More flexible API for preview job determination

python/core/qgsdataprovider.sip

@@ -380,6 +380,7 @@ Current time stamp of data source
 .. versionadded:: 3.0
 %End
 
+
   signals:
 
     void fullExtentCalculated();

src/core/qgsdataprovider.cpp

@@ -42,7 +42,7 @@ void QgsDataProvider::setListening( bool isListening )
   Q_UNUSED( isListening );
 }
 
-bool QgsDataProvider::renderInPreview( double lastRenderingTimeMS, double maxRenderingTimeMS )
+bool QgsDataProvider::renderInPreview( QgsDataProvider::PreviewContext context )
 {
-  return lastRenderingTimeMS <= maxRenderingTimeMS;
+  return context.lastRenderingTimeMs <= context.maxRenderingTimeMs;
 }

src/core/qgsdataprovider.h

@@ -461,6 +461,23 @@ class CORE_EXPORT QgsDataProvider : public QObject
      */
     virtual void setListening( bool isListening );
 
+#ifndef SIP_RUN
+
+    /**
+     * Stores settings related to the context in which a preview job runs.
+     * \note Not available in Python bindings
+     * \since QGIS 3.0
+     */
+    struct PreviewContext
+    {
+      //! Previous rendering time for the layer, in ms
+      double lastRenderingTimeMs = -1;
+
+      //! Default maximum allowable render time, in ms
+      double maxRenderingTimeMs = MAXIMUM_LAYER_PREVIEW_TIME_MS;
+    };
+#endif
+
     /**
      * Returns whether the layer must be rendered in preview jobs.
      *
@@ -474,7 +491,7 @@ class CORE_EXPORT QgsDataProvider : public QObject
      *
      * \note not available in Python bindings
      */
-    virtual bool renderInPreview( double lastRenderingTimeMS, double maxRenderingTimeMS ); // SIP_SKIP
+    virtual bool renderInPreview( QgsDataProvider::PreviewContext context ); // SIP_SKIP
 
   signals:
 

src/gui/qgsmapcanvas.cpp

@@ -2273,9 +2273,12 @@ void QgsMapCanvas::startPreviewJob( int number )
   // truncate preview layers to fast layers
   const QList<QgsMapLayer *> layers = jobSettings.layers();
   QList< QgsMapLayer * > previewLayers;
+  QgsDataProvider::PreviewContext context;
+  context.maxRenderingTimeMs = MAXIMUM_LAYER_PREVIEW_TIME_MS;
   for ( QgsMapLayer *layer : layers )
   {
-    if ( !layer->dataProvider()->renderInPreview( mLastLayerRenderTime.value( layer->id() ), MAXIMUM_LAYER_PREVIEW_TIME_MS ) )
+    context.lastRenderingTimeMs = mLastLayerRenderTime.value( layer->id() );
+    if ( !layer->dataProvider()->renderInPreview( context ) )
     {
       QgsDebugMsgLevel( QString( "Layer %1 not rendered because it does not match the renderInPreview criterion %2" ).arg( layer->id() ).arg( mLastLayerRenderTime.value( layer->id() ) ), 3 );
       continue;