Watching an object for property changes

[Joel_graff]

So I'm trying to sort out how to watch another object for changes in it's properties. Essentially, I'm creating a FeaturePython object which takes a Line object as a child. I'm using the FeaturePython object to store metadata about the line, for example, it's bearing w.r.t the unit Y axis.

So, the Bearing property of the FP object, then, needs to update whenever the Line object changes...

[TheMarkster]

Should be able to use setExpression() to do it. For example, given an object named Cylinder and another named Line, if we wanted Cylinder's Height property to always be equal to Line's Y2 - Y1 value:

Code: Select all

App.ActiveDocument.Cylinder.setExpression('Height',u'Line.Y2-Line.Y1')

[Chris_G]

So I'm trying to sort out how to watch another object for changes in it's properties. Essentially, I'm creating a FeaturePython object which takes a Line object as a child. I'm using the FeaturePython object to store metadata about the line, for example, it's bearing w.r.t the unit Y axis.

So, the Bearing property of the FP object, then, needs to update whenever the Line object changes...

What about using a App::PropertyLink property to have your FeaturePython object "triggered" when the Line object is changed :

Code: Select all

class myFeaturePythonProxy(object):
    def class __init__(self, fp):
        fp.addProperty("App::PropertyLink", "LineObject", "myFP","Link to Line object")
        fp.addProperty("App::PropertySomething", "Bearing", "myFP","Bearing")


    def execute(self, fp):
        # usually, execute is run whenever one of its child objects is touched.
        update_bearing_property(fp)
        # ... usual code here

    def update_bearing_property(self, fp):
        # check fp.LineObject properties
        fp.Bearing = ... something

[ickby]

you can simply install a document observer which is called when a object changes. I did not find a python example, but in the cpp file you can see the kind of functions that are exposed:
https://github.com/FreeCAD/FreeCAD/blob ... erPython.h

[Joel_graff]

you can simply install a document observer which is called when a object changes. I did not find a python example, but in the cpp file you can see the kind of functions that are exposed:

Fascinating. I'll keep that in mind. I'm starting to sort the problem out, but if I can't solve it, this might do the trick...

[Joel_graff]

Solved this. The trick was to create a group object and add the object to be watched as a child. This triggers an auto update of the parent.