PartDesign: Introduce ability to disable single-solid rule

This adds "SingleSolidRuleMode" enum that controls if PartDesign will
enforce singular solid. By default the single-solid is enforced so
nothing changes for the user, it must be explicitly disabled by setting
new Allow Compound boolean property on a given body.

Default for this value is controled using user parameter under
Mod/PartDesign/AllowCompoundDefault

src/Mod/Part/App/BodyBase.h

@@ -47,9 +47,8 @@ public:
     /**
      * The final feature of the body it is associated with.
      * Note: tip may either point to the BaseFeature or to some feature inside the Group list.
-     *       in case it points to the model the PartDesign::Body guaranties that it is a solid.
      */
-    App::PropertyLink       Tip;
+    App::PropertyLink Tip;
 
     /**
      * A base object of the body, serves as a base object for the first feature of the body.

src/Mod/PartDesign/App/Body.cpp

@@ -40,7 +40,17 @@ using namespace PartDesign;
 PROPERTY_SOURCE(PartDesign::Body, Part::BodyBase)
 
 Body::Body() {
-    _GroupTouched.setStatus(App::Property::Output,true);
+    ADD_PROPERTY_TYPE(AllowCompound, (false), "Experimental", App::Prop_None, "Allow multiple solids in Body (experimental)");
+
+    _GroupTouched.setStatus(App::Property::Output, true);
+
+    static Base::Reference<ParameterGrp> hGrp = App::GetApplication()
+        .GetUserParameter()
+        .GetGroup("BaseApp/Preferences/Mod/PartDesign");
+
+    auto allowCompoundDefaultValue = hGrp->GetBool("AllowCompoundDefault", false);
+
+    ADD_PROPERTY(AllowCompound, (allowCompoundDefaultValue));
 }
 
 /*
@@ -428,7 +438,7 @@ void Body::onSettingDocument() {
     Part::BodyBase::onSettingDocument();
 }
 
-void Body::onChanged (const App::Property* prop) {
+void Body::onChanged(const App::Property* prop) {
     // we neither load a project nor perform undo/redo
     if (!this->isRestoring()
             && this->getDocument()
@@ -438,7 +448,6 @@ void Body::onChanged (const App::Property* prop) {
             auto first = Group.getValues().empty() ? nullptr : Group.getValues().front();
 
             if (BaseFeature.getValue()) {
-
                 //setup the FeatureBase if needed
                 if (!first || !first->isDerivedFrom(FeatureBase::getClassTypeId())) {
                     bf = static_cast<FeatureBase*>(getDocument()->addObject("PartDesign::FeatureBase", "BaseFeature"));
@@ -452,17 +461,26 @@ void Body::onChanged (const App::Property* prop) {
                 }
             }
 
-            if (bf && (bf->BaseFeature.getValue() != BaseFeature.getValue()))
+            if (bf && (bf->BaseFeature.getValue() != BaseFeature.getValue())) {
                 bf->BaseFeature.setValue(BaseFeature.getValue());
+            }
         }
-        else if( prop == &Group ) {
-
+        else if (prop == &Group) {
             //if the FeatureBase was deleted we set the BaseFeature link to nullptr
             if (BaseFeature.getValue() &&
                (Group.getValues().empty() || !Group.getValues().front()->isDerivedFrom(FeatureBase::getClassTypeId()))) {
                     BaseFeature.setValue(nullptr);
             }
         }
+        else if (prop == &AllowCompound) {
+            // As disallowing compounds can break the model we need to recompute the whole tree.
+            // This will inform user about first place where there is more than one solid.
+            if (!AllowCompound.getValue()) {
+                for (auto feature : getFullModel()) {
+                    feature->enforceRecompute();
+                }
+            }
+        }
     }
 
     Part::BodyBase::onChanged(prop);

src/Mod/PartDesign/App/Body.h

@@ -41,6 +41,7 @@ class PartDesignExport Body : public Part::BodyBase
     PROPERTY_HEADER_WITH_OVERRIDE(PartDesign::Body);
 
 public:
+    App::PropertyBool AllowCompound;
 
     /// True if this body feature is active or was active when the document was last closed
     //App::PropertyBool IsActive;

src/Mod/PartDesign/App/Feature.cpp

@@ -91,10 +91,17 @@ short Feature::mustExecute() const
 // TODO: Toponaming April 2024 Deprecated in favor of TopoShape method.  Remove when possible.
 TopoDS_Shape Feature::getSolid(const TopoDS_Shape& shape)
 {
-    if (shape.IsNull())
+    if (shape.IsNull()) {
         Standard_Failure::Raise("Shape is null");
+    }
+
+    // If single solid rule is not enforced  we simply return the shape as is
+    if (singleSolidRuleMode() != Feature::SingleSolidRuleMode::Enforced) {
+        return shape;
+    }
+
     TopExp_Explorer xp;
-    xp.Init(shape,TopAbs_SOLID);
+    xp.Init(shape, TopAbs_SOLID);
     if (xp.More()) {
         return xp.Current();
     }
@@ -107,12 +114,19 @@ TopoShape Feature::getSolid(const TopoShape& shape)
     if (shape.isNull()) {
         throw Part::NullShapeException("Null shape");
     }
+
+    // If single solid rule is not enforced  we simply return the shape as is
+    if (singleSolidRuleMode() != Feature::SingleSolidRuleMode::Enforced) {
+        return shape;
+    }
+
     int count = shape.countSubShapes(TopAbs_SOLID);
-    if(count) {
-        auto res = shape.getSubTopoShape(TopAbs_SOLID,1);
+    if (count) {
+        auto res = shape.getSubTopoShape(TopAbs_SOLID, 1);
         res.fixSolidOrientation();
         return res;
     }
+
     return shape;
 }
 
@@ -153,12 +167,24 @@ int Feature::countSolids(const TopoDS_Shape& shape, TopAbs_ShapeEnum type)
 
 bool Feature::isSingleSolidRuleSatisfied(const TopoDS_Shape& shape, TopAbs_ShapeEnum type)
 {
+    if (singleSolidRuleMode() == Feature::SingleSolidRuleMode::Disabled) {
+        return true;
+    }
+
     int solidCount = countSolids(shape, type);
 
     return solidCount <= 1;
 }
 
 
+Feature::SingleSolidRuleMode Feature::singleSolidRuleMode()
+{
+    auto body = getFeatureBody();
+    auto areCompoundSolidsAllowed = body->AllowCompound.getValue();
+
+    return areCompoundSolidsAllowed ? SingleSolidRuleMode::Disabled : SingleSolidRuleMode::Enforced;
+}
+
 const gp_Pnt Feature::getPointFromFace(const TopoDS_Face& f)
 {
     if (!f.Infinite()) {

src/Mod/PartDesign/App/Feature.h

@@ -52,6 +52,8 @@ class PartDesignExport Feature : public Part::Feature, public App::SuppressibleE
 public:
     Feature();
 
+    enum SingleSolidRuleMode { Disabled = 0, Enforced = 1 };
+
     /// Base feature which this feature will be fused into or cut out of
     App::PropertyLink   BaseFeature;
     App::PropertyLinkHidden _Body;
@@ -96,14 +98,15 @@ protected:
      * Get a solid of the given shape. If no solid is found an exception is raised.
      */
     // TODO: Toponaming April 2024 Deprecated in favor of TopoShape method.  Remove when possible.
-    static TopoDS_Shape getSolid(const TopoDS_Shape&);
+    TopoDS_Shape getSolid(const TopoDS_Shape&);
     TopoShape getSolid(const TopoShape&);
     static int countSolids(const TopoDS_Shape&, TopAbs_ShapeEnum type = TopAbs_SOLID);
 
     /**
      * Checks if the single-solid body rule is fulfilled.
      */
-    static bool isSingleSolidRuleSatisfied(const TopoDS_Shape&, TopAbs_ShapeEnum type = TopAbs_SOLID);
+    bool isSingleSolidRuleSatisfied(const TopoDS_Shape&, TopAbs_ShapeEnum type = TopAbs_SOLID);
+    SingleSolidRuleMode singleSolidRuleMode();
 
     /// Grab any point from the given face
     static const gp_Pnt getPointFromFace(const TopoDS_Face& f);