Add explicit setter for project's transform context

And add transformContextChanged signal, so that we can detect
when the context is changed.

python/core/qgsproject.sip

@@ -128,17 +128,29 @@ Returns the QgsProject singleton instance
 %End
 
 
-
-    QgsCoordinateTransformContext &transformContext();
+    QgsCoordinateTransformContext transformContext() const;
 %Docstring
- Returns a modifiable reference to the project's coordinate transform context, which stores various
+ Returns a copy of the project's coordinate transform context, which stores various
  information regarding which datum transforms should be used when transforming points
  from a source to destination coordinate reference system.
 
 .. versionadded:: 3.0
+.. seealso:: setTransformContext()
+.. seealso:: transformContextChanged()
  :rtype: QgsCoordinateTransformContext
 %End
 
+    void setTransformContext( const QgsCoordinateTransformContext &context );
+%Docstring
+ Sets the project's coordinate transform ``context``, which stores various
+ information regarding which datum transforms should be used when transforming points
+ from a source to destination coordinate reference system.
+
+.. versionadded:: 3.0
+.. seealso:: transformContext()
+.. seealso:: transformContextChanged()
+%End
+
     void clear();
 %Docstring
  Clear the project - removes all settings and resets it back to an empty, default state.
@@ -924,6 +936,15 @@ emitted whenever the configuration for snapping has changed
 .. seealso:: :py:func:`ellipsoid()`
 %End
 
+
+    void transformContextChanged();
+%Docstring
+ Emitted when the project transformContext() is changed.
+
+.. versionadded:: 3.0
+.. seealso:: transformContext()
+%End
+
     void transactionGroupsChanged();
 %Docstring
  Emitted whenever a new transaction group has been created or a

src/core/qgsproject.cpp

@@ -476,9 +476,10 @@ QgsCoordinateTransformContext QgsProject::transformContext() const
   return mTransformContext;
 }
 
-QgsCoordinateTransformContext &QgsProject::transformContext()
+void QgsProject::setTransformContext( const QgsCoordinateTransformContext &context )
 {
-  return mTransformContext;
+  mTransformContext = context;
+  emit transformContextChanged();
 }
 
 void QgsProject::clear()

src/core/qgsproject.h

@@ -186,17 +186,21 @@ class CORE_EXPORT QgsProject : public QObject, public QgsExpressionContextGenera
      * from a source to destination coordinate reference system.
      *
      * \since QGIS 3.0
+     * \see setTransformContext()
+     * \see transformContextChanged()
      */
-    QgsCoordinateTransformContext transformContext() const SIP_SKIP;
+    QgsCoordinateTransformContext transformContext() const;
 
     /**
-     * Returns a modifiable reference to the project's coordinate transform context, which stores various
+     * Sets the project's coordinate transform \a context, which stores various
      * information regarding which datum transforms should be used when transforming points
      * from a source to destination coordinate reference system.
      *
      * \since QGIS 3.0
+     * \see transformContext()
+     * \see transformContextChanged()
      */
-    QgsCoordinateTransformContext &transformContext();
+    void setTransformContext( const QgsCoordinateTransformContext &context );
 
     /**
      * Clear the project - removes all settings and resets it back to an empty, default state.
@@ -920,6 +924,15 @@ class CORE_EXPORT QgsProject : public QObject, public QgsExpressionContextGenera
      */
     void ellipsoidChanged( const QString &ellipsoid );
 
+
+    /**
+     * Emitted when the project transformContext() is changed.
+     *
+     * \since QGIS 3.0
+     * \see transformContext()
+     */
+    void transformContextChanged();
+
     /**
      * Emitted whenever a new transaction group has been created or a
      * transaction group has been removed.

tests/src/python/test_qgscoordinatetransform.py

@@ -138,7 +138,9 @@ class TestQgsCoordinateTransform(unittest.TestCase):
         Test creating transform using convenience constructor which takes project reference
         """
         p = QgsProject()
-        p.transformContext().addSourceDatumTransform(QgsCoordinateReferenceSystem('EPSG:28356'), 1)
+        context = p.transformContext()
+        context.addSourceDatumTransform(QgsCoordinateReferenceSystem('EPSG:28356'), 1)
+        p.setTransformContext(context)
 
         transform = QgsCoordinateTransform(QgsCoordinateReferenceSystem('EPSG:28356'), QgsCoordinateReferenceSystem('EPSG:28353'), p)
         self.assertEqual(transform.sourceDatumTransform(), 1)

tests/src/python/test_qgscoordinatetransformcontext.py

@@ -20,6 +20,7 @@ from qgis.core import (QgsCoordinateReferenceSystem,
                        QgsProject)
 from qgis.testing import start_app, unittest
 from qgis.PyQt.QtXml import QDomDocument
+from qgis.PyQt.QtTest import QSignalSpy
 
 app = start_app()
 
@@ -220,7 +221,11 @@ class TestQgsCoordinateTransformContext(unittest.TestCase):
         Test project's transform context
         """
         project = QgsProject()
-        project.transformContext().addSourceDatumTransform(QgsCoordinateReferenceSystem('EPSG:3111'), 1)
+        context_changed_spy = QSignalSpy(project.transformContextChanged)
+        context = project.transformContext()
+        context.addSourceDatumTransform(QgsCoordinateReferenceSystem('EPSG:3111'), 1)
+        project.setTransformContext(context)
+        self.assertEqual(len(context_changed_spy), 1)
         self.assertEqual(project.transformContext().sourceDatumTransforms(), {'EPSG:3111': 1})