[composer] Always update map caches when zoom changes

Before this would only happen with the "render" mode, not "cache"
mode. Now updating caches is so responsive and transparent to users
we can safely trigger the background cache update whenever the
zoom changes without any performance impact.

src/app/composer/qgscomposer.cpp

@@ -763,6 +763,8 @@ void QgsComposer::connectViewSlots()
   //listen out for position updates from the QgsComposerView
   connect( mView, &QgsComposerView::cursorPosChanged, this, &QgsComposer::updateStatusCursorPos );
   connect( mView, &QgsComposerView::zoomLevelChanged, this, &QgsComposer::updateStatusZoom );
+
+  connect( mView, &QgsComposerView::zoomLevelChanged, this, &QgsComposer::invalidateCachedRenders );
 }
 
 void QgsComposer::connectCompositionSlots()
@@ -774,7 +776,6 @@ void QgsComposer::connectCompositionSlots()
 
   connect( mComposition, &QgsComposition::nameChanged, this, &QgsComposer::setWindowTitle );
   connect( mComposition, &QgsComposition::selectedItemChanged, this, &QgsComposer::showItemOptions );
-  connect( mComposition, &QgsComposition::itemAdded, this, &QgsComposer::compositionItemAdded );
   connect( mComposition, &QgsComposition::itemRemoved, this, &QgsComposer::deleteItem );
   connect( mComposition, &QgsComposition::paperSizeChanged, this, [ = ]
   {
@@ -802,6 +803,7 @@ void QgsComposer::connectOtherSlots()
   connect( mVerticalRuler, &QgsComposerRuler::cursorPosChanged, this, &QgsComposer::updateStatusCursorPos );
   //listen out for zoom updates
   connect( this, &QgsComposer::zoomLevelChanged, this, &QgsComposer::updateStatusZoom );
+  connect( this, &QgsComposer::zoomLevelChanged, this, &QgsComposer::invalidateCachedRenders );
 }
 
 void QgsComposer::open()
@@ -1007,11 +1009,15 @@ void QgsComposer::atlasFeatureChanged( QgsFeature *feature )
   mapCanvas()->expressionContextScope().addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "atlas_geometry" ), QVariant::fromValue( atlasFeature.geometry() ), true ) );
 }
 
-void QgsComposer::compositionItemAdded( QgsComposerItem *item )
+void QgsComposer::invalidateCachedRenders()
 {
-  if ( item && item->type() == QgsComposerItem::ComposerMap )
+  //redraw cached map items
+  QList< QgsComposerMap *> maps;
+  mComposition->composerItems( maps );
+
+  Q_FOREACH ( QgsComposerMap *map, maps )
   {
-    connect( this, &QgsComposer::zoomLevelChanged, static_cast< QgsComposerMap *>( item ), &QgsComposerMap::renderModeUpdateCachedImage );
+    map->invalidateCache();
   }
 }
 

src/app/composer/qgscomposer.h

@@ -614,7 +614,7 @@ class QgsComposer: public QMainWindow, private Ui::QgsComposerBase
 
     void atlasFeatureChanged( QgsFeature *feature );
 
-    void compositionItemAdded( QgsComposerItem *item );
+    void invalidateCachedRenders();
 
 
 

src/gui/qgscomposerview.cpp

@@ -2054,16 +2054,6 @@ void QgsComposerView::wheelZoom( QWheelEvent *event )
   emit zoomLevelChanged();
   updateRulers();
   update();
-  //redraw cached map items
-  QList<QGraphicsItem *> itemList = composition()->items();
-  QList<QGraphicsItem *>::iterator itemIt = itemList.begin();
-  for ( ; itemIt != itemList.end(); ++itemIt )
-  {
-    if ( QgsComposerMap *mypItem = dynamic_cast<QgsComposerMap *>( *itemIt ) )
-    {
-      mypItem->renderModeUpdateCachedImage();
-    }
-  }
 }
 
 void QgsComposerView::setZoomLevel( double zoomLevel )