Design

The design of Assembly3 (and the fork of FreeCAD) partially follows the unfinished FreeCAD Assembly project plan, in particularly, the section Infrustracture and Object model, which are summarized below,

Multi model

The forked FreeCAD core supports external object linking (with a new type of property, PropertyXLink, as a drop-in replacement of PropertyLink), displaying, editing, importing/exporting, and cross-document undo/redo.

Part-tree

Assembly3 provides the Assembly container for holding its child features (or sub-assembly), and their constraints. It also introduce a new concept of Elements for declaring geometry elements used by constraints inside parent assembly. The purpose of the Element is to minimize the problem caused by geometry topological name changing, and make the assembly easier to maintain. See the following section for more details. A single object (e.g. Part object, sketch, or another assembly), can be added to multiple parent assemblies, either within the same or located outside of the current document. Each of its appearance inside the parent assembly has independent visibility control, but shares the same placement, meaning that if you move one instance of the object, all other instances moves relative to their parent assembly container. You can have independent placement by converting a child object into a link type object (See the following section for details). Simply right click the child object in the tree view and select Link actions -> Replace with link.

Unified Drag/Drop/Copy/Paste interface

The drag and drop API has been extended to let the target object know where the dropped object located in the object hierarchy, which is taken full advantage by Assembly3. The copy and paste is extended as well to be aware of external objects, and let the user decide whether to do a shallow or deep copy.

Object model

The SubName field in Gui::SelectionObject is for holding the selected geometry sub-element, such as face, edge or vertex. The forked FreeCAD extended usage of SubName to hold the path of selected object within the object hierarchy, e.g. a selection object with Object = Assembly1 and SubName = Parts.Assembly2.Constraints002.Constraint. means the user selected the Constraint object of Assembly2, which is a child feature of the part group (Parts) in Assembly1. Notice the ending . in SubName. This is for backward compatibility purpose, so that the SubName can still be used to refer to a geometry sub-element of some sub-object without any ambiguity. The rule is that, any sub-object references must end with a ., and those names without an ending . are sub-element references. The aforementioned PropertyXLink has an optional subname field (assign/return as a tuple(obj, subname) in Python) for linking into a sub-object/element.

Gui.Selection is extended with backward compatibility to provide full object path information on each selection, which makes it possible for the same object to be included in more than one group like objects without ambiguity on selection. Several new APIs have been added to FreeCAD core to provide nested child object placement and geometry information.