Refine logic regarding vector layer rendering composition

If previous layer render was relatively quick (eg < 3 seconds), then we defer composition until the whole layer has been rendered. If not, then we defer composition until either the first feature has been rendered OR the render is taking a while and the map will benefit from progressive updates
qgis · Jan 27, 2021 · 0edd4f9 · 0edd4f9
1 parent bb746c6
commit 0edd4f9
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diff --git a/src/core/vector/qgsvectorlayerrenderer.cpp b/src/core/vector/qgsvectorlayerrenderer.cpp
@@ -44,6 +44,7 @@
 
 #include <QPicture>
 
+constexpr int MAX_TIME_TO_USE_CACHED_PREVIEW_IMAGE = 3000;
 
 QgsVectorLayerRenderer::QgsVectorLayerRenderer( QgsVectorLayer *layer, QgsRenderContext &context )
   : QgsMapLayerRenderer( layer->id(), &context )
@@ -178,10 +179,17 @@ QgsVectorLayerRenderer::QgsVectorLayerRenderer( QgsVectorLayer *layer, QgsRender
     //layer properties require rasterization
     mForceRasterRender = true;
   }
+
+  mReadyToCompose = false;
 }
 
 QgsVectorLayerRenderer::~QgsVectorLayerRenderer() = default;
 
+void QgsVectorLayerRenderer::setLayerRenderingTimeHint( int time )
+{
+  mRenderTimeHint = time;
+}
+
 QgsFeedback *QgsVectorLayerRenderer::feedback() const
 {
   return mInterruptionChecker.get();
@@ -195,19 +203,33 @@ bool QgsVectorLayerRenderer::forceRasterRender() const
 bool QgsVectorLayerRenderer::render()
 {
   if ( mGeometryType == QgsWkbTypes::NullGeometry || mGeometryType == QgsWkbTypes::UnknownGeometry )
+  {
+    mReadyToCompose = true;
     return true;
+  }
 
   if ( mRenderers.empty() )
   {
+    mReadyToCompose = true;
     mErrors.append( QObject::tr( "No renderer for drawing." ) );
     return false;
   }
 
+  // if the previous layer render was relatively quick (e.g. less than 3 seconds), the we show any previously
+  // cached version of the layer during rendering instead of the usual progressive updates
+  if ( mRenderTimeHint > 0 && mRenderTimeHint <= MAX_TIME_TO_USE_CACHED_PREVIEW_IMAGE )
+  {
+    mBlockRenderUpdates = true;
+    mElapsedTimer.start();
+  }
+
   bool res = true;
   for ( const std::unique_ptr< QgsFeatureRenderer > &renderer : mRenderers )
   {
     res = renderInternal( renderer.get() ) && res;
   }
+
+  mReadyToCompose = true;
   return res;
 }
 
@@ -440,6 +462,14 @@ void QgsVectorLayerRenderer::drawRenderer( QgsFeatureRenderer *renderer, QgsFeat
       // labeling - register feature
       if ( rendered )
       {
+        // as soon as first feature is rendered, we can start showing layer updates.
+        // but if we are blocking render updates (so that a previously cached image is being shown), we wait
+        // at most e.g. 3 seconds before we start forcing progressive updates.
+        if ( !mBlockRenderUpdates || mElapsedTimer.elapsed() > MAX_TIME_TO_USE_CACHED_PREVIEW_IMAGE )
+        {
+          mReadyToCompose = true;
+        }
+
         // new labeling engine
         if ( isMainRenderer && context.labelingEngine() && ( mLabelProvider || mDiagramProvider ) )
         {
@@ -640,6 +670,14 @@ void QgsVectorLayerRenderer::drawRendererLevels( QgsFeatureRenderer *renderer, Q
         try
         {
           renderer->renderFeature( *fit, context, layer, sel, drawMarker );
+
+          // as soon as first feature is rendered, we can start showing layer updates.
+          // but if we are blocking render updates (so that a previously cached image is being shown), we wait
+          // at most e.g. 3 seconds before we start forcing progressive updates.
+          if ( !mBlockRenderUpdates || mElapsedTimer.elapsed() > MAX_TIME_TO_USE_CACHED_PREVIEW_IMAGE )
+          {
+            mReadyToCompose = true;
+          }
         }
         catch ( const QgsCsException &cse )
         {

diff --git a/src/core/vector/qgsvectorlayerrenderer.h b/src/core/vector/qgsvectorlayerrenderer.h
@@ -32,6 +32,7 @@ class QgsMapClippingRegion;
 
 #include <QList>
 #include <QPainter>
+#include <QElapsedTimer>
 
 typedef QList<int> QgsAttributeList;
 
@@ -89,6 +90,8 @@ class QgsVectorLayerRenderer : public QgsMapLayerRenderer
 
     bool render() override;
 
+    void setLayerRenderingTimeHint( int time ) override;
+
   private:
 
     /**
@@ -172,6 +175,10 @@ class QgsVectorLayerRenderer : public QgsMapLayerRenderer
     bool mApplyLabelClipGeometries = false;
     bool mForceRasterRender = false;
 
+    int mRenderTimeHint = 0;
+    bool mBlockRenderUpdates = false;
+    QElapsedTimer mElapsedTimer;
+
 };