() {
    RouteF lWaypoints;

    QVariant varMarkers = mView->page()->mainFrame()->evaluateJavaScript(QString("getMarkers();") );
    QList<QVariant> allListObj = varMarkers.toList();
    for(int i = 0; i < allListObj.size(); i++) {
        QStringList lPoint = allListObj.at(i).toString().split(':');
        if(lPoint.size() != 2) {
            qDebug() << "Warning: Waypoint not valid!";
            continue;
        }

        double fLat = lPoint.at(0).toDouble();
        double fLon = lPoint.at(1).toDouble();
        PointF waypoint = PointF(fLat, fLon);
        lWaypoints.push_back(waypoint);
        qDebug() << "Added point: " << fLat << ":" << fLon;
    }

    return lWaypoints;
}

answered Apr 12, 2014 at 8:35

dgrat

2,2644 gold badges25 silver badges51 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Baso Over a year ago

Hello, I would like to do something similar in a small project, but I don't know where to start and I think I lake some knowledge as I couldn't know what to do just form what you provided. So Would you please provide me with a simple full working example so I can understand the whole thing. Thank you.

dgrat Over a year ago

code.google.com/p/rpicopter Look at the code of the GroundControl part.

Collectives™ on Stack Overflow

Reading the return value of a java script function with "evaluateJavaScript" from Qt

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related