Show a warning when loading a project with a transform not available locally

python/core/qgscoordinatetransformcontext.sip

@@ -137,10 +137,15 @@ class QgsCoordinateTransformContext
  :rtype: QgsCoordinateTransform.TransformPair
 %End
 
-    void readXml( const QDomElement &element, const QgsReadWriteContext &context );
+    bool readXml( const QDomElement &element, const QgsReadWriteContext &context, QStringList &missingTransforms /Out/ );
 %Docstring
  Reads the context's state from a DOM ``element``.
+
+ Returns false if transforms stored in the XML are not available. In this case ``missingTransforms`` will be
+ filled with missing datum transform strings.
+
 .. seealso:: :py:func:`writeXml()`
+ :rtype: bool
 %End
 
     void writeXml( QDomElement &element, const QgsReadWriteContext &context ) const;

python/core/qgsproject.sip

@@ -943,6 +943,12 @@ emitted whenever the configuration for snapping has changed
 
 .. versionadded:: 3.0
 .. seealso:: :py:func:`transformContext()`
+%End
+
+    void missingDatumTransforms( const QStringList &missingTransforms );
+%Docstring
+ Emitted when datum transforms stored in the project are not available locally.
+.. versionadded:: 3.0
 %End
 
     void transactionGroupsChanged();

src/app/qgisapp.cpp

@@ -3186,6 +3186,12 @@ void QgisApp::setupConnections()
   // project crs connections
   connect( QgsProject::instance(), &QgsProject::crsChanged, this, &QgisApp::projectCrsChanged );
 
+  connect( QgsProject::instance(), &QgsProject::missingDatumTransforms, this, [ = ]( const QStringList & transforms )
+  {
+    QString message = tr( "Transforms are not installed: %1 " ).arg( transforms.join( QStringLiteral( " ," ) ) );
+    messageBar()->pushWarning( tr( "Missing datum transforms" ), message );
+  } );
+
   connect( QgsProject::instance(), &QgsProject::labelingEngineSettingsChanged,
            this, [ = ]
   {

src/core/qgscoordinatetransformcontext.cpp

@@ -161,7 +161,7 @@ QgsCoordinateTransform::TransformPair QgsCoordinateTransformContext::calculateDa
 #endif
 }
 
-void QgsCoordinateTransformContext::readXml( const QDomElement &element, const QgsReadWriteContext & )
+bool QgsCoordinateTransformContext::readXml( const QDomElement &element, const QgsReadWriteContext &, QStringList &missingTransforms )
 {
   d.detach();
   d->mLock.lockForWrite();
@@ -176,9 +176,12 @@ void QgsCoordinateTransformContext::readXml( const QDomElement &element, const Q
   if ( contextNodes.count() < 1 )
   {
     d->mLock.unlock();
-    return;
+    return true;
   }
 
+  missingTransforms.clear();
+  bool result = true;
+
   const QDomElement contextElem = contextNodes.at( 0 ).toElement();
 
   // src/dest transforms
@@ -194,11 +197,26 @@ void QgsCoordinateTransformContext::readXml( const QDomElement &element, const Q
 
     int datumId1 = -1;
     int datumId2 = -1;
+    //warn if value1 or value2 is non-empty, yet no matching transform was found
     if ( !value1.isEmpty() )
+    {
       datumId1 = QgsCoordinateTransform::projStringToDatumTransformId( value1 );
+      if ( datumId1 < 0 )
+      {
+        result = false;
+        missingTransforms << value1;
+      }
+    }
     if ( !value2.isEmpty() )
+    {
       datumId2 = QgsCoordinateTransform::projStringToDatumTransformId( value2 );
-    //TODO - throw warning if value1 or value2 is non-empty, yet no matching transform was found
+      if ( datumId2 < 0 )
+      {
+        result = false;
+        missingTransforms << value2;
+      }
+    }
+
     d->mSourceDestDatumTransforms.insert( qMakePair( key1, key2 ), QgsCoordinateTransform::TransformPair( datumId1, datumId2 ) );
   }
 
@@ -235,6 +253,7 @@ void QgsCoordinateTransformContext::readXml( const QDomElement &element, const Q
 #endif
 
   d->mLock.unlock();
+  return result;
 }
 
 void QgsCoordinateTransformContext::writeXml( QDomElement &element, const QgsReadWriteContext & ) const

src/core/qgscoordinatetransformcontext.h

@@ -233,9 +233,13 @@ class CORE_EXPORT QgsCoordinateTransformContext
 
     /**
      * Reads the context's state from a DOM \a element.
+     *
+     * Returns false if transforms stored in the XML are not available. In this case \a missingTransforms will be
+     * filled with missing datum transform strings.
+     *
      * \see writeXml()
      */
-    void readXml( const QDomElement &element, const QgsReadWriteContext &context );
+    bool readXml( const QDomElement &element, const QgsReadWriteContext &context, QStringList &missingTransforms SIP_OUT );
 
     /**
      * Writes the context's state to a DOM \a element.

src/core/qgsproject.cpp

@@ -925,7 +925,11 @@ bool QgsProject::readProjectFile( const QString &filename )
   }
   mCrs = projectCrs;
 
-  mTransformContext.readXml( doc->documentElement(), context );
+  QStringList datumErrors;
+  if ( !mTransformContext.readXml( doc->documentElement(), context, datumErrors ) )
+  {
+    emit missingDatumTransforms( datumErrors );
+  }
   emit transformContextChanged();
 
   QDomNodeList nl = doc->elementsByTagName( QStringLiteral( "autotransaction" ) );

src/core/qgsproject.h

@@ -933,6 +933,12 @@ class CORE_EXPORT QgsProject : public QObject, public QgsExpressionContextGenera
      */
     void transformContextChanged();
 
+    /**
+     * Emitted when datum transforms stored in the project are not available locally.
+     * \since QGIS 3.0
+     */
+    void missingDatumTransforms( const QStringList &missingTransforms );
+
     /**
      * Emitted whenever a new transaction group has been created or a
      * transaction group has been removed.