Add method to determine whether transform context has a valid transform

for a specific src/dest CRS pair

Author: nyalldawson

python/core/qgscoordinatetransformcontext.sip

@@ -110,6 +110,14 @@ class QgsCoordinateTransformContext
 .. seealso:: addSourceDestinationDatumTransform()
 %End
 
+    bool hasTransform( const QgsCoordinateReferenceSystem &source,
+                       const QgsCoordinateReferenceSystem &destination ) const;
+%Docstring
+ Returns true if the context has a valid datum transform to use
+ when transforming from the specified ``source`` CRS to ``destination`` CRS.
+ :rtype: bool
+%End
+
     QPair< int, int > calculateDatumTransforms( const QgsCoordinateReferenceSystem &source,
         const QgsCoordinateReferenceSystem &destination ) const;
 %Docstring

src/app/qgisapp.cpp

@@ -12535,10 +12535,8 @@ bool QgisApp::askForDatumTransform( QgsCoordinateReferenceSystem sourceCrs, QgsC
   bool ok = false;
 
   QgsCoordinateTransformContext context = QgsProject::instance()->transformContext();
-  QPair<int, int> dt = context.calculateDatumTransforms( sourceCrs, destinationCrs );
-  if ( dt != qMakePair( -1, -1 ) )
+  if ( context.hasTransform( sourceCrs, destinationCrs ) )
   {
-    // already defined by user
     ok = true;
   }
   else

src/core/qgscoordinatetransformcontext.cpp

@@ -124,6 +124,12 @@ void QgsCoordinateTransformContext::removeSourceDestinationDatumTransform( const
   d->mSourceDestDatumTransforms.remove( qMakePair( sourceCrs.authid(), destinationCrs.authid() ) );
 }
 
+bool QgsCoordinateTransformContext::hasTransform( const QgsCoordinateReferenceSystem &source, const QgsCoordinateReferenceSystem &destination ) const
+{
+  QPair<int, int> t = calculateDatumTransforms( source, destination );
+  return t.first != -1 || t.second != -1;
+}
+
 QPair<int, int> QgsCoordinateTransformContext::calculateDatumTransforms( const QgsCoordinateReferenceSystem &source, const QgsCoordinateReferenceSystem &destination ) const
 {
   QString srcKey = source.authid();

src/core/qgscoordinatetransformcontext.h

@@ -209,6 +209,13 @@ class CORE_EXPORT QgsCoordinateTransformContext
     void removeSourceDestinationDatumTransform( const QgsCoordinateReferenceSystem &sourceCrs,
         const QgsCoordinateReferenceSystem &destinationCrs );
 
+    /**
+     * Returns true if the context has a valid datum transform to use
+     * when transforming from the specified \a source CRS to \a destination CRS.
+     */
+    bool hasTransform( const QgsCoordinateReferenceSystem &source,
+                       const QgsCoordinateReferenceSystem &destination ) const;
+
     /**
      * Returns the pair of source and destination datum transforms to use
      * for a transform from the specified \a source CRS to \a destination CRS.

tests/src/python/test_qgscoordinatetransformcontext.py

@@ -94,8 +94,15 @@ def testDestDatumTransforms(self):
     def testSourceDestinationDatumTransforms(self):
         context = QgsCoordinateTransformContext()
         self.assertEqual(context.sourceDestinationDatumTransforms(), {})
+        self.assertFalse(context.hasTransform(QgsCoordinateReferenceSystem('EPSG:3111'), QgsCoordinateReferenceSystem('EPSG:4283')))
         self.assertTrue(context.addSourceDestinationDatumTransform(QgsCoordinateReferenceSystem('EPSG:3111'),
                                                                    QgsCoordinateReferenceSystem('EPSG:4283'), 1, 2))
+        self.assertTrue(
+            context.hasTransform(QgsCoordinateReferenceSystem('EPSG:3111'), QgsCoordinateReferenceSystem('EPSG:4283')))
+        self.assertFalse(
+            context.hasTransform(QgsCoordinateReferenceSystem('EPSG:3111'), QgsCoordinateReferenceSystem('EPSG:4326')))
+        self.assertFalse(
+            context.hasTransform(QgsCoordinateReferenceSystem('EPSG:3113'), QgsCoordinateReferenceSystem('EPSG:4283')))
         self.assertEqual(context.sourceDestinationDatumTransforms(), {('EPSG:3111', 'EPSG:4283'): (1, 2)})
         self.assertTrue(context.addSourceDestinationDatumTransform(QgsCoordinateReferenceSystem('EPSG:28356'),
                                                                    QgsCoordinateReferenceSystem(4283), 3, 4))