add chapter on object deletion

Author: WandererFan

technical/ObjectDeletion.md

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+---
+layout: default
+---
+
+# How to control object deletion
+
+It is possible for a user to attempt to delete a DocumentObject while a Task
+Dialog is attempting to update that object.  Handling this in a Task Dialog 
+is not tidy.
+
+## Recommended reading:
+- ViewProviders
+- In/Out Lists
+
+There are two methods in the ViewProvider that deal with this situation.
+
+'''c++
+bool canDelete(App::DocumentObject *obj) 
+'''
+is called when Std_Delete is asked to delete obj, BUT it is called for each of
+obj's parent in the InList.  This is where you would decide if you will allow 
+one of your children can be deleted.  In effect, the is canDeleteMyChildObject().
+Return true to allow the child to be deleted, or false to prevent the deletion.
+
+'''c++
+bool onDelete(const std::vector<std::string> & parms)
+'''
+is also called when when Std_Delete is asked to delete a DocumentObject, but in
+this case it is the DocumentObject's ViewProvider that is called.  Return true
+to allow deletion of the DocumentObject, or false to prevent the deletion.
+
+Note that it is possible to circumvent these methods via scripting.