datum transforms have reversible source/dest

Author: 3nids

python/core/qgscoordinatetransformcontext.sip

@@ -115,6 +115,9 @@ class QgsCoordinateTransformContext
 %Docstring
  Returns true if the context has a valid datum transform to use
  when transforming from the specified ``source`` CRS to ``destination`` CRS.
+.. note::
+
+   source and destination are reversible.
  :rtype: bool
 %End
 
@@ -126,6 +129,10 @@ class QgsCoordinateTransformContext
 
  Returns -1 if a datum transform should not be used for the source or
  destination.
+
+.. note::
+
+   source and destination are reversible.
  :rtype: QPair< int, int >
 %End
 

src/core/qgscoordinatetransformcontext.cpp

@@ -127,6 +127,7 @@ void QgsCoordinateTransformContext::removeSourceDestinationDatumTransform( const
 bool QgsCoordinateTransformContext::hasTransform( const QgsCoordinateReferenceSystem &source, const QgsCoordinateReferenceSystem &destination ) const
 {
   QPair<int, int> t = calculateDatumTransforms( source, destination );
+  // calculateDatumTransforms already takes care of switching source and destination
   return t.first != -1 || t.second != -1;
 }
 
@@ -138,6 +139,12 @@ QPair<int, int> QgsCoordinateTransformContext::calculateDatumTransforms( const Q
   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 == qMakePair( -1, -1 ) )
+  {
+    // try to reverse
+    QPair< int, int > res2 = d->mSourceDestDatumTransforms.value( qMakePair( destKey, srcKey ), qMakePair( -1, -1 ) );
+    res = qMakePair( res2.second, res2.first );
+  }
   d->mLock.unlock();
   return res;
 

src/core/qgscoordinatetransformcontext.h

@@ -212,6 +212,7 @@ class CORE_EXPORT QgsCoordinateTransformContext
     /**
      * Returns true if the context has a valid datum transform to use
      * when transforming from the specified \a source CRS to \a destination CRS.
+     * \note source and destination are reversible.
      */
     bool hasTransform( const QgsCoordinateReferenceSystem &source,
                        const QgsCoordinateReferenceSystem &destination ) const;
@@ -222,6 +223,8 @@ class CORE_EXPORT QgsCoordinateTransformContext
      *
      * Returns -1 if a datum transform should not be used for the source or
      * destination.
+     *
+     * \note source and destination are reversible.
      */
     QPair< int, int > calculateDatumTransforms( const QgsCoordinateReferenceSystem &source,
         const QgsCoordinateReferenceSystem &destination ) const;