[tracer] simpler and lazier updates of tracer configuration when some…

…thing changes

src/core/qgstracer.cpp

@@ -526,7 +526,7 @@ bool QgsTracer::initGraph()
   int timeMake = t3.elapsed();
 
   QgsDebugMsg( QString( "tracer extract %1 ms, noding %2 ms (call %3 ms), make %4 ms" )
-               .arg ( timeExtract ).arg( timeNoding ).arg ( timeNodingCall ).arg( timeMake ) );
+               .arg( timeExtract ).arg( timeNoding ).arg( timeNodingCall ).arg( timeMake ) );
   return true;
 }
 
@@ -582,6 +582,9 @@ bool QgsTracer::init()
   if ( mGraph )
     return true;
 
+  // configuration from derived class?
+  configure();
+
   return initGraph();
 }
 
@@ -611,7 +614,7 @@ void QgsTracer::onGeometryChanged( QgsFeatureId fid, QgsGeometry& geom )
   invalidateGraph();
 }
 
-QVector<QgsPoint> QgsTracer::findShortestPath(const QgsPoint& p1, const QgsPoint& p2, PathError* error )
+QVector<QgsPoint> QgsTracer::findShortestPath( const QgsPoint& p1, const QgsPoint& p2, PathError* error )
 {
   init();  // does nothing if the graph exists already
   if ( !mGraph )

src/core/qgstracer.h

@@ -66,7 +66,10 @@ class CORE_EXPORT QgsTracer : public QObject
     //! depending on how big the input layers are. It is not necessary
     //! to call this method explicitly - it will be called by findShortestPath()
     //! if necessary.
-    virtual bool init();
+    bool init();
+
+    //! Whether the internal data structures have been initialized
+    bool isInitialized() const { return mGraph != nullptr; }
 
     enum PathError
     {
@@ -86,9 +89,17 @@ class CORE_EXPORT QgsTracer : public QObject
     //! Find out whether the point is snapped to a vertex or edge (i.e. it can be used for tracing start/stop)
     bool isPointSnapped( const QgsPoint& pt );
 
+  protected:
+    //! Allows derived classes to setup the settings just before the tracer is initialized.
+    //! This allows the configuration to be set in a lazy way only when it is really necessary.
+    //! Default implementation does nothing.
+    virtual void configure() {}
+
+  protected slots:
+    void invalidateGraph();
+
   private:
     bool initGraph();
-    void invalidateGraph();
 
   private slots:
     void onFeatureAdded( QgsFeatureId fid );

src/gui/qgsmapcanvastracer.cpp

@@ -20,11 +20,12 @@ QgsMapCanvasTracer::QgsMapCanvasTracer( QgsMapCanvas* canvas, QgsMessageBar* mes
 {
   sTracers.insert( canvas, this );
 
-  connect( canvas, SIGNAL( destinationCrsChanged() ), this, SLOT( updateSettings() ) );
-  connect( canvas, SIGNAL( layersChanged() ), this, SLOT( updateLayerSettings() ) );
-  connect( canvas, SIGNAL( extentsChanged() ), this, SLOT( updateSettings() ) );
+  // when things change we just invalidate the graph - and set up new parameters again only when necessary
+  connect( canvas, SIGNAL( destinationCrsChanged() ), this, SLOT( invalidateGraph() ) );
+  connect( canvas, SIGNAL( layersChanged() ), this, SLOT( invalidateGraph() ) );
+  connect( canvas, SIGNAL( extentsChanged() ), this, SLOT( invalidateGraph() ) );
   connect( canvas, SIGNAL( currentLayerChanged( QgsMapLayer* ) ), this, SLOT( onCurrentLayerChanged() ) );
-  connect( canvas->snappingUtils(), SIGNAL( configChanged() ), this, SLOT( updateLayerSettings() ) );
+  connect( canvas->snappingUtils(), SIGNAL( configChanged() ), this, SLOT( invalidateGraph() ) );
 
   mActionEnableTracing = new QAction( QIcon( QgsApplication::getThemeIcon( "/mActionTracing.png" ) ), tr( "Enable Tracing" ), this );
   mActionEnableTracing->setShortcut( Qt::Key_T );
@@ -33,22 +34,13 @@ QgsMapCanvasTracer::QgsMapCanvasTracer( QgsMapCanvas* canvas, QgsMessageBar* mes
   // arbitrarily chosen limit that should allow for fairly fast initialization
   // of the underlying graph structure
   setMaxFeatureCount( QSettings().value( "/qgis/digitizing/tracing_max_feature_count", 10000 ).toInt() );
-
-  updateSettings(); // initialize
-  updateLayerSettings();
 }
 
 QgsMapCanvasTracer::~QgsMapCanvasTracer()
 {
   sTracers.remove( mCanvas );
 }
 
-bool QgsMapCanvasTracer::init()
-{
-  bool res = QgsTracer::init();
-  return res;
-}
-
 QgsMapCanvasTracer* QgsMapCanvasTracer::tracerForCanvas( QgsMapCanvas* canvas )
 {
   return sTracers.value( canvas, 0 );
@@ -66,23 +58,23 @@ void QgsMapCanvasTracer::reportError( QgsTracer::PathError err, bool addingVerte
   QString message;
   switch ( err )
   {
-  case ErrTooManyFeatures:
-    message = tr( "Disabled - there are too many features displayed. Try zooming in or disable some layers." );
-    break;
-  case ErrPoint1:
-    message = tr( "The start point needs to be snapped and in the visible map view" );
-    break;
-  case ErrPoint2:
-    if ( addingVertex )
-      message = tr( "The end point needs to be snapped" );
-    break;
-  case ErrNoPath:
-    if ( addingVertex )
-      message = tr( "Endpoints are not connected" );
-    break;
-  case ErrNone:
-  default:
-    break;
+    case ErrTooManyFeatures:
+      message = tr( "Disabled - there are too many features displayed. Try zooming in or disable some layers." );
+      break;
+    case ErrPoint1:
+      message = tr( "The start point needs to be snapped and in the visible map view" );
+      break;
+    case ErrPoint2:
+      if ( addingVertex )
+        message = tr( "The end point needs to be snapped" );
+      break;
+    case ErrNoPath:
+      if ( addingVertex )
+        message = tr( "Endpoints are not connected" );
+      break;
+    case ErrNone:
+    default:
+      break;
   }
 
   if ( message.isEmpty() )
@@ -93,14 +85,11 @@ void QgsMapCanvasTracer::reportError( QgsTracer::PathError err, bool addingVerte
   mMessageBar->pushItem( mLastMessage );
 }
 
-void QgsMapCanvasTracer::updateSettings()
+void QgsMapCanvasTracer::configure()
 {
   setDestinationCrs( mCanvas->mapSettings().destinationCrs() );
   setExtent( mCanvas->extent() );
-}
 
-void QgsMapCanvasTracer::updateLayerSettings()
-{
   QList<QgsVectorLayer*> layers;
   QStringList visibleLayerIds = mCanvas->mapSettings().layers();
 
@@ -138,5 +127,5 @@ void QgsMapCanvasTracer::onCurrentLayerChanged()
 {
   // no need to bother if we are not snapping
   if ( mCanvas->snappingUtils()->snapToMapMode() == QgsSnappingUtils::SnapCurrentLayer )
-    updateLayerSettings();
+    invalidateGraph();
 }

src/gui/qgsmapcanvastracer.h

@@ -18,8 +18,6 @@ class GUI_EXPORT QgsMapCanvasTracer : public QgsTracer
 
     QAction* actionEnableTracing() { return mActionEnableTracing; }
 
-    virtual bool init();
-
     //! Retrieve instance of this class associated with given canvas (if any).
     //! The class keeps a simple registry of tracers associated with map canvas
     //! instances for easier access to the common tracer by various map tools
@@ -28,9 +26,10 @@ class GUI_EXPORT QgsMapCanvasTracer : public QgsTracer
     //! Report a path finding error to the user
     void reportError( PathError err, bool addingVertex );
 
+  protected:
+    virtual void configure();
+
   private slots:
-    void updateSettings();
-    void updateLayerSettings();
     void onCurrentLayerChanged();
 
   private: