Cleanup (and rename?) QgsGeometry

[nyalldawson]

Since the merge of the new geometry engine, the role of QgsGeometry is undefined. It currently acts as just an implicitly shared container for a QgsAbstractGeometryV2, plus a random bunch of methods for modifying and converting the geometry.

I'd like to see this refined in QGIS 3.0. My thoughts:

• rename QgsGeometry to QgsGeometryContainer, to better indicate what it's used for and differentiate it from QgsAbstractGeometryV2
• move all geometry modification (eg buffer, addRing, addPart, ...) and relation methods ( eg contains, crosses, ...) out of this class. Many are already implemented in either QgsGeometryEngine, QgsAbstractGeometryV2 or QgsGeometryUtils and the QgsGeometry versions are just wrappers around these
• move all the creation methods (eg fromQPointF, ... ) to QgsGeometryFactory (or to their relevant geometry class, eg QgsPointV2::fromQPointF )

What would remain in QgsGeometryContainer would be:

• methods for accessing and setting the contained QgsAbstractGeometryV2, isEmpty()
• some conversion routines, eg convertToMultiType, convertToSingleType, segmentize, and a general "bool convertTo( QgsWKBTypes::Type )" and "bool canConvertTo( QgsWKBTypes::Type )"

[raymondnijssen]

• Related to this, I think the geometry model should work in an inherited way, so you could do a QgsPoint.buffer() and QgsLine.buffer() without writing the typecasting code to (and from) QgsGeometry all the time.

• We could merge the QgsPoint and QgsPointXY classes

• Typecasting to and from QgsGeometry should have more descriptive function names than get() and set().
  (According to the documentation set() will be removed from the python api in qgis4 anyway)

[m-kuhn]

@raymondnijssen I think parts of this does not require QGIS 4. Mainly because inheritance is not involved in the way your comment suggest.

You can already right now today add a new method

QgsGeometry QgsGeometry::buffer()
{
  .... write some code that calls QgsAbstractGeometry::buffer on the contained geometry.
}

PS: and I am very much in favor of doing this!! This will mean that there is less reason to actually call get() and constGet() which will make the API way easier to use.