Reuse existing labeling results if cached labels were redrawn

python/core/qgsmaprendererjob.sip

@@ -24,6 +24,8 @@ class QgsMapRendererJob : QObject
     //! Tell whether the rendering job is currently running in background.
     virtual bool isActive() const = 0;
 
+    virtual bool usedCachedLabels() const = 0;
+
     //! Get pointer to internal labeling engine (in order to get access to the results)
     virtual QgsLabelingResults* takeLabelingResults() = 0 /Transfer/;
 

python/core/qgsmaprendererparalleljob.sip

@@ -20,6 +20,7 @@ class QgsMapRendererParallelJob : QgsMapRendererQImageJob
     virtual void cancel();
     virtual void waitForFinished();
     virtual bool isActive() const;
+    virtual bool usedCachedLabels() const;
 
     virtual QgsLabelingResults* takeLabelingResults() /Transfer/;
 

python/core/qgsmaprenderersequentialjob.sip

@@ -21,6 +21,7 @@ class QgsMapRendererSequentialJob : QgsMapRendererQImageJob
     virtual void cancel();
     virtual void waitForFinished();
     virtual bool isActive() const;
+    virtual bool usedCachedLabels() const;
 
     virtual QgsLabelingResults* takeLabelingResults() /Transfer/;
 

src/core/qgsmaprenderercustompainterjob.cpp

@@ -156,6 +156,10 @@ bool QgsMapRendererCustomPainterJob::isActive() const
   return mActive;
 }
 
+bool QgsMapRendererCustomPainterJob::usedCachedLabels() const
+{
+  return mLabelJob.cached;
+}
 
 QgsLabelingResults* QgsMapRendererCustomPainterJob::takeLabelingResults()
 {

src/core/qgsmaprenderercustompainterjob.h

@@ -41,6 +41,7 @@ class CORE_EXPORT QgsMapRendererCustomPainterJob : public QgsMapRendererJob
     virtual void cancel() override;
     virtual void waitForFinished() override;
     virtual bool isActive() const override;
+    virtual bool usedCachedLabels() const override;
     virtual QgsLabelingResults* takeLabelingResults() override;
 
     //! @note not available in python bindings

src/core/qgsmaprendererjob.h

@@ -125,7 +125,20 @@ class CORE_EXPORT QgsMapRendererJob : public QObject
     //! Tell whether the rendering job is currently running in background.
     virtual bool isActive() const = 0;
 
-    //! Get pointer to internal labeling engine (in order to get access to the results)
+    /**
+     * Returns true if the render job was able to use a cached labeling solution.
+     * If so, any previously stored labeling results (see takeLabelingResults())
+     * should be retained.
+     * @see takeLabelingResults()
+     * @note added in QGIS 3.0
+     */
+    virtual bool usedCachedLabels() const = 0;
+
+    /**
+     * Get pointer to internal labeling engine (in order to get access to the results).
+     * This should not be used if cached labeling was redrawn - see usedCachedLabels().
+     * @see usedCachedLabels()
+     */
     virtual QgsLabelingResults* takeLabelingResults() = 0;
 
     //! @note Added in QGIS 3.0

src/core/qgsmaprendererparalleljob.cpp

@@ -153,6 +153,11 @@ bool QgsMapRendererParallelJob::isActive() const
   return mStatus != Idle;
 }
 
+bool QgsMapRendererParallelJob::usedCachedLabels() const
+{
+  return mLabelJob.cached;
+}
+
 QgsLabelingResults* QgsMapRendererParallelJob::takeLabelingResults()
 {
   if ( mLabelingEngineV2 )

src/core/qgsmaprendererparalleljob.h

@@ -39,6 +39,7 @@ class CORE_EXPORT QgsMapRendererParallelJob : public QgsMapRendererQImageJob
     virtual void waitForFinished() override;
     virtual bool isActive() const override;
 
+    virtual bool usedCachedLabels() const override;
     virtual QgsLabelingResults* takeLabelingResults() override;
 
     // from QgsMapRendererJobWithPreview

src/core/qgsmaprenderersequentialjob.cpp

@@ -98,6 +98,11 @@ bool QgsMapRendererSequentialJob::isActive() const
   return nullptr != mInternalJob;
 }
 
+bool QgsMapRendererSequentialJob::usedCachedLabels() const
+{
+  return mUsedCachedLabels;
+}
+
 QgsLabelingResults* QgsMapRendererSequentialJob::takeLabelingResults()
 {
   QgsLabelingResults* tmp = mLabelingResults;
@@ -125,6 +130,7 @@ void QgsMapRendererSequentialJob::internalFinished()
   mPainter = nullptr;
 
   mLabelingResults = mInternalJob->takeLabelingResults();
+  mUsedCachedLabels = mInternalJob->usedCachedLabels();
 
   mErrors = mInternalJob->errors();
 

src/core/qgsmaprenderersequentialjob.h

@@ -41,6 +41,7 @@ class CORE_EXPORT QgsMapRendererSequentialJob : public QgsMapRendererQImageJob
     virtual void waitForFinished() override;
     virtual bool isActive() const override;
 
+    virtual bool usedCachedLabels() const override;
     virtual QgsLabelingResults* takeLabelingResults() override;
 
     // from QgsMapRendererJobWithPreview
@@ -56,6 +57,7 @@ class CORE_EXPORT QgsMapRendererSequentialJob : public QgsMapRendererQImageJob
     QImage mImage;
     QPainter* mPainter;
     QgsLabelingResults* mLabelingResults;
+    bool mUsedCachedLabels = false;
 
 };
 

src/gui/qgsmapcanvas.cpp

@@ -546,8 +546,11 @@ void QgsMapCanvas::rendererJobFinished()
   {
     // take labeling results before emitting renderComplete, so labeling map tools
     // connected to signal work with correct results
-    delete mLabelingResults;
-    mLabelingResults = mJob->takeLabelingResults();
+    if ( !mJob->usedCachedLabels() )
+    {
+      delete mLabelingResults;
+      mLabelingResults = mJob->takeLabelingResults();
+    }
 
     QImage img = mJob->renderedImage();