[Qgis-developer] Rendering single layer from mapcanvas

[bonar]

Hi guys, Tim & Dr.Marco 

I was tried to display real-time data in my mapcanvas. In my mapcanvas, i
have 4 layer, but i juz want to refresh 1 layer only(this layer retrieve
data from PostGis) from the mapcanvas without refreshing all layer in the
mapcanvas. I already searching from this forum and Google it, and the result
is i should use QgsMapCanvasItem that have that capabilities to rendering
single layer without refreshing mapcanvas. But, i don't know how to
implement it in my application. I'm using sample application from Mr.Tim
Sutton, it is a great tutorial for a beginner like me, and i think its
better to include the code in here. This is what i already done: 


//Setting my layer 

mpMapCanvas= new QgsMapCanvas(0, 0); 
  mpMapCanvas->enableAntiAliasing(true); 
  mpMapCanvas->useImageToRender(true); 
  mpMapCanvas->setCanvasColor(QColor(255, 255, 255)); 
  mpMapCanvas->freeze(false); 
  mpMapCanvas->setVisible(true); 

  QString myLayerPathNegeri         = LAYERPATH() + "klkv_state.shp"; 
  QString myLayerBaseNameNegeri     = "klkv_state"; 
  QString myProviderName            = "ogr"; 

  QString myLayerPathDistrict         = LAYERPATH() + "klkv_district.shp"; 
  QString myLayerBaseNameDistrict     = "klkv_district"; 

  QString myLayerPathMukim         = LAYERPATH() + "klkv_mukim.shp"; 
  QString myLayerBaseNameMukim     = "klkv_mukim"; 

QString myLayerPathPoi         = "location_poi"; 
  QString myLayerBaseNamePoi     = "location_poi"; 

QgsVectorLayer * mypLayerNegeri = new QgsVectorLayer(myLayerPathNegeri,
myLayerBaseNameNegeri, myProviderName); 

  QgsVectorLayer * mypLayerDistrict = new
QgsVectorLayer(myLayerPathDistrict, myLayerBaseNameDistrict,
myProviderName); 

  QgsVectorLayer * mypLayerMukim = new QgsVectorLayer(myLayerPathMukim,
myLayerBaseNameMukim, myProviderName); 

  QgsVectorLayer * mypLayerPoi = new
QgsVectorLayer(POSTGIS_LAYER_SETUP_INCIDENT_POI_FUNC(), myLayerBaseNamePoi,
"postgres"); 


QgsSingleSymbolRenderer *mypRendererNegeri = new
QgsSingleSymbolRenderer(mypLayerNegeri->geometryType()); 
  QList<QgsMapCanvasLayer> myLayerSet; 
  mypLayerNegeri->setRenderer(mypRendererNegeri); 

  QgsSingleSymbolRenderer *mypRendererDistrict = new
QgsSingleSymbolRenderer(mypLayerDistrict->geometryType()); 
  mypLayerDistrict->setRenderer(mypRendererDistrict); 

  QgsSingleSymbolRenderer *mypRendererMukim = new
QgsSingleSymbolRenderer(mypLayerMukim->geometryType()); 
  mypLayerMukim->setRenderer(mypRendererMukim); 


QgsSingleSymbolRenderer *mypRendererPoi = new
QgsSingleSymbolRenderer(mypLayerPoi->geometryType()); 
  mypLayerPoi->setRenderer(mypRendererPoi); 


QgsMapLayerRegistry::instance()->addMapLayer(mypLayerNegeri, TRUE); 
  QgsMapLayerRegistry::instance()->addMapLayer(mypLayerDistrict, TRUE); 
  QgsMapLayerRegistry::instance()->addMapLayer(mypLayerMukim, TRUE); 
QgsMapLayerRegistry::instance()->addMapLayer(mypLayerPoi, TRUE); 

myLayerSet.append(QgsMapCanvasLayer(mypLayerPoi)); 
myLayerSet.append(QgsMapCanvasLayer(mypLayerMukim)); 
  myLayerSet.append(QgsMapCanvasLayer(mypLayerDistrict)); 
  myLayerSet.append(QgsMapCanvasLayer(mypLayerNegeri)); 

mpMapCanvas->setLayerSet(myLayerSet); 


//I make a Qtimer to check if any update position for mypLayerPoi from
updatingLocationPoiLayer() function 

QTimer *timerPoiRenderer = new QTimer(this); 
  connect(timerPoiRenderer, SIGNAL(timeout()), this,
SLOT(updatingLocationPoiLayer())); 
  timerPoiRenderer->start(10 * 1000);// 10 seconds 


//this is how i tried to using QgsMapCanvasItem, sorry for the code because
i am beginner for this QGis, so i juz try to play-around with the
QgsMapCanvasItem code . 

void MyMain::updatingLocationPoiLayer() 
{ 

  QGraphicsScene *mScene = new QGraphicsScene(); 
  QgsMapCanvasMap* mMap = new QgsMapCanvasMap(mpMapCanvas); 
  mScene->addItem( mMap ); 
  mScene->update(); 
  QList<QGraphicsItem*> list = mScene->items(); 
  QList<QGraphicsItem*>::iterator it = list.begin(); 

  while ( it != list.end() ){ 
          QgsMapCanvasItem* item = dynamic_cast<QgsMapCanvasItem*>( *it ); 

          if(item){ 
                  item->updatePosition(); 
                  QMessageBox::information( this, tr( "TEST" ), tr( "Update
rendering" ) ); 
          }else{ 
//it will goes here. 
                  QMessageBox::information( this, tr( "TEST" ), tr( "NOT
Update rendering" ) ); 
          } 

          it++; 
  } 
} 

from my reading,what i understand is i should use
QgsMapCanvasItem::updatePosition() to make my mapcanvas look like real-time.
Is it true? And i really hope you guys can teach me about how to implement
it in my code. 


Thanks in advance, 
Bonar
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