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Thursday, February 03, 2011

GWT and Delayed Execution for Google Maps Geocoding

Thursday, February 03, 2011 Posted by Ismail Habib , 58 comments
Google Maps provide us with an asynchronous way of doing a geocoding, which is pretty much how GWT is dealing with all type of client-server communication. However, sometimes we need a way to synchronize them. By synchronizing I mean waiting for the required asynchronous call to return a value before continue with other execution. Using something like Timer to wait for a reply doesn't work since it will only block the whole process on the browser since JavaScript interpreter is single-threaded. Fortunately, there is a way to deal with it within GWT by using DeferredCommand.

I'm going to take an example of using Geocoding from Google Maps. What I would like to do is geocode two address into latitude, longitude coordinate and use them as parameters for another method.

Geocoder geocode = new Geocoder();
final Callback firstCallback = new Callbak();
geocode.getLatLng("Jl. Tb Ismail Bandung Indonesia", firstCallback);
final Callback secondCallback = new Callbak();
geocode.getLatLng("Jl. Ganesha Bandung Indonesia", secondCallback);
final Command command = new Command() {
   public void execute(){
      if ((firstCallback.location != null) 
            && (secondCallback.location != null)) {
         //call the method
         anotherMethod(firstCallback.location, secondCallback.location);
      } else {
         DeferredCommand.addCommand(this); //delay execution

With "Callback" as a class to temporarily store value from geocode.

class Callback implements LatLngCallback {
   public LatLng location;

   public void onFailure() {
      //put something here

   public void onSuccess(LatLng point) {
      location = point;

This works for me well, however, I do realize that DeferredCommand is now deprecated. Any other solution?