[TD] handle object deletions better

see https://forum.freecadweb.org/viewtopic.php?p=366902#p366902

src/Mod/TechDraw/Gui/ViewProviderDimension.cpp

@@ -221,3 +221,12 @@ void ViewProviderDimension::handleChangedPropertyType(Base::XMLReader &reader, c
         LineWidth.setValue(LineWidthProperty.getValue());
     }
 }
+
+bool ViewProviderDimension::canDelete(App::DocumentObject *obj) const
+{
+    // deletions of objects from a ProjGroupItem don't necesarily destroy anything
+    // thus we can pass this action
+    // we can warn the user if necessary in the object's ViewProvider in the onDelete() function
+    Q_UNUSED(obj)
+        return true;
+}

src/Mod/TechDraw/Gui/ViewProviderDimension.h

@@ -80,8 +80,9 @@ class TechDrawGuiExport ViewProviderDimension : public ViewProviderDrawingView
     double prefFontSize() const;
     double prefWeight() const;
     int prefStandardAndStyle() const;
+    virtual bool canDelete(App::DocumentObject* obj) const;
 
-protected:
+protected:
     virtual void handleChangedPropertyType(Base::XMLReader &reader, const char *TypeName, App::Property * prop);
 
 private:

src/Mod/TechDraw/Gui/ViewProviderPage.cpp

@@ -27,6 +27,8 @@
 #ifndef _PreComp_
 # include <QAction>
 # include <QMenu>
+# include <QMessageBox>
+# include <QTextStream>
 # include <QTimer>
 #include <QList>
 #include <QPointer>
@@ -191,11 +193,37 @@ void ViewProviderPage::updateData(const App::Property* prop)
     Gui::ViewProviderDocumentObject::updateData(prop);
 }
 
-bool ViewProviderPage::onDelete(const std::vector<std::string> &items)
+bool ViewProviderPage::onDelete(const std::vector<std::string> &)
 {
-    bool rc = ViewProviderDocumentObject::onDelete(items);
-    removeMDIView();
-    return rc;
+    // warn the user if the Page is not empty
+
+    // check if there are items in the group
+    auto objs = claimChildren();
+
+    if (!objs.empty())
+    {
+        // generate dialog
+        QString bodyMessage;
+        QTextStream bodyMessageStream(&bodyMessage);
+        bodyMessageStream << qApp->translate("Std_Delete",
+            "The page is not empty, therefore the\n following referencing objects might be lost.\n\n"
+            "Are you sure you want to continue?\n");
+        for (auto ObjIterator : objs)
+            bodyMessageStream << '\n' << QString::fromUtf8(ObjIterator->Label.getValue());
+        // show and evaluate the dialog
+        int DialogResult = QMessageBox::warning(Gui::getMainWindow(),
+            qApp->translate("Std_Delete", "Object dependencies"), bodyMessage,
+            QMessageBox::Yes, QMessageBox::No);
+        if (DialogResult == QMessageBox::Yes) {
+            removeMDIView(); 
+            return true;
+        } else
+            return false;
+    }
+    else {
+        removeMDIView();
+        return true;
+    }
 }
 
 void ViewProviderPage::setupContextMenu(QMenu* menu, QObject* receiver, const char* member)
@@ -421,6 +449,16 @@ void ViewProviderPage::setGraphicsView(QGVPage* gv)
     m_graphicsView = gv;
 }
 
+bool ViewProviderPage::canDelete(App::DocumentObject *obj) const
+{
+    // deletions from a page don't necesarily destroy anything
+    // thus we can pass this action
+    // if an object could break something, like e.g. the template object
+    // its ViewProvider handles this in the onDelete() function
+    Q_UNUSED(obj)
+    return true;
+}
+
 //! Redo the whole visual page
 void ViewProviderPage::onGuiRepaint(const TechDraw::DrawPage* dp) 
 {

src/Mod/TechDraw/Gui/ViewProviderPage.h

@@ -92,6 +92,7 @@ class TechDrawGuiExport ViewProviderPage : public Gui::ViewProviderDocumentObjec
     void toggleFrameState(void);
     void setTemplateMarkers(bool state);
     void setGraphicsView(QGVPage* gv);
+    virtual bool canDelete(App::DocumentObject* obj) const;
 
 protected:
     bool setEdit(int ModNum) override;

src/Mod/TechDraw/Gui/ViewProviderProjGroup.cpp

@@ -27,6 +27,8 @@
 # endif
 # include <QAction>
 # include <QMenu>
+# include <QMessageBox>
+# include <QTextStream>
 #endif
 
 #include <Base/Console.h>
@@ -48,7 +50,6 @@
 
 #include <Mod/TechDraw/App/DrawProjGroupItem.h>
 
-
 #include "TaskProjGroup.h"
 #include "ViewProviderProjGroup.h"
 
@@ -146,6 +147,45 @@ bool ViewProviderProjGroup::doubleClicked(void)
     return true;
 }
 
+bool ViewProviderProjGroup::onDelete(const std::vector<std::string> &)
+{
+    // warn the user if the ProjGroup is not empty
+
+    // check if there are items in the group
+    auto objs = claimChildren();
+
+    if (!objs.empty())
+    {
+        // generate dialog
+        QString bodyMessage;
+        QTextStream bodyMessageStream(&bodyMessage);
+        bodyMessageStream << qApp->translate("Std_Delete",
+            "The projection group is not empty, therefore\n the following referencing objects might be lost.\n\n"
+            "Are you sure you want to continue?\n");
+        for (auto ObjIterator : objs)
+            bodyMessageStream << '\n' << QString::fromUtf8(ObjIterator->Label.getValue());
+        // show and evaluate dialog
+        int DialogResult = QMessageBox::warning(Gui::getMainWindow(),
+            qApp->translate("Std_Delete", "Object dependencies"), bodyMessage,
+            QMessageBox::Yes, QMessageBox::No);
+        if (DialogResult == QMessageBox::Yes)
+            return true;
+        else
+            return false;
+    }
+    else
+        return true;
+}
+
+bool ViewProviderProjGroup::canDelete(App::DocumentObject *obj) const
+{
+    // deletions of views from a ProjGroup don't necesarily destroy anything
+    // thus we can pass this action
+    // we can warn the user if necessary in the object's ViewProvider in the onDelete() function
+    Q_UNUSED(obj)
+    return true;
+}
+
 std::vector<App::DocumentObject*> ViewProviderProjGroup::claimChildren(void) const
 {
     // Collect any child fields

src/Mod/TechDraw/Gui/ViewProviderProjGroup.h

@@ -60,7 +60,8 @@ class TechDrawGuiExport ViewProviderProjGroup : public ViewProviderDrawingView
     virtual TechDraw::DrawProjGroup* getViewObject() const;
     void unsetEdit(int ModNum);
     virtual void onChanged(const App::Property *prop);
-
+    virtual bool onDelete(const std::vector<std::string> &);
+    virtual bool canDelete(App::DocumentObject* obj) const;
 
 protected:
     bool setEdit(int ModNum);

src/Mod/TechDraw/Gui/ViewProviderProjGroupItem.cpp

@@ -22,6 +22,8 @@
 
 #include "PreCompiled.h"
 #ifndef _PreComp_
+ #include <QMessageBox>
+ #include <QTextStream>
 #endif
 
 /// Here the FreeCAD includes sorted by Base,App,Gui......
@@ -41,6 +43,8 @@
 #include <Gui/Document.h>
 #include <Gui/MainWindow.h>
 
+#include <Mod/TechDraw/App/DrawProjGroup.h>
+#include <Mod/TechDraw/App/DrawProjGroupItem.h>
 
 #include "ViewProviderProjGroupItem.h"
 
@@ -142,6 +146,49 @@ bool ViewProviderProjGroupItem::doubleClicked(void)
     return true;
 }
 
+bool ViewProviderProjGroupItem::onDelete(const std::vector<std::string> &)
+{
+    // we cannot delete the anchor view, thus check if the item is the front item
+
+    bool isAnchor = false;
+
+    // get the item and group
+    TechDraw::DrawProjGroupItem* dpgi = static_cast<TechDraw::DrawProjGroupItem*>(getViewObject());
+    TechDraw::DrawProjGroup* dpg = dpgi->getPGroup();
+    // get the projection
+    TechDraw::DrawProjGroupItem* proj = getObject();
+    // check if it is the anchor projection
+    if ((dpg != nullptr) && (dpg->hasProjection(proj->Type.getValueAsString()))
+        && (dpg->getAnchor() == dpgi))
+        isAnchor = true;
+
+   if (isAnchor)
+   {
+       // generate dialog
+        QString bodyMessage;
+        QTextStream bodyMessageStream(&bodyMessage);
+        bodyMessageStream << qApp->translate("Std_Delete",
+            "You cannot delete the anchor view of a projection group!");
+        QMessageBox::warning(Gui::getMainWindow(),
+            qApp->translate("Std_Delete", "Object dependencies"), bodyMessage,
+            QMessageBox::Ok);
+        // don't allow to delete
+        return false;
+    }
+    else {
+        return true;
+    }
+}
+
+bool ViewProviderProjGroupItem::canDelete(App::DocumentObject *obj) const
+{
+    // deletions of objects from a ProjGroupItem don't necesarily destroy anything
+    // thus we can pass this action
+    // we can warn the user if necessary in the object's ViewProvider in the onDelete() function
+    Q_UNUSED(obj)
+    return true;
+}
+
 TechDraw::DrawProjGroupItem* ViewProviderProjGroupItem::getViewObject() const
 {
     return dynamic_cast<TechDraw::DrawProjGroupItem*>(pcObject);

src/Mod/TechDraw/Gui/ViewProviderProjGroupItem.h

@@ -54,6 +54,8 @@ class TechDrawGuiExport ViewProviderProjGroupItem: public ViewProviderViewPart
     virtual TechDraw::DrawProjGroupItem* getViewObject() const;
     TechDraw::DrawProjGroupItem* getObject() const;
     void unsetEdit(int ModNum);
+    virtual bool onDelete(const std::vector<std::string> &);
+    virtual bool canDelete(App::DocumentObject* obj) const;
 
 protected:
     bool setEdit(int ModNum);

src/Mod/TechDraw/Gui/ViewProviderTemplate.cpp

@@ -24,6 +24,8 @@
 #include "PreCompiled.h"
 
 #ifndef _PreComp_
+# include <QMessageBox>
+# include <QTextStream>
 # ifdef FC_OS_WIN32
 #  include <windows.h>
 # endif
@@ -47,6 +49,7 @@
 #include <Mod/TechDraw/App/DrawSVGTemplate.h>
 #include <Mod/TechDraw/App/DrawPage.h>
 
+#include "DrawGuiUtil.h"
 #include "QGITemplate.h"
 #include "QGISVGTemplate.h"
 #include "QGVPage.h"
@@ -182,6 +185,31 @@ void ViewProviderTemplate::setMarkers(bool state)
     }
 }
 
+bool ViewProviderTemplate::onDelete(const std::vector<std::string> &)
+{
+    // deleting the template will break the page view, thus warn the user
+
+    // get the page
+    auto page = getTemplate()->getParentPage();
+
+    // generate dialog
+    QString bodyMessage;
+    QTextStream bodyMessageStream(&bodyMessage);
+    bodyMessageStream << qApp->translate("Std_Delete",
+        "The following referencing objects might break.\n\n"
+        "Are you sure you want to continue?\n");
+    bodyMessageStream << '\n' << QString::fromUtf8(page->Label.getValue());
+
+    // show and evaluate dialog
+    int DialogResult = QMessageBox::warning(Gui::getMainWindow(),
+        qApp->translate("Std_Delete", "Object dependencies"), bodyMessage,
+        QMessageBox::Yes, QMessageBox::No);
+    if (DialogResult == QMessageBox::Yes)
+        return true;
+    else
+        return false;
+}
+
 MDIViewPage* ViewProviderTemplate::getMDIViewPage(void) const
 {
     MDIViewPage* myMdi = nullptr;

src/Mod/TechDraw/Gui/ViewProviderTemplate.h

@@ -60,6 +60,7 @@ class TechDrawGuiExport ViewProviderTemplate : public Gui::ViewProviderDocumentO
     virtual Gui::MDIView *getMDIView() const override;
 
     void setMarkers(bool state);
+    virtual bool onDelete(const std::vector<std::string> &);
 };
 
 } // namespace TechDrawGui