--- a/Applications/Samples/SimpleViewerApplication.h	Mon Sep 10 12:22:26 2018 +0200
+++ b/Applications/Samples/SimpleViewerApplication.h	Fri Sep 14 16:44:01 2018 +0200
@@ -204,12 +204,7 @@
         wasmViewport1_(NULL),
         wasmViewport2_(NULL)
       {
-        DeclareIgnoredMessage(MessageType_Widget_ContentChanged);
-        DeclareHandledMessage(MessageType_Widget_GeometryChanged);
-
-        DeclareHandledMessage(MessageType_OrthancApi_GetStudyIds_Ready);
-        DeclareHandledMessage(MessageType_OrthancApi_GetStudy_Ready);
-        DeclareHandledMessage(MessageType_OrthancApi_GetSeries_Ready);
+//        DeclareIgnoredMessage(MessageType_Widget_ContentChanged);
       }
 
       virtual void Finalize() {}
@@ -249,7 +244,7 @@
           thumbnailsLayout_->SetVertical();
 
           mainWidget_ = new LayerWidget(broker_, "main-viewport");
-          mainWidget_->RegisterObserver(*this);
+          //mainWidget_->RegisterObserver(*this);
 
           // hierarchy
           mainLayout_->AddWidget(thumbnailsLayout_);
@@ -273,7 +268,7 @@
         if (parameters.count("studyId") < 1)
         {
           LOG(WARNING) << "The study ID is missing, will take the first studyId found in Orthanc";
-          orthancApiClient_->ScheduleGetStudyIds(*this);
+          orthancApiClient_->GetJsonAsync("/studies", new Callable<SimpleViewerApplication, OrthancApiClient::NewGetJsonResponseReadyMessage>(*this, &SimpleViewerApplication::OnStudyListReceived));
         }
         else
         {
@@ -281,26 +276,32 @@
         }
       }
 
-      void OnStudyListReceived(const Json::Value& response)
+      void OnStudyListReceived(const OrthancApiClient::NewGetJsonResponseReadyMessage& message)
       {
+        const Json::Value& response = message.response_;
+
         if (response.isArray() && response.size() > 1)
         {
           SelectStudy(response[0].asString());
         }
       }
-      void OnStudyReceived(const Json::Value& response)
+      void OnStudyReceived(const OrthancApiClient::NewGetJsonResponseReadyMessage& message)
       {
+        const Json::Value& response = message.response_;
+
         if (response.isObject() && response["Series"].isArray())
         {
           for (size_t i=0; i < response["Series"].size(); i++)
           {
-            orthancApiClient_->ScheduleGetSeries(*this, response["Series"][(int)i].asString());
+            orthancApiClient_->GetJsonAsync("/series/" + response["Series"][(int)i].asString(), new Callable<SimpleViewerApplication, OrthancApiClient::NewGetJsonResponseReadyMessage>(*this, &SimpleViewerApplication::OnSeriesReceived));
           }
         }
       }
 
-      void OnSeriesReceived(const Json::Value& response)
+      void OnSeriesReceived(const OrthancApiClient::NewGetJsonResponseReadyMessage& message)
       {
+        const Json::Value& response = message.response_;
+
         if (response.isObject() && response["Instances"].isArray() && response["Instances"].size() > 0)
         {
           // keep track of all instances IDs
@@ -330,34 +331,19 @@
         LayerWidget* thumbnailWidget = new LayerWidget(broker_, "thumbnail-series-" + seriesId);
         thumbnails_.push_back(thumbnailWidget);
         thumbnailsLayout_->AddWidget(thumbnailWidget);
-        thumbnailWidget->RegisterObserver(*this);
+        thumbnailWidget->RegisterObserverCallback(new Callable<SimpleViewerApplication, LayerWidget::GeometryChangedMessage>(*this, &SimpleViewerApplication::OnWidgetGeometryChanged));
         thumbnailWidget->AddLayer(smartLoader_->GetFrame(instanceId, 0));
         thumbnailWidget->SetInteractor(*thumbnailInteractor_);
       }
 
       void SelectStudy(const std::string& studyId)
       {
-        orthancApiClient_->ScheduleGetStudy(*this, studyId);
+        orthancApiClient_->GetJsonAsync("/studies/" + studyId, new Callable<SimpleViewerApplication, OrthancApiClient::NewGetJsonResponseReadyMessage>(*this, &SimpleViewerApplication::OnStudyReceived));
       }
 
-      virtual void HandleMessage(IObservable& from, const IMessage& message) {
-        switch (message.GetType()) {
-        case MessageType_Widget_GeometryChanged:
-          LOG(INFO) << "Widget geometry ready: " << dynamic_cast<LayerWidget&>(from).GetName();
-          dynamic_cast<LayerWidget&>(from).SetDefaultView();
-          break;
-        case MessageType_OrthancApi_GetStudyIds_Ready:
-          OnStudyListReceived(dynamic_cast<const OrthancApiClient::GetJsonResponseReadyMessage&>(message).response_);
-          break;
-        case MessageType_OrthancApi_GetSeries_Ready:
-          OnSeriesReceived(dynamic_cast<const OrthancApiClient::GetJsonResponseReadyMessage&>(message).response_);
-          break;
-        case MessageType_OrthancApi_GetStudy_Ready:
-          OnStudyReceived(dynamic_cast<const OrthancApiClient::GetJsonResponseReadyMessage&>(message).response_);
-          break;
-        default:
-          VLOG("unhandled message type" << message.GetType());
-        }
+      void OnWidgetGeometryChanged(const LayerWidget::GeometryChangedMessage& message)
+      {
+        message.origin_.SetDefaultView();
       }
 
       void SelectSeriesInMainViewport(const std::string& seriesId)