Scripted objects with attachment

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Introduction

The purpose of this page is to show a minimal example of Part Attachment feature using Scripted objects in Python.

See full and complete minimal example below.

The following GIF demonstrates attaching our custom box to a cylinder, and automatically updating it's position when the cylinder's position changes.

Box-attached-to-cylinder-demo.gif

NOTE: The box is our custom scripted object, and the cylinder is a regular FreeCAD object generated from the Part workbench.

Making Scripted Objects Attachable

Add Attach Extension

First, we need to add the Part::AttachExtensionPython extension to our Part::FeaturePython oject in the constructor, or __init__ method, of our custom scripted object.

 1class Box():
 2    """Custom Scripted Box Object"""
 3
 4    def __init__(self, obj):
 5        self.Type = 'Box'
 6
 7        obj.Proxy = self
 8        
 9        ... custom properties
10
11        # Needed to make this object "attachable"
12        obj.addExtension('Part::AttachExtensionPython', obj)

Without adding this code, we'll see the following warning dialog when attaching our custom scripted object to another object.

Part-attachment-warning-dialog.png

Update Position Based on Attached Object

Then, in the `execute` method of our custom scripted object, we need to call the `positionBySupport` on our `Part::FeaturePython` object.

1class Box:
2    
3    ...
4
5    def execute(self, obj):
6        obj.positionBySupport()
7        
8        # Assign a Shape to obj
9        obj.Shape = Part.makeBox(...)

Without calling positionBySupport, our custom scripted object won't update it's position when the position of the attached-to object changes.

Full and Complete Minimal Example

 1import FreeCAD as App
 2import Part
 3
 4
 5class Box():
 6    """
 7    Simple Custom Box Object
 8    See Also:
 9        https://wiki.freecadweb.org/FeaturePython_Objects
10    """
11
12    def __init__(self, obj):
13        """
14        Constructor
15        Arguments
16        ---------
17        - obj: an existing document object or an object created with FreeCAD.Document.addObject('Part::FeaturePython', '{name}').
18        """
19
20        self.Type = 'Box'
21
22        obj.Proxy = self
23        obj.addProperty('App::PropertyLength', 'Length',
24                        'Dimensions', 'Box length').Length = 10.0
25        obj.addProperty('App::PropertyLength', 'Width',
26                        'Dimensions', 'Box width').Width = 10.0
27        obj.addProperty('App::PropertyLength', 'Height',
28                        'Dimensions', 'Box height').Height = 10.0
29
30        # Needed to make this object "attachable",
31        # or able to attach parameterically to other objects
32        obj.addExtension('Part::AttachExtensionPython', obj)
33
34    def execute(self, obj):
35        """
36        Called on document recompute
37        """
38        # Needed to update position when attached-to object changes position.
39        # Reposition object based on Support, MapMode and MapPathParameter properties.
40        # Returns True if attachment calculation was successful, False if object is not attached and Placement wasn't updated,
41        obj.positionBySupport()
42
43        obj.Shape = Part.makeBox(obj.Length, obj.Width, obj.Height)
44
45
46def create_box(obj_name, document):
47    """
48    Create a Box.
49    """
50    obj = document.addObject('Part::FeaturePython', obj_name)
51    Box(obj)
52    obj.ViewObject.Proxy = 0  # Mandatory unless ViewProvider is coded
53    return obj
54
55
56document = App.ActiveDocument
57if document is None:
58    document = App.newDocument('Part Attachment Example')
59
60box = create_box('CustomBox', document)
61document.recompute()

References

Tested With the Following FreeCAD Version

Tested with the following FreeCAD version information:

OS: Ubuntu 18.04.3 LTS
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.18.16146 (Git) AppImage
Build type: Release
Branch: (HEAD detached at 0.18.4)
Hash: 980bf9060e28555fecd9e3462f68ca74007b70f8
Python version: 3.6.7
Qt version: 5.6.2
Coin version: 4.0.0a
OCC version: 7.3.0
Locale: English/UnitedStates (en_US)