import bpy
from bpy_extras import view3d_utils


def main(context, event):
    """Run this function on left mouse, execute the ray cast"""
    # get the context arguments
    scene = context.scene
    region = context.region
    rv3d = context.region_data
    coord = event.mouse_region_x, event.mouse_region_y

    # get the ray from the viewport and mouse
    view_vector = view3d_utils.region_2d_to_vector_3d(region, rv3d, coord)
    ray_origin = view3d_utils.region_2d_to_origin_3d(region, rv3d, coord)

    ray_target = ray_origin + view_vector

    def visible_objects_and_duplis():
        """Loop over (object, matrix) pairs (mesh only)"""

        depsgraph = context.evaluated_depsgraph_get()
        for dup in depsgraph.object_instances:
            if dup.is_instance:  # Real dupli instance
                obj = dup.instance_object
                yield (obj, dup.matrix_world.copy())
            else:  # Usual object
                obj = dup.object
                yield (obj, obj.matrix_world.copy())

    def obj_ray_cast(obj, matrix):
        """Wrapper for ray casting that moves the ray into object space"""

        # get the ray relative to the object
        matrix_inv = matrix.inverted()
        ray_origin_obj = matrix_inv @ ray_origin
        ray_target_obj = matrix_inv @ ray_target
        ray_direction_obj = ray_target_obj - ray_origin_obj

        # cast the ray
        success, location, normal, face_index = obj.ray_cast(ray_origin_obj, ray_direction_obj)

        if success:
            return location, normal, face_index
        else:
            return None, None, None

    # cast rays and find the closest object
    best_length_squared = -1.0
    best_obj = None

    for obj, matrix in visible_objects_and_duplis():
        if obj.type == 'MESH':
            hit, normal, face_index = obj_ray_cast(obj, matrix)
            if hit is not None:
                hit_world = matrix @ hit
                scene.cursor.location = hit_world
                length_squared = (hit_world - ray_origin).length_squared
                if best_obj is None or length_squared < best_length_squared:
                    best_length_squared = length_squared
                    best_obj = obj
                    best_face = face_index

    # now we have the object under the mouse cursor,
    # we could do lots of stuff but for the example just select.
    if best_obj is not None:
        # for selection etc. we need the original object,
        # evaluated objects are not in viewlayer
        best_original = best_obj.original
        best_original.select_set(True)
        print(best_face)


class ViewOperatorRayCast(bpy.types.Operator):
    """Modal object selection with a ray cast"""
    bl_idname = "view3d.modal_operator_raycast"
    bl_label = "RayCast View Operator"

    def modal(self, context, event):
        if event.type in {'MIDDLEMOUSE', 'WHEELUPMOUSE', 'WHEELDOWNMOUSE'}:
            # allow navigation
            return {'PASS_THROUGH'}
        elif event.type == 'LEFTMOUSE':
            main(context, event)
            return {'RUNNING_MODAL'}
        elif event.type in {'RIGHTMOUSE', 'ESC'}:
            return {'CANCELLED'}

        return {'RUNNING_MODAL'}

    def invoke(self, context, event):
        if context.space_data.type == 'VIEW_3D':
            context.window_manager.modal_handler_add(self)
            return {'RUNNING_MODAL'}
        else:
            self.report({'WARNING'}, "Active space must be a View3d")
            return {'CANCELLED'}


def register():
    bpy.utils.register_class(ViewOperatorRayCast)


def unregister():
    bpy.utils.unregister_class(ViewOperatorRayCast)


if __name__ == "__main__":
    register()