create cache only if necessary

git-svn-id: http://svn.osgeo.org/qgis/trunk@3027 c8812cc2-4d05-0410-92ff-de0c093fc19c

src/qgscomposermap.cpp

@@ -86,6 +86,7 @@ void QgsComposerMap::init ()
 
     // Cache
     mCachePixmap = new QPixmap();
+    mCacheUpdated = false;
 
     // Potemkin
     mCalculateComboBox->insertItem( "Scale" );
@@ -195,6 +196,7 @@ void QgsComposerMap::cache ( void )
     p.end();
 
     mNumCachedLayers = mMapCanvas->layerCount();
+    mCacheUpdated = true;
 }
 
 void QgsComposerMap::draw ( QPainter & painter )
@@ -205,7 +207,7 @@ void QgsComposerMap::draw ( QPainter & painter )
     if ( plotStyle() == QgsComposition::Preview &&  mPreviewMode == Cache ) { // Draw from cache
         std::cout << "use cache" << std::endl;
 
-	if ( mMapCanvas->layerCount() != mNumCachedLayers ) {
+	if ( !mCacheUpdated || mMapCanvas->layerCount() != mNumCachedLayers ) {
 	    cache();
 	}
 	
@@ -294,7 +296,7 @@ void QgsComposerMap::scaleChanged ( void )
     mSymbolScale = mSymbolScaleLineEdit->text().toDouble();
     mFontScale = mFontScaleLineEdit->text().toDouble();
 
-    cache();
+    mCacheUpdated = false;
     QCanvasRectangle::update();
     QCanvasRectangle::canvas()->update();
     
@@ -337,7 +339,7 @@ void QgsComposerMap::recalculate ( void )
     std::cout << "mExtent = " << mExtent.stringRep() << std::endl;
 
     setOptions();
-    cache();
+    mCacheUpdated = false;
 }
 
 void QgsComposerMap::frameChanged ( )

src/qgscomposermap.h

@@ -165,6 +165,9 @@ private:
     //        It is very slow, with zoom in in QCanvasView, it seems, that QCanvas is stored as a big image
     //        with resolution necessary for current zoom and so always a big image mus be redrawn. 
     QPixmap *mCachePixmap; 
+
+    // Is cache up to date
+    bool mCacheUpdated;
     
     // Resize schema
     QgsComposition::Calculate mCalculate;