Create a FeaturePython object part II

App::FeaturePython vs. Part::FeaturePython
To this point, we've focused on the internal aspects of a Python class built around a FeaturePython object - specifically, an App::FeaturePython object. We've created the object, defined some properties, and added some document-level event callbacks that allow our object to respond to a document recompute with the method.

But we still don't have a box.

We can easily create one, however, by adding just two lines.

First, add a new import to the top of the file:

Then, in, add the following line (you can delete the statement):

These commands execute python scripts that come with FreeCAD as a default.
 * The  method generates a new box Shape.
 * The enclosing call to adds the Shape to the document tree and makes it visible.

If you have FreeCAD open, reload the box module and create a new box object using (delete any existing box objects, just to keep things clean).

Notice how a box immediately appears on the screen. that's because the execute method is called immediately after the box is created, because we force the document to recompute at the end of



But there's a problem.

It should be pretty obvious. The box itself is represented by an entirely different object than our FeaturePython object. The reason is because creates a separate box object and adds it to the document. In fact, if you go to your FeaturePython object and change the dimensions, you'll see another box shape gets created and the old one is left in place. That's not good! Additionally, if you have the Report View open, you may notice an error stating 'Nested recomputes of a document are not allowed". This has to do with using the Part.show method inside a FeaturePython object.  We want to avoid doing that.

Tip

The problem is, we're relying on the methods, which only generate non-parametric Part::Feature objects (simple, dumb shapes), just as you'd get if you copied a parametric object using Part Simple Copy.

What we want, of course, is a parametric box object that resizes the existing box as we change it's properties. We could delete the previous Part::Feature object and regenerate it every time we change a property, but we still have two objects to manage - our custom FeaturePython object and the Part::Feature object it generates.

So how do we solve this problem?

First, we need to use the right type of object.

To this point we've been using objects. They're great, but they're not intended for use as geometry objects. Rather, they are better used as document objects which do not require a visual representation in the 3D view. So we need to use a object instead.

In, change the following line:

obj = App.ActiveDocument.addObject('App::FeaturePython', obj_name)

to read:

obj = App.ActiveDocument.addObject('', obj_name)

To finish making the changes we need, the following line in the method needs to be changed:

Part.show(Part.makeBox(obj.Length, obj.Width, obj.Height))

to:

obj.Shape = Part.makeBox(obj.Length, obj.Width, obj.Height)

Your code should look like this:

import FreeCAD as App import Part def create(obj_name): """  Object creation method   """ obj = App.ActiveDocument.addObject('Part::FeaturePython', obj_name) fpo = box(obj) App.ActiveDocument.recompute return fpo class box: def __init__(self, obj): """      Default Constructor       """ self.Type = 'box' obj.addProperty('App::PropertyString', 'Description', 'Base', 'Box description').Description = "" obj.addProperty('App::PropertyLength', 'Length', 'Dimensions', 'Box length').Length = 10.0 obj.addProperty('App::PropertyLength', 'Width', 'Dimensions', 'Box width').Width = '10 mm' obj.addProperty('App::PropertyLength', 'Height', 'Dimensions', 'Box height').Height = '1 cm' obj.Proxy = self self.Object = obj def execute(self, obj): """      Called on document recompute       """ obj.Shape = Part.makeBox(obj.Length, obj.Width, obj.Height)



Now, save your changes and switch back to FreeCAD. Delete any existing objects, reload the box module, and create a new box object.

The results may seem a bit mixed. The icon in the treeview is different - it's a box, now. But there's no cube. And the icon is still gray!

What happened? Although we've properly created our box shape and assigned it to a object, before we can make it show up in our 3D view, we need to assign a ViewProvider.

Writing a ViewProvider
A View Provider is the component of an object which allows it to have visual representation in the GUI - specifically in the 3D view. FreeCAD uses an application structure known as 'model-view', which is designed to separate the data (the 'model') from it's visual representation (the 'view'). If you've spent any time working with FreeCAD in Python, you'll likely already be aware of this through the use of two core Python modules: FreeCAD and FreeCADGui (often aliased as 'App' and 'Gui' repectively).

Thus, our FeaturePython Box implementation also requires these elements. Thus far, we've focused purely on the 'model' portion, so now it's time to write the 'view'. Fortunately, most view implementations are simple and require little effort to write, at least to get started. Here's an example ViewProvider, borrowed and slightly modified from

class ViewProviderBox: def __init__(self, obj): """       Set this object to the proxy object of the actual view provider        """ obj.Proxy = self def attach(self, obj): """       Setup the scene sub-graph of the view provider, this method is mandatory        """ return def updateData(self, fp, prop): """       If a property of the handled feature has changed we have the chance to handle this here        """ return def getDisplayModes(self,obj): """       Return a list of display modes.        """ return None def getDefaultDisplayMode(self): """       Return the name of the default display mode. It must be defined in getDisplayModes.        """ return "Shaded" def setDisplayMode(self,mode): """       Map the display mode defined in attach with those defined in getDisplayModes.        Since they have the same names nothing needs to be done.        This method is optional.        """ return mode def onChanged(self, vp, prop): """       Print the name of the property that has changed        """ App.Console.PrintMessage("Change property: " + str(prop) + "\n") def getIcon(self): """       Return the icon in XMP format which will appear in the tree view. This method is optional and if not defined a default icon is shown.        """ return """           /* XPM */            static const char * ViewProviderBox_xpm[] = {            "16 16 6 1",            " 	c None",            ".	c #141010",            "+	c #615BD2",            "@	c #C39D55",            "#	c #000000",            "$	c #57C355",            "        ........",            "   ......++..+..",            "   .@@@@.++..++.",            "   .@@@@.++..++.",            "   .@@  .++++++.",            "  ..@@  .++..++.",            "###@@@@ .++..++.",            "##$.@@$#.++++++.",            "#$#$.$$$........",            "#$$#######      ",            "#$$#$$$$$#      ",            "#$$#$$$$$#      ",            "#$$#$$$$$#      ",            " #$#$$$$$#      ",            "  ##$$$$$#      ",            "   #######      "};            """ def __getstate__(self): """       Called during document saving.        """ return None def __setstate__(self,state): """"       Called during document restore. """"       return None

Note in the above code, we also define an XMP icon for this object. Icon design is beyond the scope of this tutorial, but basic icon designs can be managed using open source tools like GIMP, Krita, and Inkscape. The getIcon method is optional, as well. If it is not provided, FreeCAD will provide a default icon.

With out ViewProvider defined, we now need to put it to use to give our object the gift of visualization.

Return to the method in your code and add the following near the end:

ViewProviderBox(obj.ViewObject)

This instances the custom ViewProvider class and passes the FeaturePython's built-in ViewObject to it. The ViewObject won't do anything without our custom class implementation, so when the ViewProvider class initializes, it saves a reference to itself in the FeaturePython's ViewObject.Proxy attribute. This way, when FreeCAD needs to render our Box visually, it can find the ViewProvider class to do that.

Now, save the changes and return to FreeCAD. Import or reload the Box module and call.

We still don't see anything, but notice what happened to the icon next to the box object. It's in color! And it has a shape! That's a clue that our ViewProvider is working as expected.

So now, it's time to actually *add* a box.

Return to the code and add the following line to the execute method:

obj.Shape = Part.makeBox(obj.Length, obj.Width, obj.Height)

Now save, reload the box module in FreeCAD and create a new box.

You should see a box appear on screen! If it's too big (or maybe doesn't show up at all), click one of the 'ViewFit' buttons to fit it to the 3D view.

Note what we did here that differs from the way it was implemented for App::FeaturePython above. In the method, we called the  macro and passed it the box properties as before. But rather than call on the resulting object (which created a new, separate box object), we simply assigned it to the  property of our Part::FeaturePython object instead.

You can even alter the box dimensions by changing the values in the FreeCAD property panel. Give it a try!

Trapping Events
To this point, we haven't explicitly addressed event trapping. Nearly every method of a FeaturePython class serves as a callback accessible to the FeaturePython object (which gets access to our class instance through the attribute, if you recall).

Below is a list of the callbacks that may be implemented in the basic FeaturePython object:

In addition, there are two callbacks in the ViewProvider class that may occasionally prove useful:

It is not uncommon to encounter a situation where the Python callbacks are not being triggered as they should. Beginners in this area need to rest assured that the FeaturePython callback system is not fragile or broken. Invariably, when callbacks fail to run, it is because a reference is lost or undefined in the underlying code. If, however, callbacks appear to be breaking with no explanation, providing object / proxy references in the callback (as noted in the first table above) may alleviate these problems. Until you are comfortable with the callback system, it may be useful to add print statements in each callback to print messages to the console as a diagnostic during development.

The Code
The complete code for this example:

import FreeCAD as App import Part def create(obj_name): """   Object creation method    """ obj = App.ActiveDocument.addObject('Part::FeaturePython', obj_name) fpo = box(obj) ViewProviderBox(obj.ViewObject) App.ActiveDocument.recompute return fpo class box: def __init__(self, obj): """       Default Constructor        """ self.Type = 'box' self.ratios = None obj.addProperty('App::PropertyString', 'Description', 'Base', 'Box description').Description = 'Hello World!' obj.addProperty('App::PropertyLength', 'Length', 'Dimensions', 'Box length').Length = 10.0 obj.addProperty('App::PropertyLength', 'Width', 'Dimensions', 'Box width'). Width = '10 mm' obj.addProperty('App::PropertyLength', 'Height', 'Dimensions', 'Box Height').Height = '1 cm' obj.addProperty('App::PropertyBool', 'Aspect Ratio', 'Dimensions', 'Lock the box aspect ratio').Aspect_Ratio = False obj.Proxy = self self.Object = obj def __getstate__(self): return self.Type def __setstate__(self, state): if state: self.Type = state def execute(self, obj): """       Called on document recompute        """ obj.Shape = Part.makeBox(obj.Length, obj.Width, obj.Height) class ViewProviderBox: def __init__(self, obj): """       Set this object to the proxy object of the actual view provider        """ obj.Proxy = self def attach(self, obj): """       Setup the scene sub-graph of the view provider, this method is mandatory        """ return def updateData(self, fp, prop): """       If a property of the handled feature has changed we have the chance to handle this here        """ return def getDisplayModes(self,obj): """       Return a list of display modes.        """ return None def getDefaultDisplayMode(self): """       Return the name of the default display mode. It must be defined in getDisplayModes.        """ return "Shaded" def setDisplayMode(self,mode): """       Map the display mode defined in attach with those defined in getDisplayModes.        Since they have the same names nothing needs to be done.        This method is optional.        """ return mode def onChanged(self, vobj, prop): """       Print the name of the property that has changed        """ App.Console.PrintMessage("Change property: " + str(prop) + "\n") def getIcon(self): """       Return the icon in XMP format which will appear in the tree view. This method is optional and if not defined a default icon is shown.        """ return """           /* XPM */            static const char * ViewProviderBox_xpm[] = {            "16 16 6 1",            " 	c None",            ".	c #141010",            "+	c #615BD2",            "@	c #C39D55",            "#	c #000000",            "$	c #57C355",            "        ........",            "   ......++..+..",            "   .@@@@.++..++.",            "   .@@@@.++..++.",            "   .@@  .++++++.",            "  ..@@  .++..++.",            "###@@@@ .++..++.",            "##$.@@$#.++++++.",            "#$#$.$$$........",            "#$$#######      ",            "#$$#$$$$$#      ",            "#$$#$$$$$#      ",            "#$$#$$$$$#      ",            " #$#$$$$$#      ",            "  ##$$$$$#      ",            "   #######      "};            """ def __getstate__(self): return None def __setstate__(self,state): return None