import hou
import drawable2d as d2d

MY_LINE_DRAWABLE = "line"
MY_MARKER_DRAWABLE = "marker"

TX = "tx"
TY = "ty"

def __init__(self, **kwargs):
    self.__dict__.update(kwargs)

    self._line = None
    self._marker = None

    # handle dragger
    self._dragger = d2d.Dragger2D(self.scene_viewer)

def onActivate(self, kwargs):
    # Drawables creation omitted for brievity
    pass

def onMouseEvent(self, kwargs):
    """ Drags a marker position and update the line point #2
        with the new position.
    """
    ui_event = kwargs["ui_event"]
    reason = ui_event.reason()

    if reason == hou.uiEventReason.Changed:
        # End the current drag operation on mouse up
        self._dragger.endDrag()
        return True

    if not self._dragger.update(kwargs, self._marker):
        return False

    drawable_name = self.handle_context.name()

    consumed = True
    if reason == hou.uiEventReason.Start:
        if drawable_name == MY_MARKER_DRAWABLE:
            self._dragger.startDrag(p1=self.handle_parms[TX], p2=self.handle_parms[TY], 
                transform_mode=d2d.Drawable2D.TransformMode.PARAMS)

    elif reason in [hou.uiEventReason.Active, hou.uiEventReason.Changed]:
        if drawable_name == MY_MARKER_DRAWABLE:
            # Drag the drawable
            self._dragger.dragXY()

            # Change the drawable positions  
            dx, dy = self._dragger.delta()
            delta = hou.Vector3(dx,dy,0)

            # Adjust the delta with the handle scaling factor
            handle_scale = self.handle_context.handleScaleValue(delta)
            delta /= handle_scale

            dx = delta[0]
            dy = delta[1]

            su.addToDrawablePos(self._marker, dx, dy)                                
            su.addToDrawablePoint(self._line, 1, dx, dy)

    return consumed

    ...