[composer] Don't redraw map if layers change and map is set to cached

Previously map was being redrawn every time a layer was added or
removed from the registry, which blocked the ui until the map
redraw is complete.

Now the map will only be redrawn as a result of layer changes
if it's set to Render mode.

Possible fix for 12234 and 12125. (refs #12234, #12125)

Author: nyalldawson

python/core/composer/qgscomposermap.sip

@@ -757,9 +757,12 @@ class QgsComposerMap : QgsComposerItem
 
   public slots:
 
-    /**Called if map canvas has changed*/
+    /**Forces an update of the cached map image*/
     void updateCachedImage();
-    /**Call updateCachedImage if item is in render mode*/
+
+    /**Updates the cached map image if the map is set to Render mode
+     * @see updateCachedImage
+    */
     void renderModeUpdateCachedImage();
 
     /**Updates the bounding rect of this item. Call this function before doing any changes related to annotation out of the map rectangle */
@@ -769,4 +772,13 @@ class QgsComposerMap : QgsComposerItem
     void overviewExtentChanged();
 
     virtual void refreshDataDefinedProperty( const QgsComposerObject::DataDefinedProperty property = QgsComposerObject::AllProperties );
+
+  protected slots:
+
+    /**Called when layers are added or removed from the layer registry. Updates the maps
+     * layer set and redraws the map if required.
+     * @note added in QGIS 2.9
+    */
+    void layersChanged();
+
 };

src/core/composer/qgscomposermap.cpp

@@ -486,9 +486,8 @@ bool QgsComposerMap::shouldDrawPart( PartType part ) const
   return true; // for Layer
 }
 
-void QgsComposerMap::updateCachedImage( void )
+void QgsComposerMap::updateCachedImage()
 {
-  syncLayerSet(); //layer list may have changed
   mCacheUpdated = false;
   cache();
   QGraphicsRectItem::update();
@@ -502,6 +501,12 @@ void QgsComposerMap::renderModeUpdateCachedImage()
   }
 }
 
+void QgsComposerMap::layersChanged()
+{
+  syncLayerSet();
+  renderModeUpdateCachedImage();
+}
+
 void QgsComposerMap::setCacheUpdated( bool u )
 {
   mCacheUpdated = u;
@@ -1169,8 +1174,8 @@ void QgsComposerMap::connectUpdateSlot()
   QgsMapLayerRegistry* layerRegistry = QgsMapLayerRegistry::instance();
   if ( layerRegistry )
   {
-    connect( layerRegistry, SIGNAL( layerWillBeRemoved( QString ) ), this, SLOT( updateCachedImage() ) );
-    connect( layerRegistry, SIGNAL( layerWasAdded( QgsMapLayer* ) ), this, SLOT( updateCachedImage() ) );
+    connect( layerRegistry, SIGNAL( layerWillBeRemoved( QString ) ), this, SLOT( layersChanged() ) );
+    connect( layerRegistry, SIGNAL( layerWasAdded( QgsMapLayer* ) ), this, SLOT( layersChanged() ) );
   }
 }
 

src/core/composer/qgscomposermap.h

@@ -796,9 +796,12 @@ class CORE_EXPORT QgsComposerMap : public QgsComposerItem
 
   public slots:
 
-    /**Called if map canvas has changed*/
+    /**Forces an update of the cached map image*/
     void updateCachedImage();
-    /**Call updateCachedImage if item is in render mode*/
+
+    /**Updates the cached map image if the map is set to Render mode
+     * @see updateCachedImage
+    */
     void renderModeUpdateCachedImage();
 
     /**Updates the bounding rect of this item. Call this function before doing any changes related to annotation out of the map rectangle */
@@ -809,6 +812,14 @@ class CORE_EXPORT QgsComposerMap : public QgsComposerItem
 
     virtual void refreshDataDefinedProperty( const QgsComposerObject::DataDefinedProperty property = QgsComposerObject::AllProperties ) override;
 
+  protected slots:
+
+    /**Called when layers are added or removed from the layer registry. Updates the maps
+     * layer set and redraws the map if required.
+     * @note added in QGIS 2.9
+    */
+    void layersChanged();
+
   private:
 
     /**Unique identifier*/