Add QgsCoordinateTransformContext arguments to QgsCoordinateTransform constructor

And automatically retrieve correct datum transforms for the
transform, based on the information in the context.

Also add a convenience constructor which takes a QgsProject
instance instead of a QgsCoordinateTransformContext and which
automatically retrieves the project's transform context and uses
that. This is designed to make it easy to upgrade existing
Python code (by adding just QgsProject.instance() to the transform
constructors).

Author: nyalldawson

python/core/qgscoordinatetransform.sip

@@ -45,14 +45,42 @@ class QgsCoordinateTransform
 Default constructor, creates an invalid QgsCoordinateTransform.
 %End
 
-    QgsCoordinateTransform( const QgsCoordinateReferenceSystem &source,
-                            const QgsCoordinateReferenceSystem &destination );
+    explicit QgsCoordinateTransform( const QgsCoordinateReferenceSystem &source,
+                                     const QgsCoordinateReferenceSystem &destination );
 %Docstring
  Constructs a QgsCoordinateTransform using QgsCoordinateReferenceSystem objects.
  \param source source CRS, typically of the layer's coordinate system
  \param destination CRS, typically of the map canvas coordinate system
 %End
 
+    explicit QgsCoordinateTransform( const QgsCoordinateReferenceSystem &source,
+                                     const QgsCoordinateReferenceSystem &destination,
+                                     const QgsCoordinateTransformContext &context );
+%Docstring
+ Constructs a QgsCoordinateTransform to transform from the ``source``
+ to ``destination`` coordinate reference system.
+
+ The ``context`` argument specifies the context under which the transform
+ will be applied, and is used for calculating necessary datum transforms
+ to utilise.
+
+.. versionadded:: 3.0
+%End
+
+    explicit QgsCoordinateTransform( const QgsCoordinateReferenceSystem &source,
+                                     const QgsCoordinateReferenceSystem &destination,
+                                     const QgsProject *project );
+%Docstring
+ Constructs a QgsCoordinateTransform to transform from the ``source``
+ to ``destination`` coordinate reference system, when used with the
+ given ``project``.
+
+ No reference to ``project`` is stored or utilised outside of the constructor,
+ and it is used to retrieve the project's transform context only.
+
+.. versionadded:: 3.0
+%End
+
     QgsCoordinateTransform( const QgsCoordinateTransform &o );
 %Docstring
  Copy constructor
@@ -86,6 +114,13 @@ Default constructor, creates an invalid QgsCoordinateTransform.
  \param crs CRS to transform coordinates to
 .. seealso:: :py:func:`destinationCrs()`
 .. seealso:: :py:func:`setSourceCrs()`
+%End
+
+    void setContext( const QgsCoordinateTransformContext &context );
+%Docstring
+ Sets the ``context`` in which the coordinate transform should be
+ calculated.
+.. versionadded:: 3.0
 %End
 
     QgsCoordinateReferenceSystem sourceCrs() const;
@@ -213,14 +248,53 @@ Default constructor, creates an invalid QgsCoordinateTransform.
 
     int sourceDatumTransform() const;
 %Docstring
+ Returns the index of the datum transform to use when projecting from the source
+ CRS.
+
+ This is usually calculated automatically from the transform's QgsCoordinateTransformContext,
+ but can be manually overwritten by a call to setSourceDatumTransform().
+
+.. seealso:: setSourceDatumTransform()
+.. seealso:: destinationDatumTransform()
  :rtype: int
 %End
-    void setSourceDatumTransform( int dt );
+
+    void setSourceDatumTransform( int datum );
+%Docstring
+ Sets the index of the ``datum`` transform to use when projecting from the source
+ CRS.
+
+ This is usually calculated automatically from the transform's QgsCoordinateTransformContext.
+ Calling this method will overwrite any automatically calculated datum transform.
+
+.. seealso:: sourceDatumTransform()
+.. seealso:: setDestinationDatumTransform()
+%End
+
     int destinationDatumTransform() const;
 %Docstring
+ Returns the index of the datum transform to use when projecting to the destination
+ CRS.
+
+ This is usually calculated automatically from the transform's QgsCoordinateTransformContext,
+ but can be manually overwritten by a call to setDestinationDatumTransform().
+
+.. seealso:: setDestinationDatumTransform()
+.. seealso:: sourceDatumTransform()
  :rtype: int
 %End
-    void setDestinationDatumTransform( int dt );
+
+    void setDestinationDatumTransform( int datum );
+%Docstring
+ Sets the index of the ``datum`` transform to use when projecting to the destination
+ CRS.
+
+ This is usually calculated automatically from the transform's QgsCoordinateTransformContext.
+ Calling this method will overwrite any automatically calculated datum transform.
+
+.. seealso:: destinationDatumTransform()
+.. seealso:: setSourceDatumTransform()
+%End
 
     void initialize();
 %Docstring

src/core/qgscoordinatetransform.cpp

@@ -22,6 +22,7 @@
 #include "qgspointxy.h"
 #include "qgsrectangle.h"
 #include "qgsexception.h"
+#include "qgsproject.h"
 
 //qt includes
 #include <QDomNode>
@@ -47,7 +48,17 @@ QgsCoordinateTransform::QgsCoordinateTransform()
 
 QgsCoordinateTransform::QgsCoordinateTransform( const QgsCoordinateReferenceSystem &source, const QgsCoordinateReferenceSystem &destination )
 {
-  d = new QgsCoordinateTransformPrivate( source, destination );
+  d = new QgsCoordinateTransformPrivate( source, destination, QgsCoordinateTransformContext() );
+}
+
+QgsCoordinateTransform::QgsCoordinateTransform( const QgsCoordinateReferenceSystem &source, const QgsCoordinateReferenceSystem &destination, const QgsCoordinateTransformContext &context )
+{
+  d = new QgsCoordinateTransformPrivate( source, destination, context );
+}
+
+QgsCoordinateTransform::QgsCoordinateTransform( const QgsCoordinateReferenceSystem &source, const QgsCoordinateReferenceSystem &destination, const QgsProject *project )
+{
+  d = new QgsCoordinateTransformPrivate( source, destination, project ? project->transformContext() : QgsCoordinateTransformContext() );
 }
 
 QgsCoordinateTransform::QgsCoordinateTransform( const QgsCoordinateTransform &o )
@@ -67,12 +78,22 @@ void QgsCoordinateTransform::setSourceCrs( const QgsCoordinateReferenceSystem &c
 {
   d.detach();
   d->mSourceCRS = crs;
+  d->calculateTransforms();
   d->initialize();
 }
 void QgsCoordinateTransform::setDestinationCrs( const QgsCoordinateReferenceSystem &crs )
 {
   d.detach();
   d->mDestCRS = crs;
+  d->calculateTransforms();
+  d->initialize();
+}
+
+void QgsCoordinateTransform::setContext( const QgsCoordinateTransformContext &context )
+{
+  d.detach();
+  d->mContext = context;
+  d->calculateTransforms();
   d->initialize();
 }
 

src/core/qgscoordinatetransform.h

@@ -27,6 +27,8 @@ class QgsCoordinateTransformPrivate;
 class QgsPointXY;
 class QgsRectangle;
 class QPolygonF;
+class QgsCoordinateTransformContext;
+class QgsProject;
 
 /**
  * \ingroup core
@@ -63,8 +65,36 @@ class CORE_EXPORT QgsCoordinateTransform
      * \param source source CRS, typically of the layer's coordinate system
      * \param destination CRS, typically of the map canvas coordinate system
      */
-    QgsCoordinateTransform( const QgsCoordinateReferenceSystem &source,
-                            const QgsCoordinateReferenceSystem &destination );
+    explicit QgsCoordinateTransform( const QgsCoordinateReferenceSystem &source,
+                                     const QgsCoordinateReferenceSystem &destination );
+
+    /**
+     * Constructs a QgsCoordinateTransform to transform from the \a source
+     * to \a destination coordinate reference system.
+     *
+     * The \a context argument specifies the context under which the transform
+     * will be applied, and is used for calculating necessary datum transforms
+     * to utilise.
+     *
+     * \since QGIS 3.0
+     */
+    explicit QgsCoordinateTransform( const QgsCoordinateReferenceSystem &source,
+                                     const QgsCoordinateReferenceSystem &destination,
+                                     const QgsCoordinateTransformContext &context );
+
+    /**
+     * Constructs a QgsCoordinateTransform to transform from the \a source
+     * to \a destination coordinate reference system, when used with the
+     * given \a project.
+     *
+     * No reference to \a project is stored or utilised outside of the constructor,
+     * and it is used to retrieve the project's transform context only.
+     *
+     * \since QGIS 3.0
+     */
+    explicit QgsCoordinateTransform( const QgsCoordinateReferenceSystem &source,
+                                     const QgsCoordinateReferenceSystem &destination,
+                                     const QgsProject *project );
 
     /**
      * Copy constructor
@@ -101,6 +131,13 @@ class CORE_EXPORT QgsCoordinateTransform
      */
     void setDestinationCrs( const QgsCoordinateReferenceSystem &crs );
 
+    /**
+     * Sets the \a context in which the coordinate transform should be
+     * calculated.
+     * \since QGIS 3.0
+     */
+    void setContext( const QgsCoordinateTransformContext &context );
+
     /**
      * Returns the source coordinate reference system, which the transform will
      * transform coordinates from.
@@ -271,10 +308,53 @@ class CORE_EXPORT QgsCoordinateTransform
         \returns epsgNr epsg code of the transformation (or 0 if not in epsg db)*/
     static bool datumTransformCrsInfo( int datumTransform, int &epsgNr, QString &srcProjection, QString &dstProjection, QString &remarks, QString &scope, bool &preferred, bool &deprecated );
 
+    /**
+     * Returns the index of the datum transform to use when projecting from the source
+     * CRS.
+     *
+     * This is usually calculated automatically from the transform's QgsCoordinateTransformContext,
+     * but can be manually overwritten by a call to setSourceDatumTransform().
+     *
+     * \see setSourceDatumTransform()
+     * \see destinationDatumTransform()
+     */
     int sourceDatumTransform() const;
-    void setSourceDatumTransform( int dt );
+
+    /**
+     * Sets the index of the \a datum transform to use when projecting from the source
+     * CRS.
+     *
+     * This is usually calculated automatically from the transform's QgsCoordinateTransformContext.
+     * Calling this method will overwrite any automatically calculated datum transform.
+     *
+     * \see sourceDatumTransform()
+     * \see setDestinationDatumTransform()
+     */
+    void setSourceDatumTransform( int datum );
+
+    /**
+     * Returns the index of the datum transform to use when projecting to the destination
+     * CRS.
+     *
+     * This is usually calculated automatically from the transform's QgsCoordinateTransformContext,
+     * but can be manually overwritten by a call to setDestinationDatumTransform().
+     *
+     * \see setDestinationDatumTransform()
+     * \see sourceDatumTransform()
+     */
     int destinationDatumTransform() const;
-    void setDestinationDatumTransform( int dt );
+
+    /**
+     * Sets the index of the \a datum transform to use when projecting to the destination
+     * CRS.
+     *
+     * This is usually calculated automatically from the transform's QgsCoordinateTransformContext.
+     * Calling this method will overwrite any automatically calculated datum transform.
+     *
+     * \see destinationDatumTransform()
+     * \see setSourceDatumTransform()
+     */
+    void setDestinationDatumTransform( int datum );
 
     //!initialize is used to actually create the Transformer instance
     void initialize();

src/core/qgscoordinatetransform_p.cpp

@@ -50,11 +50,15 @@ QgsCoordinateTransformPrivate::QgsCoordinateTransformPrivate()
   setFinder();
 }
 
-QgsCoordinateTransformPrivate::QgsCoordinateTransformPrivate( const QgsCoordinateReferenceSystem &source, const QgsCoordinateReferenceSystem &destination )
+QgsCoordinateTransformPrivate::QgsCoordinateTransformPrivate( const QgsCoordinateReferenceSystem &source,
+    const QgsCoordinateReferenceSystem &destination,
+    const QgsCoordinateTransformContext &context )
   : mSourceCRS( source )
   , mDestCRS( destination )
+  , mContext( context )
 {
   setFinder();
+  calculateTransforms();
   initialize();
 }
 
@@ -64,6 +68,7 @@ QgsCoordinateTransformPrivate::QgsCoordinateTransformPrivate( const QgsCoordinat
   , mShortCircuit( other.mShortCircuit )
   , mSourceCRS( other.mSourceCRS )
   , mDestCRS( other.mDestCRS )
+  , mContext( other.mContext )
   , mSourceDatumTransform( other.mSourceDatumTransform )
   , mDestinationDatumTransform( other.mDestinationDatumTransform )
 {
@@ -101,7 +106,9 @@ bool QgsCoordinateTransformPrivate::initialize()
 
   mIsValid = true;
 
-  bool useDefaultDatumTransform = ( mSourceDatumTransform == - 1 && mDestinationDatumTransform == -1 );
+  int sourceDatumTransform = mSourceDatumTransform;
+  int destDatumTransform = mDestinationDatumTransform;
+  bool useDefaultDatumTransform = ( sourceDatumTransform == - 1 && destDatumTransform == -1 );
 
   // init the projections (destination and source)
   freeProj();
@@ -111,24 +118,24 @@ bool QgsCoordinateTransformPrivate::initialize()
   {
     mSourceProjString = stripDatumTransform( mSourceProjString );
   }
-  if ( mSourceDatumTransform != -1 )
+  if ( sourceDatumTransform != -1 )
   {
-    mSourceProjString += ( ' ' + datumTransformString( mSourceDatumTransform ) );
+    mSourceProjString += ( ' ' + datumTransformString( sourceDatumTransform ) );
   }
 
   mDestProjString = mDestCRS.toProj4();
   if ( !useDefaultDatumTransform )
   {
     mDestProjString = stripDatumTransform( mDestProjString );
   }
-  if ( mDestinationDatumTransform != -1 )
+  if ( destDatumTransform != -1 )
   {
-    mDestProjString += ( ' ' +  datumTransformString( mDestinationDatumTransform ) );
+    mDestProjString += ( ' ' +  datumTransformString( destDatumTransform ) );
   }
 
   if ( !useDefaultDatumTransform )
   {
-    addNullGridShifts( mSourceProjString, mDestProjString );
+    addNullGridShifts( mSourceProjString, mDestProjString, sourceDatumTransform, destDatumTransform );
   }
 
   // create proj projections for current thread
@@ -185,6 +192,14 @@ bool QgsCoordinateTransformPrivate::initialize()
   return mIsValid;
 }
 
+void QgsCoordinateTransformPrivate::calculateTransforms()
+{
+  // recalculate datum transforms from context
+  QPair< int, int > transforms = mContext.calculateDatumTransforms( mSourceCRS, mDestCRS );
+  mSourceDatumTransform = transforms.first;
+  mDestinationDatumTransform = transforms.second;
+}
+
 QPair<projPJ, projPJ> QgsCoordinateTransformPrivate::threadLocalProjData()
 {
   mProjLock.lockForRead();
@@ -301,27 +316,28 @@ QString QgsCoordinateTransformPrivate::datumTransformString( int datumTransform
   return transformString;
 }
 
-void QgsCoordinateTransformPrivate::addNullGridShifts( QString &srcProjString, QString &destProjString ) const
+void QgsCoordinateTransformPrivate::addNullGridShifts( QString &srcProjString, QString &destProjString,
+    int sourceDatumTransform, int destinationDatumTransform ) const
 {
   //if one transformation uses ntv2, the other one needs to be null grid shift
-  if ( mDestinationDatumTransform == -1 && srcProjString.contains( QLatin1String( "+nadgrids" ) ) ) //add null grid if source transformation is ntv2
+  if ( destinationDatumTransform == -1 && srcProjString.contains( QLatin1String( "+nadgrids" ) ) ) //add null grid if source transformation is ntv2
   {
     destProjString += QLatin1String( " +nadgrids=@null" );
     return;
   }
-  if ( mSourceDatumTransform == -1 && destProjString.contains( QLatin1String( "+nadgrids" ) ) )
+  if ( sourceDatumTransform == -1 && destProjString.contains( QLatin1String( "+nadgrids" ) ) )
   {
     srcProjString += QLatin1String( " +nadgrids=@null" );
     return;
   }
 
   //add null shift grid for google mercator
   //(see e.g. http://trac.osgeo.org/proj/wiki/FAQ#ChangingEllipsoidWhycantIconvertfromWGS84toGoogleEarthVirtualGlobeMercator)
-  if ( mSourceCRS.authid().compare( QLatin1String( "EPSG:3857" ), Qt::CaseInsensitive ) == 0 && mSourceDatumTransform == -1 )
+  if ( mSourceCRS.authid().compare( QLatin1String( "EPSG:3857" ), Qt::CaseInsensitive ) == 0 && sourceDatumTransform == -1 )
   {
     srcProjString += QLatin1String( " +nadgrids=@null" );
   }
-  if ( mDestCRS.authid().compare( QLatin1String( "EPSG:3857" ), Qt::CaseInsensitive ) == 0 && mDestinationDatumTransform == -1 )
+  if ( mDestCRS.authid().compare( QLatin1String( "EPSG:3857" ), Qt::CaseInsensitive ) == 0 && destinationDatumTransform == -1 )
   {
     destProjString += QLatin1String( " +nadgrids=@null" );
   }