When the canvas temporal range is changed, we need to invalidate any

cached images we have of temporal enabled layers

Otherwise these will not be re-rendered using the update temporal range
when the canvas is redrawn (and we want to avoid a full map redraw during
animations, which is expensive, so we want to be able to reuse as many
cached images as possible!)

python/core/auto_generated/qgsmaprenderercache.sip.in

@@ -91,6 +91,13 @@ Returns a list of map layers on which an image in the cache depends.
 Removes an image from the cache with matching ``cacheKey``.
 
 .. seealso:: :py:func:`clear`
+%End
+
+    void invalidateCacheForLayer( QgsMapLayer *layer );
+%Docstring
+Invalidates cached images which relate to the specified map ``layer``.
+
+.. versionadded:: 3.14
 %End
 
 };

src/core/qgsmaprenderercache.cpp

@@ -145,9 +145,15 @@ QList< QgsMapLayer * > QgsMapRendererCache::dependentLayers( const QString &cach
   return QList< QgsMapLayer * >();
 }
 
+
 void QgsMapRendererCache::layerRequestedRepaint()
 {
   QgsMapLayer *layer = qobject_cast<QgsMapLayer *>( sender() );
+  invalidateCacheForLayer( layer );
+}
+
+void QgsMapRendererCache::invalidateCacheForLayer( QgsMapLayer *layer )
+{
   if ( !layer )
     return;
 
@@ -175,3 +181,4 @@ void QgsMapRendererCache::clearCacheImage( const QString &cacheKey )
   mCachedImages.remove( cacheKey );
   dropUnusedConnections();
 }
+

src/core/qgsmaprenderercache.h

@@ -97,6 +97,13 @@ class CORE_EXPORT QgsMapRendererCache : public QObject
      */
     void clearCacheImage( const QString &cacheKey );
 
+    /**
+     * Invalidates cached images which relate to the specified map \a layer.
+     *
+     * \since QGIS 3.14
+     */
+    void invalidateCacheForLayer( QgsMapLayer *layer );
+
   private slots:
     //! Remove layer (that emitted the signal) from the cache
     void layerRequestedRepaint();

src/gui/qgsmapcanvas.cpp

@@ -75,6 +75,7 @@ email                : sherman at mrcc.com
 #include "qgscustomdrophandler.h"
 #include "qgsreferencedgeometry.h"
 #include "qgsprojectviewsettings.h"
+#include "qgsmaplayertemporalproperties.h"
 
 /**
  * \ingroup gui
@@ -734,6 +735,19 @@ void QgsMapCanvas::setTemporalRange( const QgsDateTimeRange &dateTimeRange )
     return;
 
   mSettings.setTemporalRange( dateTimeRange );
+
+  if ( mCache )
+  {
+    // we need to discard any previously cached images which have temporal properties enabled, so that these will be updated when
+    // the canvas is redrawn
+    const QList<QgsMapLayer *> layers;
+    for ( QgsMapLayer *layer : layers )
+    {
+      if ( layer->temporalProperties() && layer->temporalProperties()->isActive() )
+        mCache->invalidateCacheForLayer( layer );
+    }
+  }
+
   emit temporalRangeChanged();
 }
 

tests/src/python/test_qgsmaprenderercache.py

@@ -123,6 +123,27 @@ def testRequestRepaintSimple(self):
         layer.triggerRepaint(True)
         self.assertFalse(cache.hasCacheImage('xxx'))
 
+    def testInvalidateCacheForLayer(self):
+        """ test invalidating the cache for a layer """
+        layer = QgsVectorLayer("Point?field=fldtxt:string",
+                               "layer", "memory")
+        QgsProject.instance().addMapLayers([layer])
+        self.assertTrue(layer.isValid())
+
+        # add image to cache
+        cache = QgsMapRendererCache()
+        im = QImage(200, 200, QImage.Format_RGB32)
+        cache.setCacheImage('xxx', im, [layer])
+        self.assertFalse(cache.cacheImage('xxx').isNull())
+        self.assertTrue(cache.hasCacheImage('xxx'))
+
+        # invalidate cache for layer
+        cache.invalidateCacheForLayer(layer)
+        # cache image should be cleared
+        self.assertTrue(cache.cacheImage('xxx').isNull())
+        self.assertFalse(cache.hasCacheImage('xxx'))
+        QgsProject.instance().removeMapLayer(layer.id())
+
     def testRequestRepaintMultiple(self):
         """ test requesting repaint with multiple dependent layers """
         layer1 = QgsVectorLayer("Point?field=fldtxt:string",