Fix regeneration of cached maps when zoom level changes

python/core/layout/qgslayoutitem.sip

@@ -551,6 +551,11 @@ class QgsLayoutItem : QgsLayoutObject, QGraphicsRectItem, QgsLayoutUndoObjectInt
  recalculation of its position and size.
 %End
 
+    virtual void invalidateCache();
+%Docstring
+ Forces a deferred update of any cached image the item uses.
+%End
+
     virtual void redraw();
 %Docstring
  Triggers a redraw (update) of the item.

python/core/layout/qgslayoutitemmap.sip

@@ -447,10 +447,8 @@ Is emitted when the map has been prepared for atlas rendering, just before actua
 
   public slots:
 
-    void invalidateCache();
-%Docstring
- Forces a deferred update of the cached map image on next paint.
-%End
+    virtual void invalidateCache();
+
 
     void updateBoundingRect();
 %Docstring

src/core/layout/qgslayoutitem.cpp

@@ -945,6 +945,16 @@ void QgsLayoutItem::refresh()
   refreshDataDefinedProperty();
 }
 
+void QgsLayoutItem::invalidateCache()
+{
+  if ( !mItemCachedImage.isNull() )
+  {
+    mItemCachedImage = QImage();
+    mItemCacheDpi = -1;
+    update();
+  }
+}
+
 void QgsLayoutItem::redraw()
 {
   update();

src/core/layout/qgslayoutitem.h

@@ -548,6 +548,11 @@ class CORE_EXPORT QgsLayoutItem : public QgsLayoutObject, public QGraphicsRectIt
      */
     void refresh() override;
 
+    /**
+     * Forces a deferred update of any cached image the item uses.
+     */
+    virtual void invalidateCache();
+
     /**
      * Triggers a redraw (update) of the item.
      */

src/core/layout/qgslayoutitemmap.h

@@ -495,10 +495,7 @@ class CORE_EXPORT QgsLayoutItemMap : public QgsLayoutItem
 
   public slots:
 
-    /**
-     * Forces a deferred update of the cached map image on next paint.
-     */
-    void invalidateCache();
+    void invalidateCache() override;
 
     //! Updates the bounding rect of this item. Call this function before doing any changes related to annotation out of the map rectangle
     void updateBoundingRect();

src/gui/layout/qgslayoutitemwidget.cpp

@@ -327,20 +327,8 @@ void QgsLayoutItemPropertiesWidget::mBackgroundColorButton_colorChanged( const Q
   }
   mItem->layout()->undoStack()->beginCommand( mItem, tr( "Change Background Color" ), QgsLayoutItem::UndoBackgroundColor );
   mItem->setBackgroundColor( newBackgroundColor );
-
-#if 0 //TODO
-  //if the item is a composer map, we need to regenerate the map image
-  //because it usually is cached
-  if ( QgsComposerMap *cm = qobject_cast<QgsComposerMap *>( mItem ) )
-  {
-    cm->invalidateCache();
-  }
-  else
-  {
-    mItem->updateItem();
-  }
-#endif
   mItem->layout()->undoStack()->endCommand();
+  mItem->invalidateCache();
 }
 
 void QgsLayoutItemPropertiesWidget::changeItemPosition()
@@ -477,20 +465,8 @@ void QgsLayoutItemPropertiesWidget::mBackgroundGroupBox_toggled( bool state )
 
   mItem->layout()->undoStack()->beginCommand( mItem, state ? tr( "Enable Background" ) : tr( "Disable Background" ) );
   mItem->setBackgroundEnabled( state );
-
-#if 0 //TODO
-  //if the item is a composer map, we need to regenerate the map image
-  //because it usually is cached
-  if ( QgsComposerMap *cm = qobject_cast<QgsComposerMap *>( mItem ) )
-  {
-    cm->invalidateCache();
-  }
-  else
-  {
-    mItem->updateItem();
-  }
-#endif
   mItem->layout()->undoStack()->endCommand();
+  mItem->invalidateCache();
 }
 
 

src/gui/layout/qgslayoutview.cpp

@@ -58,6 +58,8 @@ QgsLayoutView::QgsLayoutView( QWidget *parent )
 
   mPreviewEffect = new QgsPreviewEffect( this );
   viewport()->setGraphicsEffect( mPreviewEffect );
+
+  connect( this, &QgsLayoutView::zoomLevelChanged, this, &QgsLayoutView::invalidateCachedRenders );
 }
 
 QgsLayout *QgsLayoutView::currentLayout()
@@ -899,6 +901,21 @@ void QgsLayoutView::scrollContentsBy( int dx, int dy )
   viewChanged();
 }
 
+void QgsLayoutView::invalidateCachedRenders()
+{
+  if ( !currentLayout() )
+    return;
+
+  //redraw cached map items
+  QList< QgsLayoutItem *> items;
+  currentLayout()->layoutItems( items );
+
+  for ( QgsLayoutItem *item : qgis::as_const( items ) )
+  {
+    item->invalidateCache();
+  }
+}
+
 void QgsLayoutView::viewChanged()
 {
   if ( mHorizontalRuler )

src/gui/layout/qgslayoutview.h

@@ -451,6 +451,8 @@ class GUI_EXPORT QgsLayoutView: public QGraphicsView
 
   private slots:
 
+    void invalidateCachedRenders();
+
   private:
 
     //! Zoom layout from a mouse wheel event