ViewProvider/ViewProviderExtension: various new APIs

Some of these new APIs are not strictly needed for Link to work, but
good to have.

Summary of the API changes:

* getModeSwitch/getTransformNode() exposes view provider mode switch
  and transformation coin3D node, required by ViewProviderLink to
  override placement and visibility.

* canAddToSceneGraph() inform 3D viewer whether to add the root node to
  scenegraph. Some object only exists as a child of some coordinate
  system. Not adding them can simplify scenegraph and visibility
  management.

* showInTree() inform tree view whether to show the corresponding tree
  item.

* getDefaultMode() to expose the current active mode regardless of the
  view provider's visibility

* (can)DropObjectEx() superseds (can)DropObject() with additional
  support of subname reference, which makes it easy to support linking
  to sub-objects. One of the use case is cross coordinate system
  linking.

* getDropPrefix() is used to tell tree view where the object is dropped
  into. A non empty return means the object is actually dropped into a
  sub-object. For example, Assembly3 container puts all dropped object
  into its child container PartGroup.

* canDragAndDropObject() is used to inform tree view whether the object
  dropped need to be dragged out of its original parent, which usually
  does not make sure for Link type object.

* beforeDelete() will be called by Gui::Document when either the
  object itself is being deleted or when the document is being
  destoried.

* is/setLinkVisibility() allows to show/hide a link to this object.
  This may be used during editing, to prevent showing editing geometry
  in multiple places.

* update() is made a virtual function, it will be overridden by
  ViewProviderDocumentObject in future patch.

* startEditing() now becomes virtual, and return a ViewProvider that
  actually handles the editing. This is for future Link type object to
  forward editing request

* covert(), convenience function to convert between Coin3D and FC matrix

ViewProviderExtension also gains many relavant extension point of the
new API. There is also the new extensionModeSwitchChange() for notifying
mode switch changes

ViewProviderPy exposes several method/attribute for the new API as well.

src/Gui/ViewProvider.cpp

@@ -41,6 +41,7 @@
 /// Here the FreeCAD includes sorted by Base,App,Gui......
 #include <Base/Console.h>
 #include <Base/Exception.h>
+#include <Base/BoundBox.h>
 #include <Base/Matrix.h>
 #include <App/PropertyGeo.h>
 
@@ -107,11 +108,13 @@ ViewProvider::~ViewProvider()
         pcAnnotation->unref();
 }
 
-bool ViewProvider::startEditing(int ModNum)
+ViewProvider *ViewProvider::startEditing(int ModNum)
 {
-    bool ok = setEdit(ModNum);
-    if (ok) _iEditMode = ModNum;
-    return ok;
+    if(setEdit(ModNum)) {
+        _iEditMode = ModNum;
+        return this;
+    }
+    return 0;
 }
 
 int ViewProvider::getEditingMode() const
@@ -298,7 +301,7 @@ void ViewProvider::setTransformation(const SbMatrix &rcMatrix)
     pcTransform->setMatrix(rcMatrix);
 }
 
-SbMatrix ViewProvider::convert(const Base::Matrix4D &rcMatrix) const
+SbMatrix ViewProvider::convert(const Base::Matrix4D &rcMatrix)
 {
     double dMtrx[16];
     rcMatrix.getGLMatrix(dMtrx);
@@ -308,6 +311,15 @@ SbMatrix ViewProvider::convert(const Base::Matrix4D &rcMatrix) const
                     dMtrx[12],dMtrx[13],dMtrx[14], dMtrx[15]);
 }
 
+Base::Matrix4D ViewProvider::convert(const SbMatrix &smat)
+{
+    Base::Matrix4D mat;
+    for(int i=0;i<4;++i)
+        for(int j=0;j<4;++j)
+            mat[i][j] = smat[j][i];
+    return mat;
+}
+
 void ViewProvider::addDisplayMaskMode(SoNode *node, const char* type)
 {
     _sDisplayMaskModes[type] = pcModeSwitch->getNumChildren();
@@ -381,11 +393,16 @@ std::string ViewProvider::getActiveDisplayMode(void) const
 
 void ViewProvider::hide(void)
 {
-    pcModeSwitch->whichChild = -1;
+    auto exts = getExtensionsDerivedFromType<Gui::ViewProviderExtension>();
+
+    if(pcModeSwitch->whichChild.getValue() >= 0) {
+        pcModeSwitch->whichChild = -1;
+        for(auto ext : exts)
+            ext->extensionModeSwitchChange();
+    }
 
     //tell extensions that we hide
-    auto vector = getExtensionsDerivedFromType<Gui::ViewProviderExtension>();
-    for (Gui::ViewProviderExtension* ext : vector)
+    for (Gui::ViewProviderExtension* ext : exts)
         ext->extensionHide();
 }
 
@@ -429,6 +446,10 @@ void ViewProvider::setOverrideMode(const std::string &mode)
     }
     if (pcModeSwitch->whichChild.getValue() != -1)
         setModeSwitch();
+    else {
+        for(auto ext : getExtensionsDerivedFromType<Gui::ViewProviderExtension>())
+            ext->extensionModeSwitchChange();
+    }
 }
 
 const string ViewProvider::getOverrideMode() {
@@ -442,16 +463,28 @@ void ViewProvider::setModeSwitch()
         pcModeSwitch->whichChild = _iActualMode;
     else if (viewOverrideMode < pcModeSwitch->getNumChildren())
         pcModeSwitch->whichChild = viewOverrideMode;
+    else
+        return;
+    for(auto ext : getExtensionsDerivedFromType<Gui::ViewProviderExtension>())
+        ext->extensionModeSwitchChange();
 }
 
 void ViewProvider::setDefaultMode(int val)
 {
     _iActualMode = val;
+    for(auto ext : getExtensionsDerivedFromType<Gui::ViewProviderExtension>())
+        ext->extensionModeSwitchChange();
+}
+
+int ViewProvider::getDefaultMode() const {
+    return viewOverrideMode>=0?viewOverrideMode:_iActualMode;
 }
 
 void ViewProvider::onChanged(const App::Property* prop)
 {
     Application::Instance->signalChangedObject(*this, *prop);
+
+    App::TransactionalObject::onChanged(prop);
 }
 
 std::string ViewProvider::toString() const
@@ -716,8 +749,18 @@ bool ViewProvider::canDropObjects() const {
     return false;
 }
 
-void ViewProvider::dropObject(App::DocumentObject* obj)
-{
+bool ViewProvider::canDragAndDropObject(App::DocumentObject* obj) const {
+
+    auto vector = getExtensionsDerivedFromType<Gui::ViewProviderExtension>();
+    for(Gui::ViewProviderExtension* ext : vector){
+        if(!ext->extensionCanDragAndDropObject(obj))
+            return false;
+    }
+
+    return true;
+}
+
+void ViewProvider::dropObject(App::DocumentObject* obj) {
     auto vector = getExtensionsDerivedFromType<Gui::ViewProviderExtension>();
     for (Gui::ViewProviderExtension* ext : vector) {
         if (ext->extensionCanDropObject(obj)) {
@@ -729,24 +772,52 @@ void ViewProvider::dropObject(App::DocumentObject* obj)
     throw Base::RuntimeError("ViewProvider::dropObject: no extension for dropping given object available.");
 }
 
-void ViewProvider::replaceObject(App::DocumentObject* oldValue, App::DocumentObject* newValue)
+bool ViewProvider::canDropObjectEx(App::DocumentObject* obj, App::DocumentObject *owner, 
+        const char *subname, const std::vector<std::string> &elements) const
+{
+    auto vector = getExtensionsDerivedFromType<Gui::ViewProviderExtension>();
+    for(Gui::ViewProviderExtension* ext : vector){
+        if(ext->extensionCanDropObjectEx(obj,owner,subname, elements))
+            return true;
+    }
+    return canDropObject(obj);
+}
+
+std::string ViewProvider::dropObjectEx(App::DocumentObject* obj, App::DocumentObject *owner, 
+        const char *subname, const std::vector<std::string> &elements) 
+{
+    auto vector = getExtensionsDerivedFromType<Gui::ViewProviderExtension>();
+    for(Gui::ViewProviderExtension* ext : vector) {
+        if(ext->extensionCanDropObjectEx(obj, owner, subname, elements))
+            return ext->extensionDropObjectEx(obj, owner, subname, elements);
+    }
+    dropObject(obj);
+    return std::string();
+}
+
+int ViewProvider::replaceObject(App::DocumentObject* oldValue, App::DocumentObject* newValue)
 {
     auto vector = getExtensionsDerivedFromType<Gui::ViewProviderExtension>();
     for (Gui::ViewProviderExtension* ext : vector) {
         if (ext->extensionCanDropObject(newValue)) {
-            ext->extensionReplaceObject(oldValue, newValue);
-            return;
+            int ret = ext->extensionReplaceObject(oldValue, newValue);
+            if(ret>=0)
+                return !!ret;
         }
     }
-
-    throw Base::RuntimeError("ViewProvider::dropObject: no extension for dropping given object available.");
+    return -1;
 }
 
-void ViewProvider::Restore(Base::XMLReader& reader)
-{
-    setStatus(Gui::isRestoring, true);
+void ViewProvider::Restore(Base::XMLReader& reader) {
+    // Because some PropertyLists type properties are stored in a separate file,
+    // and is thus restored outside this function. So we rely on Gui::Document
+    // to set the isRestoring flags for us.
+    //
+    // setStatus(Gui::isRestoring, true);
+
     TransactionalObject::Restore(reader);
-    setStatus(Gui::isRestoring, false);
+
+    // setStatus(Gui::isRestoring, false);
 }
 
 void ViewProvider::updateData(const App::Property* prop)
@@ -812,3 +883,22 @@ std::vector< App::DocumentObject* > ViewProvider::claimChildren3D(void) const
     }
     return vec;
 }
+
+void ViewProvider::beforeDelete() {
+    auto vector = getExtensionsDerivedFromType<Gui::ViewProviderExtension>();
+    for(Gui::ViewProviderExtension* ext : vector)
+        ext->extensionBeforeDelete();
+}
+
+bool ViewProvider::isLinkVisible() const {
+    auto ext = getExtensionByType<ViewProviderLinkObserver>(true);
+    if(!ext)
+        return true;
+    return ext->isLinkVisible();
+}
+
+void ViewProvider::setLinkVisible(bool visible) {
+    auto ext = getExtensionByType<ViewProviderLinkObserver>(true);
+    if(ext)
+        ext->setLinkVisible(visible);
+}