Subscriber, that will add additional button to Glyph Editor "Display..." menu

__myExtensionName__Settings.py

# ===================================
# INFO
# this is file, where you define your settings.
#
# - It is important to change __myExtensionName__ part in the file's name to to the actual name of your extension.
# - You should change all __myExtensionName__ entries to the actual name of your extension.
# - Add all your custom defaults to "defaults" variable.
#
# Imports below ARE MANDATORY:
# ===================================
from mojo.extensions import (
    registerExtensionDefaults,
    getExtensionDefault,
    setExtensionDefault
)

extensionID = "com.__myName__.__myExtensionName__"
extensionKeyStub = extensionID + "."

defaults = {
    extensionKeyStub + "isVisible": False,
}

registerExtensionDefaults(defaults)

def internalGetDefault(key):
    key = extensionKeyStub + key
    return getExtensionDefault(key)


def internalSetDefault(key, value):
    key = extensionKeyStub + key
    setExtensionDefault(key, value)

__myExtensionName__Subscriber.py

# ===================================
# INFO
# this is main file, where you define your functionality.
#
# - It is important to change __myExtensionName__ part in the file's name to to the actual name of your extension.
# - You should change all __myExtensionName__ entries to the actual name of your extension.
# - You should define "title" class attribute (35 line of the code). It will show up as this in the display menu.
# - Your subscriber class has to have a DisplaySuscriber set as parent in order to make the whole thing work.
# - If you define glyphEditorDidOpen event, it has to call super() of itself (look at line no 8e of the code).
#
# Three imports below ARE MANDATORY:
# ===================================
from displaySubscriber import DisplaySuscriber
from mojo import subscriber
from __myExtensionName__Settings import (
    internalGetDefault,
    internalSetDefault,
    extensionID,
    extensionKeyStub
)

from fontTools.pens.transformPen import TransformPointPen, TransformPen
from fontTools.misc.transform import Transform
from merz import MerzPen

# ====================================
# Example
# ====================================       



class __myExtensionName__Subscriber(DisplaySuscriber):
    debug = True

    title = "My Extension Name" 
    curvaturePath = None
    
    def build(self):
        self.showMe = internalGetDefault("isVisible")
        window = self.getGlyphEditor()
        self.backgroundContainer = window.extensionContainer(
            identifier=extensionKeyStub + "background",
            location="background",
            clear=True,
        )
        self.bgBaseLayer = self.backgroundContainer.appendBaseSublayer()
        self.curvaturePath = self.bgBaseLayer.appendPathSublayer(
            fillColor=(1, 0, 0, 0.5), visible=False
        )

    def toggleOn(self):
        if self.curvaturePath is None:
            return
        self.curvaturePath.setVisible(True)

    def toggleOff(self):
        if self.curvaturePath is None:
            return
        self.curvaturePath.setVisible(False)

    def glyphEditorDidKeyDown(self, info):
        self.drawPath(info)

    def glyphEditorDidUndo(self, info):
        self.drawPath(info)

    def glyphEditorDidMouseDrag(self, info):
        self.drawPath(info)

    def drawPath(self, info):
        glyph = info["glyph"]
        merzPen = MerzPen()
        n_glyph = RGlyph()
        n_glyphPen = n_glyph.getPen()
        glyph.draw(n_glyphPen)
        n_glyph.moveBy((100,100))
        n_glyph.draw(merzPen)

        self.curvaturePath.setPath( merzPen.path )


    def glyphEditorDidOpen(self, info):
        super().glyphEditorDidOpen(info)
        self.drawPath(info)
        

subscriber.registerGlyphEditorSubscriber(ExampleSubscriber)

displaySubscriber.py

# ===================================
# ATTENCION!!!!
# dont' touch this file except __myExtensionName__ parts!!!
# ===================================

from vanilla.vanillaBase import VanillaCallbackWrapper
from mojo.subscriber import Subscriber
from AppKit import NSMenuItem
from __myExtensionName__Settings import (
    internalGetDefault,
    internalSetDefault,
    extensionID
)


def _getDisplayPopUpMenuFromEditWindow(editWindow):
    if editWindow is None:
        return
    return editWindow.getGlyphStatusBar().displayButton

def _createCustomVisualizerSeparator(displayPopUp):
    if len(displayPopUp.getItems()) == 27:
        menu = displayPopUp.getNSPopUpButton().menu()
        menu.addItem_(NSMenuItem.separatorItem())

    elif displayPopUp.getNSPopUpButton().itemArray()[27].isSeparatorItem():
        return

# =========================================
#  S U B S C R I B E R (with basic events)
# =========================================

class DisplaySuscriber(Subscriber):
    debug = True

    title = None

    def menuButtonWasPressed(self, nsMenuItem):
        if self.getButtonState():
            self.showMe = True
        else:
            self.showMe = False

    @property
    def showMe(self):
        return self._showMe

    @showMe.setter
    def showMe(self, value):
        if value:
            self.toggleOn()
        
        else:
            self.toggleOff()

        internalSetDefault("isVisible", value)
        self._showMe = value

    def getButtonState(self):
        return bool(self._menuItem.state())

    def _menuItemCallback(self, nsMenuItem):
        self._menuItem.setState_(not self._menuItem.state())
        self.menuButtonWasPressed(nsMenuItem)
        print(self.title)

    def _appendMenuItemToDisplayMenu(self, window):
        assert self.title is not None, "title wasn't set"
        assert isinstance(self.title, str), "title must be a string"
        displayPopUp = _getDisplayPopUpMenuFromEditWindow(window)
        if displayPopUp is None:
            return
        
        # creating the separator in Display... menu
        _createCustomVisualizerSeparator(displayPopUp)

        self._menuItem = NSMenuItem.alloc().initWithTitle_action_keyEquivalent_(self.title, "", "")
        wrapper = VanillaCallbackWrapper(self._menuItemCallback)
        self._menuItemCallbackWrappers = []
        self._menuItemCallbackWrappers.append(wrapper)
        self._menuItem.setTarget_(wrapper)
        self._menuItem.setAction_("action:")
        menu = displayPopUp.getNSPopUpButton().menu()
        menu.addItem_(self._menuItem)

    def toggleOff(self):
        NotImplemented

    def toggleOn(self):
        NotImplemented

    def glyphEditorDidOpen(self, info):
        """
        Whenever you want to use glyphEditorDidOpen event in your own DisplaySuscriber class, please
        use super method of glyphEditorDidOpen
        """
        self._appendMenuItemToDisplayMenu(info["glyphEditor"])
        self.showMe = internalGetDefault("isVisible")
        self._menuItem.setState_(self.showMe)