[FEATURE] First experiment with preloading

After main map canvas render is complete, render a separate image
for each adjacent map "tile" (in the background). These are shown
when panning the map to display a preview of what will be visible
when the panning operation ends.

src/gui/qgsmapcanvas.cpp

@@ -503,6 +503,7 @@ void QgsMapCanvas::refreshMap()
   QgsDebugMsgLevel( "CANVAS refresh!", 3 );
 
   stopRendering(); // if any...
+  stopPreviewJobs();
 
   //build the expression context
   QgsExpressionContext expressionContext;
@@ -624,6 +625,7 @@ void QgsMapCanvas::rendererJobFinished()
     p.end();
 
     mMap->setContent( img, imageRect( img, mSettings ) );
+    startPreviewJobs();
   }
 
   // now we are in a slot called from mJob - do not delete it immediately
@@ -634,6 +636,20 @@ void QgsMapCanvas::rendererJobFinished()
   emit mapCanvasRefreshed();
 }
 
+void QgsMapCanvas::previewJobFinished()
+{
+  QgsMapRendererQImageJob *job = qobject_cast<QgsMapRendererQImageJob *>( sender() );
+  if ( !job )
+  {
+    return;
+  }
+
+  if ( mMap )
+  {
+    mMap->addPreviewImage( job->renderedImage(), job->mapSettings().extent() );
+  }
+}
+
 QgsRectangle QgsMapCanvas::imageRect( const QImage &img, const QgsMapSettings &mapSettings )
 {
   // This is a hack to pass QgsMapCanvasItem::setRect what it
@@ -2109,3 +2125,54 @@ const QgsLabelingEngineSettings &QgsMapCanvas::labelingEngineSettings() const
 {
   return mSettings.labelingEngineSettings();
 }
+
+void QgsMapCanvas::startPreviewJobs()
+{
+  stopPreviewJobs(); //just in case still running
+
+  QgsRectangle mapRect = mSettings.visibleExtent();
+
+  for ( int j = 0; j < 3; ++j )
+  {
+    for ( int i = 0; i < 3; ++i )
+    {
+      if ( i == 1 && j == 1 )
+      {
+        continue;
+      }
+
+
+      //copy settings, only update extent
+      QgsMapSettings jobSettings = mSettings;
+
+      double dx = ( i - 1 ) * mapRect.width();
+      double dy = ( 1 - j ) * mapRect.height();
+      QgsRectangle jobExtent = mapRect;
+      jobExtent.setXMaximum( jobExtent.xMaximum() + dx );
+      jobExtent.setXMinimum( jobExtent.xMinimum() + dx );
+      jobExtent.setYMaximum( jobExtent.yMaximum() + dy );
+      jobExtent.setYMinimum( jobExtent.yMinimum() + dy );
+
+      jobSettings.setExtent( jobExtent );
+
+      QgsMapRendererQImageJob *job = new QgsMapRendererParallelJob( jobSettings );
+      mPreviewJobs.append( job );
+      connect( job, SIGNAL( finished() ), this, SLOT( previewJobFinished() ) );
+      job->start();
+    }
+  }
+}
+
+void QgsMapCanvas::stopPreviewJobs()
+{
+  QList< QgsMapRendererQImageJob * >::iterator it = mPreviewJobs.begin();
+  for ( ; it != mPreviewJobs.end(); ++it )
+  {
+    if ( *it )
+    {
+      ( *it )->cancel();
+    }
+    delete ( *it );
+  }
+  mPreviewJobs.clear();
+}

src/gui/qgsmapcanvas.h

@@ -586,6 +586,9 @@ class GUI_EXPORT QgsMapCanvas : public QGraphicsView
     //! called when a renderer job has finished successfully or when it was canceled
     void rendererJobFinished();
 
+    //! called when a preview job has been finished
+    void previewJobFinished();
+
     void mapUpdateTimeout();
 
     void refreshMap();
@@ -820,6 +823,8 @@ class GUI_EXPORT QgsMapCanvas : public QGraphicsView
 
     QgsSnappingUtils *mSnappingUtils = nullptr;
 
+    QList< QgsMapRendererQImageJob * > mPreviewJobs;
+
     //! lock the scale, so zooming can be performed using magnication
     bool mScaleLocked;
 
@@ -868,6 +873,9 @@ class GUI_EXPORT QgsMapCanvas : public QGraphicsView
 
     void setLayersPrivate( const QList<QgsMapLayer *> &layers );
 
+    void startPreviewJobs();
+    void stopPreviewJobs();
+
     friend class TestQgsMapCanvas;
 
 }; // class QgsMapCanvas

src/gui/qgsmapcanvasmap.cpp

@@ -16,7 +16,9 @@
 #include "qgslogger.h"
 #include "qgsmapcanvas.h"
 #include "qgsmapcanvasmap.h"
+#include "qgsmaprendererjob.h"
 #include "qgsmapsettings.h"
+#include "qgsmaplayer.h"
 
 #include <QPainter>
 
@@ -30,6 +32,8 @@ QgsMapCanvasMap::QgsMapCanvasMap( QgsMapCanvas *canvas )
 
 void QgsMapCanvasMap::setContent( const QImage &image, const QgsRectangle &rect )
 {
+  mPreviewImages.clear();
+
   mImage = image;
 
   // For true retro fans: this is approximately how the graphics looked like in 1990
@@ -40,16 +44,45 @@ void QgsMapCanvasMap::setContent( const QImage &image, const QgsRectangle &rect
   setRect( rect );
 }
 
+void QgsMapCanvasMap::addPreviewImage( const QImage &image, const QgsRectangle &rect )
+{
+  mPreviewImages.append( qMakePair( image, rect ) );
+  update();
+}
+
+QRectF QgsMapCanvasMap::boundingRect() const
+{
+  double width = mItemSize.width();
+  double height = mItemSize.height();
+
+  return QRectF( -width, -height, 3 * width, 3 * height );
+}
+
 void QgsMapCanvasMap::paint( QPainter *painter )
 {
-  int w = qRound( boundingRect().width() ) - 2, h = qRound( boundingRect().height() ) - 2; // setRect() makes the size +2 :-(
+  int w = qRound( mItemSize.width() ) - 2, h = qRound( mItemSize.height() ) - 2; // setRect() makes the size +2 :-(
   if ( mImage.size() != QSize( w, h ) )
   {
     QgsDebugMsg( QString( "map paint DIFFERENT SIZE: img %1,%2  item %3,%4" ).arg( mImage.width() ).arg( mImage.height() ).arg( w ).arg( h ) );
     // This happens on zoom events when ::paint is called before
     // the renderer has completed
   }
 
+  /*Offset between 0/0 and mRect.xMinimum/mRect.yMinimum.
+  We need to consider the offset, because mRect is not updated yet and there might be an offset*/
+  QgsPoint pt = toMapCoordinates( QPoint( 0, 0 ) );
+  double offsetX = pt.x() - mRect.xMinimum();
+  double offsetY = pt.y() - mRect.yMaximum();
+
+  //draw preview images first
+  QMap< QgsRectangle, QImage >::const_iterator previewIt = mPreviewImages.constBegin();
+  for ( ; previewIt != mPreviewImages.constEnd(); ++previewIt )
+  {
+    QPointF ul = toCanvasCoordinates( QgsPoint( previewIt.key().xMinimum() + offsetX, previewIt.key().yMaximum() + offsetY ) );
+    QPointF lr = toCanvasCoordinates( QgsPoint( previewIt.key().xMaximum() + offsetX, previewIt.key().yMinimum() + offsetY ) );
+    painter->drawImage( QRectF( ul.x(), ul.y(), lr.x() - ul.x(), lr.y() - ul.y() ), previewIt.value(), QRect( 0, 0, previewIt.value().width(), previewIt.value().height() ) );
+  }
+
   painter->drawImage( QRect( 0, 0, w, h ), mImage );
 
   // For debugging:

src/gui/qgsmapcanvasmap.h

@@ -45,9 +45,16 @@ class QgsMapCanvasMap : public QgsMapCanvasItem
 
     virtual void paint( QPainter *painter ) override;
 
+    void addPreviewImage( const QImage &image, const QgsRectangle &rect );
+
+    QRectF boundingRect() const;
+
   private:
 
     QImage mImage;
+
+    //! Preview images for panning. Usually cover area around the rendered image
+    QList< QPair< QImage, QgsRectangle > > mPreviewImages;
 };
 
 /// @endcond