Add convience method to block while canvas is rendering

NOT to be called from anything but unit tests and standalone
scripts!!

python/gui/qgsmapcanvas.sip

@@ -30,6 +30,7 @@ class QgsMapCanvas : QGraphicsView
     bool isCachingEnabled() const;
     void clearCache();
     void refreshAllLayers();
+    void waitWhileRendering();
     void setParallelRenderingEnabled( bool enabled );
     bool isParallelRenderingEnabled() const;
     void setMapUpdateInterval( int timeMilliseconds );

src/gui/qgsmapcanvas.cpp

@@ -2016,6 +2016,14 @@ void QgsMapCanvas::refreshAllLayers()
   refresh();
 }
 
+void QgsMapCanvas::waitWhileRendering()
+{
+  while ( mRefreshScheduled || mJob )
+  {
+    QgsApplication::processEvents();
+  }
+}
+
 void QgsMapCanvas::setSegmentationTolerance( double tolerance )
 {
   mSettings.setSegmentationTolerance( tolerance );

src/gui/qgsmapcanvas.h

@@ -128,6 +128,17 @@ class GUI_EXPORT QgsMapCanvas : public QGraphicsView
     //! @note added in 2.9
     void refreshAllLayers();
 
+    /**
+     * Blocks until the rendering job has finished.
+     *
+     * In almost all cases you do NOT want to call this, as it will hang the UI
+     * until the rendering job is complete. It's included in API solely for
+     * unit testing and standalone python scripts.
+     *
+     * @note added in QGIS 3.0
+     */
+    void waitWhileRendering();
+
     //! Set whether the layers are rendered in parallel or sequentially
     //! @note added in 2.4
     void setParallelRenderingEnabled( bool enabled );

tests/src/python/test_qgsmapcanvas.py

@@ -58,13 +58,10 @@ def testDeferredUpdate(self):
         canvas.setLayers([layer])
         canvas.setExtent(QgsRectangle(10, 30, 20, 35))
         canvas.show()
-
         # need to wait until first redraw can occur (note that we first need to wait till drawing starts!)
         while not canvas.isDrawing():
             app.processEvents()
-        while canvas.isDrawing():
-            app.processEvents()
-
+        canvas.waitWhileRendering()
         self.assertTrue(self.canvasImageCheck('empty_canvas', 'empty_canvas', canvas))
 
         # add polygon to layer
@@ -83,10 +80,7 @@ def testDeferredUpdate(self):
 
         # refresh canvas
         canvas.refresh()
-        while not canvas.isDrawing():
-            app.processEvents()
-        while canvas.isDrawing():
-            app.processEvents()
+        canvas.waitWhileRendering()
 
         # now we expect the canvas check to fail (since they'll be a new polygon rendered over it)
         self.assertFalse(self.canvasImageCheck('empty_canvas', 'empty_canvas', canvas))
@@ -110,9 +104,7 @@ def testRefreshOnTimer(self):
         # need to wait until first redraw can occur (note that we first need to wait till drawing starts!)
         while not canvas.isDrawing():
             app.processEvents()
-        while canvas.isDrawing():
-            app.processEvents()
-
+        canvas.waitWhileRendering()
         self.assertTrue(self.canvasImageCheck('empty_canvas', 'empty_canvas', canvas))
 
         # add polygon to layer
@@ -210,9 +202,7 @@ def testMapTheme(self):
         # need to wait until first redraw can occur (note that we first need to wait till drawing starts!)
         while not canvas.isDrawing():
             app.processEvents()
-        while canvas.isDrawing():
-            app.processEvents()
-
+        canvas.waitWhileRendering()
         self.assertTrue(self.canvasImageCheck('theme1', 'theme1', canvas))
 
         # add some styles
@@ -223,18 +213,12 @@ def testMapTheme(self):
         layer.styleManager().addStyleFromLayer('style2')
 
         canvas.refresh()
-        while not canvas.isDrawing():
-            app.processEvents()
-        while canvas.isDrawing():
-            app.processEvents()
+        canvas.waitWhileRendering()
         self.assertTrue(self.canvasImageCheck('theme2', 'theme2', canvas))
 
         layer.styleManager().setCurrentStyle('style1')
         canvas.refresh()
-        while not canvas.isDrawing():
-            app.processEvents()
-        while canvas.isDrawing():
-            app.processEvents()
+        canvas.waitWhileRendering()
         self.assertTrue(self.canvasImageCheck('theme1', 'theme1', canvas))
 
         # ok, so all good with setting/rendering map styles
@@ -258,18 +242,12 @@ def testMapTheme(self):
 
         canvas.setTheme('theme2')
         canvas.refresh()
-        while not canvas.isDrawing():
-            app.processEvents()
-        while canvas.isDrawing():
-            app.processEvents()
+        canvas.waitWhileRendering()
         self.assertTrue(self.canvasImageCheck('theme2', 'theme2', canvas))
 
         canvas.setTheme('theme1')
         canvas.refresh()
-        while not canvas.isDrawing():
-            app.processEvents()
-        while canvas.isDrawing():
-            app.processEvents()
+        canvas.waitWhileRendering()
         self.assertTrue(self.canvasImageCheck('theme1', 'theme1', canvas))
 
         # add another layer
@@ -286,17 +264,11 @@ def testMapTheme(self):
 
         # rerender canvas - should NOT show new layer
         canvas.refresh()
-        while not canvas.isDrawing():
-            app.processEvents()
-        while canvas.isDrawing():
-            app.processEvents()
+        canvas.waitWhileRendering()
         self.assertTrue(self.canvasImageCheck('theme1', 'theme1', canvas))
         # test again - this time refresh all layers
         canvas.refreshAllLayers()
-        while not canvas.isDrawing():
-            app.processEvents()
-        while canvas.isDrawing():
-            app.processEvents()
+        canvas.waitWhileRendering()
         self.assertTrue(self.canvasImageCheck('theme1', 'theme1', canvas))
 
         # add layer 2 to theme1
@@ -305,10 +277,7 @@ def testMapTheme(self):
         QgsProject.instance().mapThemeCollection().update('theme1', theme1)
 
         canvas.refresh()
-        while not canvas.isDrawing():
-            app.processEvents()
-        while canvas.isDrawing():
-            app.processEvents()
+        canvas.waitWhileRendering()
         self.assertTrue(self.canvasImageCheck('theme3', 'theme3', canvas))
 
         # change the appearance of an active style
@@ -320,42 +289,29 @@ def testMapTheme(self):
         QgsProject.instance().mapThemeCollection().update('theme1', theme1)
 
         canvas.refresh()
-        while not canvas.isDrawing():
-            app.processEvents()
-        while canvas.isDrawing():
-            app.processEvents()
+        canvas.waitWhileRendering()
         self.assertTrue(self.canvasImageCheck('theme3', 'theme3', canvas))
 
         layer2.styleManager().setCurrentStyle('style4')
         sym3 = QgsFillSymbol.createSimple({'color': '#b200b2'})
         layer2.renderer().setSymbol(sym3)
         canvas.refresh()
-
-        while not canvas.isDrawing():
-            app.processEvents()
-        while canvas.isDrawing():
-            app.processEvents()
+        canvas.waitWhileRendering()
         self.assertTrue(self.canvasImageCheck('theme4', 'theme4', canvas))
 
         # try setting layers while a theme is in place
         canvas.setLayers([layer])
         canvas.refresh()
 
         # should be no change... setLayers should be ignored if canvas is following a theme!
-        while not canvas.isDrawing():
-            app.processEvents()
-        while canvas.isDrawing():
-            app.processEvents()
+        canvas.waitWhileRendering()
         self.assertTrue(self.canvasImageCheck('theme4', 'theme4', canvas))
 
         # setLayerStyleOverrides while theme is in place
         canvas.setLayerStyleOverrides({layer2.id(): 'original'})
         # should be no change... setLayerStyleOverrides should be ignored if canvas is following a theme!
         canvas.refresh()
-        while not canvas.isDrawing():
-            app.processEvents()
-        while canvas.isDrawing():
-            app.processEvents()
+        canvas.waitWhileRendering()
         self.assertTrue(self.canvasImageCheck('theme4', 'theme4', canvas))
 
     def canvasImageCheck(self, name, reference_image, canvas):