SAPessi

I love calculating driving directions on Google Maps and then drag the blue line marking my directions to change the route. Everything is updated automatically on the page and the directions are re-calculated to go through the new point I defined.

I’ve been playing around with the Google Maps APIs recently and imagine my disappointment when I found out that Google does not allow directions calculated through the APIs to be dragged around.

Not put off by this I decided to try and replicate the directions-dragging myself. How hard can it be?
As it turns out. Very, at least if you want to make it look as smooth as Google’s own solution.

When generating directions Google Maps adds a GPolyline element overlay to your map. You can set the returned line to be editable but this makes an awful lot of vertices appear on it, which makes reading your directions quite hard. Even so, once you dragged one of this vertices around you are not editing the route but just changing the shape of the line.

Mine is not a complete solution and I’m interested in feedback and ideas on how to improve it.

First off calculate your directions:

map = new google.maps.Map2(document.getElementById('map_canvas'));
var wayPoints = [];
wayPoints.push(startPoint.getLatLng());
wayPoints.push(endPoint.getLatLng());

var myDir = new google.maps.Directions(map)
myDir.loadFromWaypoints(wayPoints, { travelMode: G_TRAVEL_MODE_DRIVING });

This will calculate your driving directions and plot a GPolyline on your map. The line is easily accessible in the GDirections object once the directions are calculated. To intercept this I have decided to use the addoverlay event on the GMap object. This event is triggered every time something is plotted over the map (says on the tin).

google.maps.Event.addListener(myDir, "addoverlay", function() {
var dirLine = myDir.getPolyline(); // Get the polyline from the directions object
});

At this point we can ask the APIs to make the GPolyline editable. This will make the vertices appear on the line and make then draggable. As I said before this only changes the shape of the line and doesn’t actually affect your directions object. Luckily the GPolyline comes with a nifty event called lineupdated.
This is triggered once the user has finished dragging a vertex. By intercepting this we can look through the vertices and know what’s been changed on the line and where the vertex has been moved to.
In order to do this we must also know the previous position of the vertices (latitude and longitude) to be able to compare the old and the new “edited” line.
Another challenge is the fact that the GDirections object can accept only so many waypoints (25 if I’m not wrong). Which means we’ll have to add only the vertex that has changed to the directions and not all of them.

// In the addoverlay event also save the original vertices of the line
var origLine = [];
for (var i = 0; i < dirLine.getVertexCount(); i++) {
origLine.push(dirLine.getVertex(i));
}
// DONE saving vertices

// Now intercept the lineupdated event and add the new waypoints
google.maps.Event.addListener(dirLine, "lineupdated", function() {
routePoints = [];
for (var i = 0; i < dirLine.getVertexCount(); i++) {
var savedPoint = origLine[i];
if (!savedPoint || (savedPoint.lat() != dirLine.getVertex(i).lat() && savedPoint.lng() != dirLine.getVertex(i).lng())) {
routePoints.push(dirLine.getVertex(i));
}
}

// Now we remove the previous directions and recalculate the route
map.removeOverlay(dirLine)
calcRoute();
});

This works quite well but does not look as smooth as Google’s solution.
The problem is that while you are dragging a vertex only that bit of the GPolyline moves and the rest stays in its original position. which makes the shape of your directions quite awkward while you are dragging. Unforunately the GPolyline does not come with a “startdragging” event, otherwise we could just recalculate the route every few seconds while the vertex is being dragged.

This is not the most elegant of solutions but it does the job.