[Core] Add visibility toggling to ViewProvider

With this extension of the API, view providers can indicate whether
document objects should be able to be toggled for visibility.  There is
both a C++ and Python interface, idiomatic for FreeCAD code.

src/Gui/FreeCADGuiInit.py

@@ -29,7 +29,7 @@
 
 # imports the one and only
 import FreeCAD, FreeCADGui
-from enum import IntEnum
+from enum import IntEnum, Enum
 
 # shortcuts
 Gui = FreeCADGui
@@ -52,6 +52,11 @@ class SelectionStyle(IntEnum):
     NormalSelection = 0
     GreedySelection = 1
 
+# The values must match with that of the Python enum class in ViewProvider.pyi
+class ToggleVisibilityMode(Enum):
+    CanToggleVisibility = "CanToggleVisibility"
+    NoToggleVisibility = "NoToggleVisibility"
+
 Gui.Selection.SelectionStyle = SelectionStyle
 
 # Important definitions

src/Gui/Tree.cpp

@@ -5534,7 +5534,9 @@ void DocumentObjectItem::testStatus(bool resetStatus, QIcon& icon1, QIcon& icon2
                 QPainter pt;
                 pt.begin(&px);
                 pt.setPen(Qt::NoPen);
-                pt.drawPixmap(0, 0, px_org.width(), px_org.height(), (currentStatus & Status::Visible) ? pxVisible : pxInvisible);
+                if (object()->canToggleVisibility()) {
+                    pt.drawPixmap(0, 0, px_org.width(), px_org.height(), (currentStatus & Status::Visible) ? pxVisible : pxInvisible);
+                }
                 pt.drawPixmap(px_org.width() + spacing, 0, px_org.width(), px_org.height(), px_org);
                 pt.end();
 

src/Gui/ViewProvider.cpp

@@ -93,6 +93,7 @@ PROPERTY_SOURCE_ABSTRACT(Gui::ViewProvider, App::TransactionalObject)
 
 ViewProvider::ViewProvider()
     : overrideMode("As Is")
+    , toggleVisibilityMode(ToggleVisibilityMode::CanToggleVisibility)
 {
     setStatus(UpdateData, true);
 

src/Gui/ViewProvider.h

@@ -121,6 +121,11 @@ class GuiExport ViewProvider : public App::TransactionalObject
     PROPERTY_HEADER_WITH_OVERRIDE(Gui::ViewProvider);
 
 public:
+    enum class ToggleVisibilityMode : bool {
+        CanToggleVisibility = true,
+        NoToggleVisibility = false
+    };
+
     /// constructor.
     ViewProvider();
 
@@ -247,6 +252,22 @@ public:
     /// deliver the icon shown in the tree view
     virtual QIcon getIcon() const;
 
+    /**
+     * @brief Whether the viewprovider should allow to toggle the visibility.
+     *
+     * Some document objects are not rendered and for those document objects,
+     * it makes no sense to be able to toggle the visibility.  Examples are
+     * VarSet and Spreadsheet.
+     *
+     * Note that "rendered" should be seen broadly here.  Objects such as
+     * TechDraw pages, templates, views, and dimensions are not rendered by
+     * Coin but are "rendered" on the TechDraw page and hence this function can
+     * return true for those items.
+     */
+    bool canToggleVisibility() const {
+        return toggleVisibilityMode == ToggleVisibilityMode::CanToggleVisibility;
+    }
+
      /** @name Methods used by the Tree
      * If you want to take control over the
      * viewprovider specific overlay icons that will be drawn with color
@@ -571,6 +592,8 @@ protected:
     /// Turn on mode switch
     virtual void setModeSwitch();
 
+    void setToggleVisibility(ToggleVisibilityMode mode) { toggleVisibilityMode = mode; }
+
 protected:
     /// The root Separator of the ViewProvider
     SoSeparator *pcRoot;
@@ -584,6 +607,10 @@ protected:
     ViewProviderPy* pyViewObject{nullptr};
     std::string overrideMode;
     std::bitset<32> StatusBits;
+    /// whether visibility can toggled
+    ToggleVisibilityMode toggleVisibilityMode;
+
+    friend class ViewProviderPy;
 
 private:
     int _iActualMode{-1};

src/Gui/ViewProvider.pyi

@@ -2,6 +2,7 @@ from Base.Metadata import constmethod
 from Base.BoundBox import BoundBox
 from App.ExtensionContainer import ExtensionContainer
 from typing import Any, Final, List, Optional
+import enum
 
 
 class ViewProvider(ExtensionContainer):
@@ -12,6 +13,11 @@ class ViewProvider(ExtensionContainer):
     Licence: LGPL
     """
 
+    class ToggleVisibilityMode(enum.Enum):
+        CanToggleVisibility = "CanToggleVisibility"
+        NoToggleVisibility = "NoToggleVisibility"
+
+
     def addProperty(
         self,
         type: str,
@@ -367,3 +373,7 @@ class ViewProvider(ExtensionContainer):
 
     DropPrefix: Final[str] = ""
     """Subname referencing the sub-object for holding dropped object."""
+
+    ToggleVisibility: ToggleVisibilityMode = ToggleVisibilityMode.CanToggleVisibility
+    """Get/set whether the viewprovider can toggle the visibility of
+    the object."""

src/Gui/ViewProviderPyImp.cpp

@@ -706,3 +706,34 @@ Py::String ViewProviderPy::getDropPrefix() const
 {
     return {getViewProviderPtr()->getDropPrefix()};
 }
+
+void ViewProviderPy::setToggleVisibility(Py::Object arg)
+{
+    std::string val;
+
+    if (PyObject_HasAttrString(arg.ptr(), "value")) {
+        // we are dealing with the enum
+        val = Py::String(arg.getAttr("value"));
+    }
+    else {
+        // we are dealing with a string
+        val = Py::String(arg);
+    }
+
+    if (val == "CanToggleVisibility") {
+        getViewProviderPtr()->setToggleVisibility(ViewProvider::ToggleVisibilityMode::CanToggleVisibility);
+    }
+    else if (val == "NoToggleVisibility") {
+        getViewProviderPtr()->setToggleVisibility(ViewProvider::ToggleVisibilityMode::NoToggleVisibility);
+    }
+    else {
+        throw Py::ValueError("Invalid ToggleVisibility mode. Use 'CanToggleVisibility' or 'NoToggleVisibility'.");
+    }
+}
+
+Py::Object ViewProviderPy::getToggleVisibility() const
+{
+    bool canToggleVisibility = getViewProviderPtr()->canToggleVisibility();
+
+    return Py::String(canToggleVisibility ? "CanToggleVisibility" : "NoToggleVisibility");
+}