PartDesign: Enable selecting a sketch as base plane of another sketch (#23428)

* PartDesign: Enable selecting a sketch as base plane of another sketch

* to squash

* Part: Attacher: enable attaching to empty objects such as empty Sketch or Body.

* Update SketchWorkflow.cpp

src/Mod/Part/App/Attacher.cpp

@@ -842,9 +842,19 @@ void AttachEngine::readLinks(const std::vector<App::DocumentObject*>& objs,
 
         auto shape = extractSubShape(objs[i], subs[i]);
         if (shape.isNull()) {
-            FC_THROWM(AttachEngineException,
-                      "AttachEngine3D: null subshape " << objs[i]->getNameInDocument() << '.'
-                                                       << subs[i]);
+            if (subs[i].length() == 0) {
+                storage.emplace_back(TopoShape());
+                shapes[i] = &storage.back();
+                types[i] = eRefType(rtPart | rtFlagHasPlacement);
+                continue;
+            }
+            else {
+                // This case should now be unreachable because extractSubShape would have thrown
+                // for a missing subname. But it's good defensive programming.
+                FC_THROWM(AttachEngineException,
+                          "AttachEngine3D: null subshape " << objs[i]->getNameInDocument() << '.'
+                                                           << subs[i]);
+            }
         }
 
         storage.emplace_back(shape);
@@ -889,9 +899,22 @@ TopoShape AttachEngine::extractSubShape(App::DocumentObject* obj, const std::str
 
         for (;;) {
             if (shape.isNull()) {
-                FC_THROWM(AttachEngineException,
-                          "AttachEngine3D: subshape not found " << obj->getNameInDocument() << '.'
-                                                                << subname);
+                // Shape is null. Let's see if this is an acceptable null.
+                // (i.e., an empty object was selected, not a broken link to a sub-element).
+                if (subname.empty()) {
+                    // The user selected the whole object, and it has no shape.
+                    // This is the empty sketch or empty body case.
+                    // Instead of throwing an error, we return a null TopoShape.
+                    // The caller (readLinks) will then handle this null shape.
+                    return TopoShape();  // Return a default-constructed (null) shape
+                }
+                else {
+                    // The user specified a subname (e.g., "Edge1"), but it couldn't be found.
+                    // This is a genuine error.
+                    FC_THROWM(AttachEngineException,
+                              "AttachEngine3D: subshape not found " << obj->getNameInDocument()
+                                                                    << '.' << subname);
+                }
             }
 
             if (shape.shapeType() != TopAbs_COMPOUND || shape.countSubShapes(TopAbs_SHAPE) != 1) {

src/Mod/PartDesign/Gui/SketchWorkflow.cpp

@@ -197,17 +197,18 @@ class SketchPreselection
 {
 public:
     SketchPreselection(Gui::Document* guidocument, PartDesign::Body* activeBody,
-                       std::tuple<Gui::SelectionFilter, Gui::SelectionFilter> filter)
+                       std::tuple<Gui::SelectionFilter, Gui::SelectionFilter, Gui::SelectionFilter> filter)
         : guidocument(guidocument)
         , activeBody(activeBody)
         , faceFilter(std::get<0>(filter))
         , planeFilter(std::get<1>(filter))
+        , sketchFilter(std::get<2>(filter))
     {
     }
 
     bool matches()
     {
-        return faceFilter.match() || planeFilter.match();
+        return faceFilter.match() || planeFilter.match() || sketchFilter.match();
     }
 
     std::string getSupport() const
@@ -231,11 +232,17 @@ public:
             selectedObject = validator.getObject();
             supportString = validator.getSupport();
         }
-        else {
+        else if (planeFilter.match()) {
             SupportPlaneValidator validator(planeFilter.Result[0][0]);
             selectedObject = validator.getObject();
             supportString = validator.getSupport();
         }
+        else {
+            // For a sketch, the support is the object itself with no sub-element.
+            Gui::SelectionObject sketchSelObject = sketchFilter.Result[0][0];
+            selectedObject = sketchSelObject.getObject();
+            supportString = sketchSelObject.getAsPropertyLinkSubString();
+        }
 
         handleIfSupportOutOfBody(selectedObject);
     }
@@ -250,7 +257,16 @@ public:
         FCMD_OBJ_CMD(activeBody, "newObject('Sketcher::SketchObject','" << FeatName << "')");
         auto Feat = activeBody->getDocument()->getObject(FeatName.c_str());
         FCMD_OBJ_CMD(Feat, "AttachmentSupport = " << supportString);
-        FCMD_OBJ_CMD(Feat, "MapMode = '" << Attacher::AttachEngine::getModeName(Attacher::mmFlatFace)<<"'");
+        if (sketchFilter.match()) {
+            FCMD_OBJ_CMD(Feat,
+                         "MapMode = '" << Attacher::AttachEngine::getModeName(Attacher::mmObjectXY)
+                                       << "'");
+        }
+        else {  // For Face or Plane
+            FCMD_OBJ_CMD(Feat,
+                         "MapMode = '" << Attacher::AttachEngine::getModeName(Attacher::mmFlatFace)
+                                       << "'");
+        }
         Gui::Command::updateActive();
         PartDesignGui::setEdit(Feat, activeBody);
     }
@@ -346,6 +362,7 @@ private:
     PartDesign::Body* activeBody;
     Gui::SelectionFilter faceFilter;
     Gui::SelectionFilter planeFilter;
+    Gui::SelectionFilter sketchFilter;
     std::string supportString;
 };
 
@@ -757,8 +774,8 @@ void SketchWorkflow::tryCreateSketch()
         return;
     }
 
-    auto faceOrPlaneFilter = getFaceAndPlaneFilter();
-    SketchPreselection sketchOnFace{ guidocument, activeBody, faceOrPlaneFilter };
+    auto filters = getFilters();
+    SketchPreselection sketchOnFace {guidocument, activeBody, filters};
 
     if (sketchOnFace.matches()) {
         // create Sketch on Face or Plane
@@ -804,7 +821,7 @@ bool SketchWorkflow::shouldAbort(bool shouldMakeBody) const
     return !shouldMakeBody && !activeBody;
 }
 
-std::tuple<Gui::SelectionFilter, Gui::SelectionFilter> SketchWorkflow::getFaceAndPlaneFilter() const
+std::tuple<Gui::SelectionFilter, Gui::SelectionFilter, Gui::SelectionFilter> SketchWorkflow::getFilters() const
 {
     // Hint:
     // The behaviour of this command has changed with respect to a selected sketch:
@@ -815,9 +832,11 @@ std::tuple<Gui::SelectionFilter, Gui::SelectionFilter> SketchWorkflow::getFaceAn
     Gui::SelectionFilter FaceFilter  ("SELECT Part::Feature SUBELEMENT Face COUNT 1");
     Gui::SelectionFilter PlaneFilter ("SELECT App::Plane COUNT 1", activeBody);
     Gui::SelectionFilter PlaneFilter2("SELECT PartDesign::Plane COUNT 1", activeBody);
+    Gui::SelectionFilter SketchFilter("SELECT Part::2DObject COUNT 1", activeBody);
 
     if (PlaneFilter2.match()) {
         PlaneFilter = PlaneFilter2;
     }
-    return std::make_tuple(FaceFilter, PlaneFilter);
+
+    return std::make_tuple(FaceFilter, PlaneFilter, SketchFilter);
 }

src/Mod/PartDesign/Gui/SketchWorkflow.h

@@ -50,7 +50,7 @@ private:
     void tryCreateSketch();
     std::tuple<bool, PartDesign::Body*> shouldCreateBody();
     bool shouldAbort(bool) const;
-    std::tuple<Gui::SelectionFilter, Gui::SelectionFilter> getFaceAndPlaneFilter() const;
+    std::tuple<Gui::SelectionFilter, Gui::SelectionFilter, Gui::SelectionFilter> getFilters() const;
 
 private:
     Gui::Document* guidocument;