[Sketcher] Only move one piece of B-spline when dragging

src/Mod/Sketcher/App/Sketch.cpp

@@ -4021,6 +4021,66 @@ void Sketch::resetInitMove()
     isInitMove = false;
 }
 
+int Sketch::initBSplinePieceMove(int geoId, PointPos pos, const Base::Vector3d& firstPoint, bool fine)
+{
+    isFine = fine;
+
+    geoId = checkGeoId(geoId);
+
+    clearTemporaryConstraints();
+
+    // don't try to move sketches that contain conflicting constraints
+    if (hasConflicts()) {
+        isInitMove = false;
+        return -1;
+    }
+
+    // this is only meant for B-Splines
+    if (Geoms[geoId].type != BSpline || pos == PointPos::start || pos == PointPos::end) {
+        return -1;
+    }
+
+    // Find the closest knot
+    auto partBsp = static_cast<GeomBSplineCurve*>(Geoms[geoId].geo);
+    double uNear;
+    partBsp->closestParameter(firstPoint, uNear);
+    GCS::BSpline &bsp = BSplines[Geoms[geoId].index];
+    auto& knots = bsp.knots;
+    auto upperknot = std::upper_bound(
+        knots.begin(), knots.end(), uNear,
+        [](double u, double* element) {
+            return u < *element;
+        });
+
+    size_t idx = 0;
+    // skipping the first knot for adjustment
+    // TODO: ensure this works for periodic as well
+    for (size_t i=1; i<bsp.mult.size() && knots[i]!=*upperknot; ++i)
+        idx += bsp.mult[i];
+
+    MoveParameters.resize(2*(bsp.degree+1)); // x[idx],y[idx],x[idx+1],y[idx+1],...
+
+    size_t mvindex = 0;
+    auto lastIt = (idx + bsp.degree + 1) % bsp.poles.size();
+    for (size_t i = idx; i != lastIt; i=(i+1)%bsp.poles.size(), ++mvindex) {
+        GCS::Point p1;
+        p1.x = &MoveParameters[mvindex];
+        ++mvindex;
+        p1.y = &MoveParameters[mvindex];
+
+        *p1.x = *bsp.poles[i].x;
+        *p1.y = *bsp.poles[i].y;
+
+        GCSsys.addConstraintP2PCoincident(p1,bsp.poles[i],GCS::DefaultTemporaryConstraint);
+    }
+
+    InitParameters = MoveParameters;
+
+    GCSsys.initSolution();
+    isInitMove = true;
+    return 0;
+}
+
 int Sketch::movePoint(int geoId, PointPos pos, Base::Vector3d toPoint, bool relative)
 {
     geoId = checkGeoId(geoId);

src/Mod/Sketcher/App/Sketch.h

@@ -133,10 +133,19 @@ public:
       */
     int initMove(int geoId, PointPos pos, bool fine=true);
 
+    /** Initializes a B-spline piece drag by setting the current
+      * sketch status as a reference. Only moves piece around `firstPoint`.
+      */
+    int initBSplinePieceMove(int geoId, PointPos pos, const Base::Vector3d& firstPoint, bool fine=true);
+
     /** Resets the initialization of a point or curve drag
      */
     void resetInitMove();
 
+    /** Limits a b-spline drag to the segment around `firstPoint`.
+     */
+    int limitBSplineMove(int geoId, PointPos pos, const Base::Vector3d& firstPoint);
+
     /** move this point (or curve) to a new location and solve.
       * This will introduce some additional weak constraints expressing
       * a condition for satisfying the new point location!

src/Mod/Sketcher/App/SketchObject.h

@@ -450,6 +450,8 @@ public: /* Solver exposed interface */
         {solvedSketch.setRecalculateInitialSolutionWhileMovingPoint(recalculateInitialSolutionWhileMovingPoint);}
     /// Forwards a request for a temporary initMove to the solver using the current sketch state as a reference (enables dragging)
     inline int initTemporaryMove(int geoId, PointPos pos, bool fine=true);
+    /// Forwards a request for a temporary initBSplinePieceMove to the solver using the current sketch state as a reference (enables dragging)
+    inline int initTemporaryBSplinePieceMove(int geoId, PointPos pos, const Base::Vector3d& firstPoint, bool fine=true);
     /** Forwards a request for point or curve temporary movement to the solver using the current state as a reference (enables dragging).
      *  NOTE: A temporary move operation must always be preceded by a initTemporaryMove() operation.
      */
@@ -692,6 +694,17 @@ inline int SketchObject::initTemporaryMove(int geoId, PointPos pos, bool fine/*=
     return solvedSketch.initMove(geoId,pos,fine);
 }
 
+inline int SketchObject::initTemporaryBSplinePieceMove(int geoId, PointPos pos, const Base::Vector3d& firstPoint, bool fine)
+{
+    // if a previous operation did not update the geometry (including geometry extensions)
+    // or constraints (including any deleted pointer, as in renameConstraint) of the solver,
+    // here we update them before starting a temporary operation.
+    if(solverNeedsUpdate)
+        solve();
+
+    return solvedSketch.initBSplinePieceMove(geoId,pos,firstPoint,fine);
+}
+
 inline int SketchObject::moveTemporaryPoint(int geoId, PointPos pos, Base::Vector3d toPoint, bool relative/*=false*/)
 {
     return solvedSketch.movePoint(geoId, pos, toPoint, relative);

src/Mod/Sketcher/Gui/ViewProviderSketch.cpp

@@ -1234,8 +1234,13 @@ bool ViewProviderSketch::mouseMove(const SbVec2s &cursorPos, Gui::View3DInventor
                     drag.resetVector();
                 }
 
-                getSketchObject()->initTemporaryMove(drag.DragCurve, Sketcher::PointPos::none, false);
-
+                if (geo->getTypeId() == Part::GeomBSplineCurve::getClassTypeId()) {
+                    getSketchObject()->initTemporaryBSplinePieceMove(
+                        drag.DragCurve, Sketcher::PointPos::none,
+                        Base::Vector3d(drag.xInit, drag.yInit, 0.0), false);
+                } else {
+                    getSketchObject()->initTemporaryMove(drag.DragCurve, Sketcher::PointPos::none, false);
+                }
             } else {
                 Mode = STATUS_NONE;
             }