Implement a cache for item content renders

Speeds up redraw of items, making use of layout designer much
faster with slow to redraw items.

This will also make it possible to use live effects on layout
items without killing performance of the designer.

src/core/layout/qgslayoutitem.cpp

@@ -20,6 +20,8 @@
 #include <QPainter>
 #include <QStyleOptionGraphicsItem>
 
+#define CACHE_SIZE_LIMIT 5000
+
 QgsLayoutItem::QgsLayoutItem( QgsLayout *layout )
   : QgsLayoutObject( layout )
   , QGraphicsRectItem( 0 )
@@ -44,23 +46,89 @@ void QgsLayoutItem::paint( QPainter *painter, const QStyleOptionGraphicsItem *it
   }
 
   //TODO - remember to disable saving/restoring on graphics view!!
-  painter->save();
-  preparePainter( painter );
 
   if ( shouldDrawDebugRect() )
   {
     drawDebugRect( painter );
+    return;
+  }
+
+  double destinationDpi = itemStyle->matrix.m11() * 25.4;
+  bool useImageCache = true;
+
+  if ( useImageCache )
+  {
+    double widthInPixels = boundingRect().width() * itemStyle->matrix.m11();
+    double heightInPixels = boundingRect().height() * itemStyle->matrix.m11();
+
+    // limit size of image for better performance
+    double scale = 1.0;
+    if ( widthInPixels > CACHE_SIZE_LIMIT || heightInPixels > CACHE_SIZE_LIMIT )
+    {
+      if ( widthInPixels > heightInPixels )
+      {
+        scale = widthInPixels / CACHE_SIZE_LIMIT;
+        widthInPixels = CACHE_SIZE_LIMIT;
+        heightInPixels /= scale;
+      }
+      else
+      {
+        scale = heightInPixels / CACHE_SIZE_LIMIT;
+        heightInPixels = CACHE_SIZE_LIMIT;
+        widthInPixels /= scale;
+      }
+      destinationDpi = destinationDpi / scale;
+    }
+
+    if ( !mItemCachedImage.isNull() && qgsDoubleNear( mItemCacheDpi, destinationDpi ) )
+    {
+      // can reuse last cached image
+      QgsRenderContext context = QgsLayoutUtils::createRenderContextForMap( nullptr, painter, destinationDpi );
+      painter->save();
+      preparePainter( painter );
+      double cacheScale = destinationDpi / mItemCacheDpi;
+      painter->scale( cacheScale / context.scaleFactor(), cacheScale / context.scaleFactor() );
+      painter->drawImage( boundingRect().x() * context.scaleFactor() / cacheScale,
+                          boundingRect().y() * context.scaleFactor() / cacheScale, mItemCachedImage );
+      painter->restore();
+      return;
+    }
+    else
+    {
+      mItemCacheDpi = destinationDpi;
+
+      mItemCachedImage = QImage( widthInPixels, heightInPixels, QImage::Format_ARGB32 );
+      mItemCachedImage.fill( Qt::transparent );
+      mItemCachedImage.setDotsPerMeterX( 1000 * destinationDpi * 25.4 );
+      mItemCachedImage.setDotsPerMeterY( 1000 * destinationDpi * 25.4 );
+      QPainter p( &mItemCachedImage );
+
+      preparePainter( &p );
+      QgsRenderContext context = QgsLayoutUtils::createRenderContextForMap( nullptr, &p, destinationDpi );
+      // painter is already scaled to dots
+      // need to translate so that item origin is at 0,0 in painter coordinates (not bounding rect origin)
+      p.translate( -boundingRect().x() * context.scaleFactor(), -boundingRect().y() * context.scaleFactor() );
+      draw( context, itemStyle );
+      p.end();
+
+      painter->save();
+      // scale painter from mm to dots
+      painter->scale( 1.0 / context.scaleFactor(), 1.0 / context.scaleFactor() );
+      painter->drawImage( boundingRect().x() * context.scaleFactor(),
+                          boundingRect().y() * context.scaleFactor(), mItemCachedImage );
+      painter->restore();
+    }
   }
   else
   {
-    double destinationDpi = itemStyle->matrix.m11() * 25.4;
+    // no caching or flattening
+    painter->save();
     QgsRenderContext context = QgsLayoutUtils::createRenderContextForMap( nullptr, painter, destinationDpi );
     // scale painter from mm to dots
     painter->scale( 1.0 / context.scaleFactor(), 1.0 / context.scaleFactor() );
     draw( context, itemStyle );
+    painter->restore();
   }
-
-  painter->restore();
 }
 
 void QgsLayoutItem::setReferencePoint( const QgsLayoutItem::ReferencePoint &point )

src/core/layout/qgslayoutitem.h

@@ -252,6 +252,9 @@ class CORE_EXPORT QgsLayoutItem : public QgsLayoutObject, public QGraphicsRectIt
     QgsLayoutPoint mItemPosition;
     double mItemRotation = 0.0;
 
+    QImage mItemCachedImage;
+    double mItemCacheDpi = -1;
+
     void initConnectionsToLayout();
 
     //! Prepares a painter by setting rendering flags

src/core/layout/qgslayoutitemregistry.cpp

@@ -95,6 +95,16 @@ TestLayoutItem::TestLayoutItem( QgsLayout *layout )
   int s = ( qrand() % ( 200 - 100 + 1 ) ) + 100;
   int v = ( qrand() % ( 130 - 255 + 1 ) ) + 130;
   mColor = QColor::fromHsv( h, s, v );
+
+  QgsStringMap properties;
+  properties.insert( QStringLiteral( "color" ), mColor.name() );
+  properties.insert( QStringLiteral( "style" ), QStringLiteral( "solid" ) );
+  properties.insert( QStringLiteral( "style_border" ), QStringLiteral( "solid" ) );
+  properties.insert( QStringLiteral( "color_border" ), QStringLiteral( "black" ) );
+  properties.insert( QStringLiteral( "width_border" ), QStringLiteral( "0.3" ) );
+  properties.insert( QStringLiteral( "joinstyle" ), QStringLiteral( "miter" ) );
+  mShapeStyleSymbol = QgsFillSymbol::createSimple( properties );
+
 }
 
 void TestLayoutItem::draw( QgsRenderContext &context, const QStyleOptionGraphicsItem *itemStyle )
@@ -114,9 +124,15 @@ void TestLayoutItem::draw( QgsRenderContext &context, const QStyleOptionGraphics
   painter->setBrush( mColor );
 
   double scale = context.convertToPainterUnits( 1, QgsUnitTypes::RenderMillimeters );
-  QRectF r = QRectF( rect().left() * scale, rect().top() * scale,
-                     rect().width() * scale, rect().height() * scale );
-  painter->drawRect( r );
+
+  QPolygonF shapePolygon = QPolygonF( QRectF( 0, 0, rect().width() * scale, rect().height() * scale ) );
+  QList<QPolygonF> rings; //empty list
+
+  mShapeStyleSymbol->startRender( context );
+  mShapeStyleSymbol->renderPolygon( shapePolygon, &rings, nullptr, context );
+  mShapeStyleSymbol->stopRender( context );
+
+// painter->drawRect( r );
   painter->restore();
   stack.end( context );
 }

src/core/layout/qgslayoutitemregistry.h

@@ -29,6 +29,7 @@
 class QgsLayout;
 class QgsLayoutView;
 class QgsLayoutItem;
+class QgsFillSymbol;
 
 /**
  * \ingroup core
@@ -271,8 +272,10 @@ class TestLayoutItem : public QgsLayoutItem
 
   private:
     QColor mColor;
+    QgsFillSymbol *mShapeStyleSymbol = nullptr;
 };
 
+
 ///@endcond
 #endif
 

src/gui/layout/qgslayoutviewtooladditem.cpp

@@ -86,7 +86,8 @@ void QgsLayoutViewToolAddItem::layoutReleaseEvent( QgsLayoutViewMouseEvent *even
   Q_UNUSED( clickOnly );
 
   QgsLayoutItem *item = QgsApplication::layoutItemRegistry()->createItem( mItemType, layout() );
-  item->setRect( rect );
+  item->attemptResize( QgsLayoutSize( rect.width(), rect.height(), QgsUnitTypes::LayoutMillimeters ) );
+  item->attemptMove( QgsLayoutPoint( rect.left(), rect.top(), QgsUnitTypes::LayoutMillimeters ) );
   layout()->addItem( item );
 }