[Solved]Help me understand Selection Observer

[alberich]

I'm trying to build a macro that analyzes selected edges of a body and sends the analysis result on to Draft and then Techdraw. I've got all the bits and pieces working but the edge selecting process bothers me because, although it works, I don't understand how it works. Here's the relevent chunk of code:

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"""Stripped down from MyDialog.FCMacro by openBrain

      https://github.com/0penBrain/FreeCAD-macros/blob/MyDialog/Examples/MyDialog.FCMacro

   Demonstrate the selection  and reporting of edges.

"""
import re

from PySide2 import QtCore, QtWidgets
import FreeCADGui as Gui

MSG = App.Console.PrintMessage

class MyDialog(QtWidgets.QDialog):

    class MySelObs(QtCore.QObject):
        """Selection Observer"""

        selection = QtCore.Signal(str)
        clear = QtCore.Signal(str)

        def __init__(self):
            super().__init__()

        def addSelection(self, doc, obj, sub, pnt):
            print(sub, sub[:4])
            if sub[:4] == "Edge":  # Is sub-element an Edge? Check head of string.
                x = sub[4:]  # get numerical value of sub-element from tail of string.
                self.selection.emit(f'Edge {x[0]}')
            else:
                print("\nNot an Edge. Deselect and try again.\n")

        def clearSelection(self,doc):
            self.clear.emit(f'Selection cleared in document {doc}')


    def __init__(self, parent=Gui.getMainWindow()):
        super().__init__(parent, QtCore.Qt.Tool)
        self.setAttribute(QtCore.Qt.WA_DeleteOnClose, True)
        self.setAttribute(QtCore.Qt.WA_WindowPropagation, True)
        self.initUI()


        if Gui.activeView() and repr(Gui.activeView()) == 'View3DInventor':
            self.view = Gui.activeView()
        else:
            pass  # Only running this in Gui active anyway

        self.sel_observer = self.MySelObs()
        self.sel_observer.selection.connect(self.writeSel)
        self.sel_observer.clear.connect(self.writeSel)
        Gui.Selection.addObserver(self.sel_observer)

    def initUI(self):
        self.setWindowTitle('Edge Gatherer Test')

        my_lay = QtWidgets.QGridLayout(self)
        self.setLayout(my_lay)

        my_sel_ob_w = QtWidgets.QWidget(self)
        my_lay.addWidget(my_sel_ob_w, 0, 1)
        my_button_box = QtWidgets.QDialogButtonBox(QtWidgets.QDialogButtonBox.Ok | QtWidgets.QDialogButtonBox.Cancel, self)
        my_lay.addWidget(my_button_box, 2, 0, 1, -1)


        my_sel_ob_lay = QtWidgets.QVBoxLayout(my_sel_ob_w)
        my_sel_ob_w.setLayout(my_sel_ob_lay)
        my_sel_ob_lay.addWidget(QtWidgets.QLabel('Selection observer:', my_sel_ob_w))
        self.my_sel_ob = QtWidgets.QPlainTextEdit(my_sel_ob_w)
        self.my_sel_ob.setReadOnly(True)
        self.my_sel_ob.setMaximumBlockCount(100)
        my_sel_ob_lay.addWidget(self.my_sel_ob)


        my_button_box.accepted.connect(self.accept)
        my_button_box.rejected.connect(self.reject)


    def writeSel(self, text):
        self.my_sel_ob.appendPlainText(text)

    def accept(self):
        MSG('Good bye\n')
        super().accept()

    def reject(self):
        print('\nSorry.  [Cancel] doesn't work yet.\n')

    def hideEvent(self, event):
        if hasattr(self, 'view_callback'):
            try:
                self.view.removeEventCallback('SoEvent', self.view_callback)
            except RuntimeError:
                pass
        Gui.Selection.removeObserver(self.sel_observer)
        super().hideEvent(event)


MyDialog().show()

Where my understanding has a gap is at the line

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def addSelection(self, doc, obj, sub, pnt):

How do the parameters here get populated? I know what they are but I don't see how they get here. I can also see that they are the same parameters I see when mousing over a body, reported as

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"Preselected: <doc>.<obj>.<sub> (pnt)

and also in the python console when clicked on as

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# Gui.Selection.addSelection(doc,obj,sub,pnt.x,pnt.y,pnt.z)

Does it tie together here?

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        self.sel_observer = self.MySelObs()  # <<<<------<<<<
        self.sel_observer.selection.connect(self.writeSel)
        self.sel_observer.clear.connect(self.writeSel)
        Gui.Selection.addObserver(self.sel_observer)  # <<<<------<<<<

Edit: added credit in code for openBrain, removed misleading comment in code.

[Chris_G]

In your code, you have this line at the end of the __init__ of class MyDialog :

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Gui.Selection.addObserver(self.sel_observer)

This adds self.sel_observer to the list of objects to which the Selection mechanism of FreeCAD Gui will transmit its information.
As such, these objects are supposed to have defined special methods that will be called by Gui.Selection.
They are addSelection(), clearSelection() and so on...

[alberich]

So Gui.Selection.addObserver connects me to the Selection API.

When I look on the wiki https://wiki.freecadweb.org/Selection_API I see

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addSelection(FreeCAD.Object)

Is that addSelection my addSelection? So that in my

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addSelection(self, doc, obj, sub, pnt)

the parameters self,doc,obj,sub,pnt is the API's FreeCAD.Object?

[Chris_G]

Is that addSelection my addSelection? So that in my

This addSelection() is a method of FreeCADGui.Selection module :

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FreeCADGui.Selection.addSelection(a_FreeCAD_object)

It is also possible to specify subNames :

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FreeCADGui.Selection.addSelection(a_FreeCAD_object, ["Face6", "Face8"])

Whenever FreeCADGui.Selection receives a selection action (either by the above code, by a mouse action in the Gui, in the Tree-View, or by a click on a subshape, in the 3D View), it loops over its list of Observer objects, and call their own functions corresponding to the action (addSelection, clearSelection, removeSelection).

[alberich]

Ok. I think I've got a bit better handle on this.

By rummaging in the source code, first at Sourceforge then GitHub, I found in Selection.h there is

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struct SelObj {
        const char* DocName;
        const char* FeatName;
        const char* SubName;
        const char* TypeName;
        App::Document* pDoc;
        App::DocumentObject*  pObject;
        App::DocumentObject* pResolvedObject;
        float x,y,z;
};

that has the bits I'm trying to track. I surmise that the python Selection API handles
passing this struct (or parts of it) around.

In my example, the line

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Gui.Selection.addObserver(self.sel_observer) 

installs an instance of my class MySelObj as an Observer that is privy to the struct elements
managed by the underlying C++ library. It is through this installation that my method addSelection
becomes aware of the Gui.Selection.AddSelection method and can, thus, share the contents of that C++ struct.

[Kunda1]

In your rummaging, Please consider adding some source code comments aka doxygen documentation as it is sorely missing

[openBrain]

In your rummaging, Please consider adding some source code comments aka doxygen documentation as it is sorely missing

OP copied my tutorial code before I added the comments.

@alberich, maybe it would be good to tell that you get it working by copying and adapting existing code. Not that I care you took my code (that's the purpose, and I'm glad it serves), but it may help forumers understand why you can get a bit lost with it.

[openBrain]

Ok. I think I've got a bit better handle on this.

By rummaging in the source code, first at Sourceforge then GitHub, I found in Selection.h there is

Available methods for Selection observer are listed in Selection.h, but care to look at the SelectionObserver class : https://github.com/FreeCAD/FreeCAD/blob ... #L276-L283

[alberich]

OP copied my tutorial code before I added the comments.

@alberich, maybe it would be good to tell that you get it working by copying and adapting existing code. Not that I care you took my code (that's the purpose, and I'm glad it serves), but it may help forumers understand why you can get a bit lost with it.

@openBrain, I'm glad you popped in. I wanted to credit your help in OP but honestly couldn't find my way back to where I originally found the code. I have it saved as SampleDialog.FCMacro but I've been DGGing for an hour and can't find a link back to its origin. I also had in mind that it was from a different FCer

If you can give me a link I'll edit the OP.

Cancel that. I just found it. It's much changed from when I grabbed it before. (New name, hence the failed search?) The new comments are clarifying.

The link is here

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https://github.com/0penBrain/FreeCAD-macros/blob/MyDialog/Examples/MyDialog.FCMacro

I'll edit the OP to add it there as well.

As an aside, I did also find FreeCAD_Mod_Dev_Guide/chapters at master · qingfengxia/FreeCAD_Mod_Dev_Guide at https://github.com/qingfengxia/FreeCAD_ ... r/chapters. While not up to date with current FC development it was a great one-stop shopping help for a FC code overview.

[alberich]

In your rummaging, Please consider adding some source code comments aka doxygen documentation as it is sorely missing

@Kunda1, I'd be more than happy to do that but my tenuous grasp on the technicalities wouldn't support it. I'd be more qualified to edit for grammar.