Implement logic for calculating datum transforms to use

src/core/qgscoordinatetransformcontext.cpp

@@ -129,5 +129,21 @@ bool QgsCoordinateTransformContext::addSourceDestinationDatumTransform( const Qg
 
 QPair<int, int> QgsCoordinateTransformContext::calculateDatumTransforms( const QgsCoordinateReferenceSystem &source, const QgsCoordinateReferenceSystem &destination ) const
 {
+  QString srcKey = source.authid();
+  QString destKey = destination.authid();
 
+  d->mLock.lockForRead();
+  // highest priority is exact match for source/dest pair
+  QPair< int, int > res = d->mSourceDestDatumTransforms.value( qMakePair( srcKey, destKey ), qMakePair( -1, -1 ) );
+  if ( res.first != -1 && res.second != -1 )
+  {
+    d->mLock.unlock();
+    return res;
+  }
+
+  // fallback to checking src and dest separately
+  int srcTransform = d->mSourceDatumTransforms.value( srcKey, -1 );
+  int destTransform = d->mDestDatumTransforms.value( destKey, -1 );
+  d->mLock.unlock();
+  return qMakePair( srcTransform, destTransform );
 }

tests/src/python/test_qgscoordinatetransformcontext.py

@@ -134,6 +134,52 @@ def testSourceDestinationDatumTransforms(self):
         context.clear()
         self.assertEqual(context.sourceDestinationDatumTransforms(), {})
 
+    def testCalculate(self):
+        context = QgsCoordinateTransformContext()
+
+        #empty context
+        self.assertEqual(context.calculateDatumTransforms(QgsCoordinateReferenceSystem('EPSG:3111'),
+                                                          QgsCoordinateReferenceSystem('EPSG:4283')),
+                         (-1, -1))
+
+        #add src transform
+        context.addSourceDatumTransform(QgsCoordinateReferenceSystem('EPSG:28356'), 1)
+        self.assertEqual(context.calculateDatumTransforms(QgsCoordinateReferenceSystem('EPSG:3111'),
+                                                          QgsCoordinateReferenceSystem('EPSG:4283')),
+                         (-1, -1))
+        self.assertEqual(context.calculateDatumTransforms(QgsCoordinateReferenceSystem('EPSG:28356'),
+                                                          QgsCoordinateReferenceSystem('EPSG:4283')),
+                         (1, -1))
+        self.assertEqual(context.calculateDatumTransforms(QgsCoordinateReferenceSystem('EPSG:4283'),
+                                                          QgsCoordinateReferenceSystem('EPSG:28356')),
+                         (-1, -1))
+
+        #add dest transform
+        context.addDestinationDatumTransform(QgsCoordinateReferenceSystem('EPSG:4283'), 2)
+        self.assertEqual(context.calculateDatumTransforms(QgsCoordinateReferenceSystem('EPSG:3111'),
+                                                          QgsCoordinateReferenceSystem('EPSG:4326')),
+                         (-1, -1))
+        self.assertEqual(context.calculateDatumTransforms(QgsCoordinateReferenceSystem('EPSG:3111'),
+                                                          QgsCoordinateReferenceSystem('EPSG:4283')),
+                         (-1, 2))
+        self.assertEqual(context.calculateDatumTransforms(QgsCoordinateReferenceSystem('EPSG:4283'),
+                                                          QgsCoordinateReferenceSystem('EPSG:3111')),
+                         (-1, -1))
+
+        #add specific source/dest pair - should take precedence
+        context.addSourceDestinationDatumTransform(QgsCoordinateReferenceSystem('EPSG:28356'),
+                                                   QgsCoordinateReferenceSystem('EPSG:4283'),
+                                                   3, 4)
+        self.assertEqual(context.calculateDatumTransforms(QgsCoordinateReferenceSystem('EPSG:28356'),
+                                                          QgsCoordinateReferenceSystem('EPSG:4283')),
+                         (3, 4))
+        self.assertEqual(context.calculateDatumTransforms(QgsCoordinateReferenceSystem('EPSG:3111'),
+                                                          QgsCoordinateReferenceSystem('EPSG:4283')),
+                         (-1, 2))
+        self.assertEqual(context.calculateDatumTransforms(QgsCoordinateReferenceSystem('EPSG:28356'),
+                                                          QgsCoordinateReferenceSystem('EPSG:3111')),
+                         (1, -1))
+
 
 if __name__ == '__main__':
     unittest.main()