Merge pull request #51 from WandererFan/propertychanges

add chapter on object deletion

Author: WandererFan

technical/ObjectDeletion.md

@@ -0,0 +1,32 @@
+---
+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.

technical/index.md

@@ -30,6 +30,7 @@ Technical information of interest to Contributors.
 - [Creating a WorkBench](https://wiki.freecad.org/Workbench_creation)
 - The [PowerUser's hub](https://wiki.freecad.org/Power_users_hub) in the FreeCAD wiki contains good information about Python scripting and workbench creation
 - [Accessing User Preferences](./preferences.md)
+- [Controlling Object Deletion](./ObjectDeletion.md)
 - [Create a Python Binding for C++ Class](./CreatePythonBindingForCpp.md)
 - [Checklist for Adding a Feature to a Workbench in C++](./ChecklistForNewFeatureC++.md)