Cura/plugins/CuraEngineBackend/CuraEngineBackend.py

#  Copyright (c) 2022 UltiMaker
#  Cura is released under the terms of the LGPLv3 or higher.

import argparse #To run the engine in debug mode if the front-end is in debug mode.
from collections import defaultdict
import os
from PyQt6.QtCore import QObject, QTimer, QUrl, pyqtSlot
import sys
from time import time
from typing import Any, cast, Dict, List, Optional, Set, TYPE_CHECKING

from PyQt6.QtGui import QDesktopServices, QImage

from UM.Backend.Backend import Backend, BackendState
from UM.Scene.SceneNode import SceneNode
from UM.Signal import Signal
from UM.Logger import Logger
from UM.Message import Message
from UM.PluginRegistry import PluginRegistry
from UM.Platform import Platform
from UM.Qt.Duration import DurationFormat
from UM.Scene.Iterator.DepthFirstIterator import DepthFirstIterator
from UM.Settings.Interfaces import DefinitionContainerInterface
from UM.Settings.SettingInstance import SettingInstance #For typing.
from UM.Tool import Tool #For typing.

from cura.CuraApplication import CuraApplication
from cura.Settings.ExtruderManager import ExtruderManager
from cura.Snapshot import Snapshot
from cura.Utils.Threading import call_on_qt_thread
from .ProcessSlicedLayersJob import ProcessSlicedLayersJob
from .StartSliceJob import StartSliceJob, StartJobResult

import pyArcus as Arcus

if TYPE_CHECKING:
    from cura.Machines.Models.MultiBuildPlateModel import MultiBuildPlateModel
    from cura.Machines.MachineErrorChecker import MachineErrorChecker
    from UM.Scene.Scene import Scene
    from UM.Settings.ContainerStack import ContainerStack

from UM.i18n import i18nCatalog
catalog = i18nCatalog("cura")


class CuraEngineBackend(QObject, Backend):
    backendError = Signal()

    printDurationMessage = Signal()
    """Emitted when we get a message containing print duration and material amount.

    This also implies the slicing has finished.
    :param time: The amount of time the print will take.
    :param material_amount: The amount of material the print will use.
    """
    slicingStarted = Signal()
    """Emitted when the slicing process starts."""

    slicingCancelled = Signal()
    """Emitted when the slicing process is aborted forcefully."""

    def __init__(self) -> None:
        """Starts the back-end plug-in.

        This registers all the signal listeners and prepares for communication
        with the back-end in general.
        CuraEngineBackend is exposed to qml as well.
        """

        super().__init__()
        # Find out where the engine is located, and how it is called.
        # This depends on how Cura is packaged and which OS we are running on.
        executable_name = "CuraEngine"
        if Platform.isWindows():
            executable_name += ".exe"
        self._default_engine_location = executable_name

        search_path = [
            os.path.abspath(os.path.dirname(sys.executable)),
            os.path.abspath(os.path.join(os.path.dirname(sys.executable), "bin")),
            os.path.abspath(os.path.join(os.path.dirname(sys.executable), "..")),

            os.path.join(CuraApplication.getInstallPrefix(), "bin"),
            os.path.dirname(os.path.abspath(sys.executable)),
        ]
        self._last_backend_plugin_port = self._port + 1000
        for path in search_path:
            engine_path = os.path.join(path, executable_name)
            if os.path.isfile(engine_path):
                self._default_engine_location = engine_path
                break

        if Platform.isLinux() and not self._default_engine_location:
            if not os.getenv("PATH"):
                raise OSError("There is something wrong with your Linux installation.")
            for pathdir in cast(str, os.getenv("PATH")).split(os.pathsep):
                execpath = os.path.join(pathdir, executable_name)
                if os.path.exists(execpath):
                    self._default_engine_location = execpath
                    break

        application: CuraApplication = CuraApplication.getInstance()
        self._multi_build_plate_model: Optional[MultiBuildPlateModel] = None
        self._machine_error_checker: Optional[MachineErrorChecker] = None

        if not self._default_engine_location:
            raise EnvironmentError("Could not find CuraEngine")

        Logger.log("i", "Found CuraEngine at: %s", self._default_engine_location)

        self._default_engine_location = os.path.abspath(self._default_engine_location)
        application.getPreferences().addPreference("backend/location", self._default_engine_location)

        # Workaround to disable layer view processing if layer view is not active.
        self._layer_view_active: bool = False
        self._onActiveViewChanged()

        self._stored_layer_data: List[Arcus.PythonMessage] = []

        # key is build plate number, then arrays are stored until they go to the ProcessSlicesLayersJob
        self._stored_optimized_layer_data: Dict[int, List[Arcus.PythonMessage]] = {}

        self._scene: Scene = application.getController().getScene()
        self._scene.sceneChanged.connect(self._onSceneChanged)

        # Triggers for auto-slicing. Auto-slicing is triggered as follows:
        #  - auto-slicing is started with a timer
        #  - whenever there is a value change, we start the timer
        #  - sometimes an error check can get scheduled for a value change, in that case, we ONLY want to start the
        #    auto-slicing timer when that error check is finished
        # If there is an error check, stop the auto-slicing timer, and only wait for the error check to be finished
        # to start the auto-slicing timer again.
        #
        self._global_container_stack: Optional[ContainerStack] = None

        # Listeners for receiving messages from the back-end.
        self._message_handlers["cura.proto.Layer"] = self._onLayerMessage
        self._message_handlers["cura.proto.LayerOptimized"] = self._onOptimizedLayerMessage
        self._message_handlers["cura.proto.Progress"] = self._onProgressMessage
        self._message_handlers["cura.proto.GCodeLayer"] = self._onGCodeLayerMessage
        self._message_handlers["cura.proto.GCodePrefix"] = self._onGCodePrefixMessage
        self._message_handlers["cura.proto.SliceUUID"] = self._onSliceUUIDMessage
        self._message_handlers["cura.proto.PrintTimeMaterialEstimates"] = self._onPrintTimeMaterialEstimates
        self._message_handlers["cura.proto.SlicingFinished"] = self._onSlicingFinishedMessage

        self._start_slice_job: Optional[StartSliceJob] = None
        self._start_slice_job_build_plate: Optional[int] = None
        self._slicing: bool = False  # Are we currently slicing?
        self._restart: bool = False  # Back-end is currently restarting?
        self._tool_active: bool = False  # If a tool is active, some tasks do not have to do anything
        self._always_restart: bool = True # Always restart the engine when starting a new slice. Don't keep the process running. TODO: Fix engine statelessness.
        self._process_layers_job: Optional[ProcessSlicedLayersJob] = None # The currently active job to process layers, or None if it is not processing layers.
        self._build_plates_to_be_sliced: List[int] = []  # what needs slicing?
        self._engine_is_fresh: bool = True  # Is the newly started engine used before or not?

        self._backend_log_max_lines: int = 20000  # Maximum number of lines to buffer
        self._error_message: Optional[Message] = None  # Pop-up message that shows errors.

        # Count number of objects to see if there is something changed
        self._last_num_objects: Dict[int, int] = defaultdict(int)
        self._postponed_scene_change_sources: List[SceneNode] = []   # scene change is postponed (by a tool)

        self._time_start_process: Optional[float] = None
        self._is_disabled: bool = False

        application.getPreferences().addPreference("general/auto_slice", False)

        self._use_timer: bool = False

        # When you update a setting and other settings get changed through inheritance, many propertyChanged
        # signals are fired. This timer will group them up, and only slice for the last setting changed signal.
        # TODO: Properly group propertyChanged signals by whether they are triggered by the same user interaction.
        self._change_timer: QTimer = QTimer()
        self._change_timer.setSingleShot(True)
        self._change_timer.setInterval(500)
        self.determineAutoSlicing()
        application.getPreferences().preferenceChanged.connect(self._onPreferencesChanged)

        self._slicing_error_message = Message(
            text = catalog.i18nc("@message", "Slicing failed with an unexpected error. Please consider reporting a bug on our issue tracker."),
            title = catalog.i18nc("@message:title", "Slicing failed"),
            message_type = Message.MessageType.ERROR
        )
        self._slicing_error_message.addAction(
            action_id = "report_bug",
            name = catalog.i18nc("@message:button", "Report a bug"),
            description = catalog.i18nc("@message:description", "Report a bug on UltiMaker Cura's issue tracker."),
            icon = "[no_icon]"
        )
        self._slicing_error_message.actionTriggered.connect(self._reportBackendError)

        self._resetLastSliceTimeStats()
        self._snapshot: Optional[QImage] = None 

        application.initializationFinished.connect(self.initialize)

    def startPlugins(self) -> None:
        """
        Ensure that all backend plugins are started
        It assigns unique ports to each plugin to avoid conflicts.
        :return:
        """
        self.stopPlugins()
        backend_plugins = CuraApplication.getInstance().getBackendPlugins()
        for backend_plugin in backend_plugins:
            # Set the port to prevent plugins from using the same one.
            if backend_plugin.getPort() < 1:
                backend_plugin.setPort(self._last_backend_plugin_port)
                self._last_backend_plugin_port += 1
            backend_plugin.start()

    def stopPlugins(self) -> None:
        """
        Ensure that all backend plugins will be terminated.
        """
        backend_plugins = CuraApplication.getInstance().getBackendPlugins()
        for backend_plugin in backend_plugins:
            if backend_plugin.isRunning():
                backend_plugin.stop()

    def _resetLastSliceTimeStats(self) -> None:
        self._time_start_process = None
        self._time_send_message = None
        self._time_end_slice = None

    def resetAndReturnLastSliceTimeStats(self) -> Dict[str, float]:
        last_slice_data = {
            "time_start_process": self._time_start_process,
            "time_send_message": self._time_send_message,
            "time_end_slice": self._time_end_slice,
        }
        self._resetLastSliceTimeStats()
        return last_slice_data

    def initialize(self) -> None:
        application = CuraApplication.getInstance()
        self._multi_build_plate_model = application.getMultiBuildPlateModel()

        application.getController().activeViewChanged.connect(self._onActiveViewChanged)

        if self._multi_build_plate_model:
            self._multi_build_plate_model.activeBuildPlateChanged.connect(self._onActiveViewChanged)

        application.getMachineManager().globalContainerChanged.connect(self._onGlobalStackChanged)
        self._onGlobalStackChanged()

        # Extruder enable / disable. Actually wanted to use machine manager here,
        # but the initialization order causes it to crash
        ExtruderManager.getInstance().extrudersChanged.connect(self._extruderChanged)

        self.backendQuit.connect(self._onBackendQuit)
        self.backendConnected.connect(self._onBackendConnected)

        # When a tool operation is in progress, don't slice. So we need to listen for tool operations.
        application.getController().toolOperationStarted.connect(self._onToolOperationStarted)
        application.getController().toolOperationStopped.connect(self._onToolOperationStopped)

        self._machine_error_checker = application.getMachineErrorChecker()
        self._machine_error_checker.errorCheckFinished.connect(self._onStackErrorCheckFinished)

    def close(self) -> None:
        """Terminate the engine process.

        This function should terminate the engine process.
        Called when closing the application.
        """

        # Terminate CuraEngine if it is still running at this point
        self._terminate()

    def getEngineCommand(self) -> List[str]:
        """Get the command that is used to call the engine.

        This is useful for debugging and used to actually start the engine.
        :return: list of commands and args / parameters.
        """
        from cura import ApplicationMetadata
        if ApplicationMetadata.IsEnterpriseVersion:
            command = [self._default_engine_location]
        else:
            command = [CuraApplication.getInstance().getPreferences().getValue("backend/location")]
        command += ["connect", "127.0.0.1:{0}".format(self._port), ""]

        parser = argparse.ArgumentParser(prog = "cura", add_help = False)
        parser.add_argument("--debug", action = "store_true", default = False,
                            help = "Turn on the debug mode by setting this option.")
        known_args = vars(parser.parse_known_args()[0])
        if known_args["debug"]:
            command.append("-vvv")

        return command

    @pyqtSlot()
    def stopSlicing(self) -> None:
        self.setState(BackendState.NotStarted)
        if self._slicing:  # We were already slicing. Stop the old job.
            self._terminate()
            self._createSocket()

        if self._process_layers_job is not None:
            # We were processing layers. Stop that, the layers are going to change soon.
            Logger.log("i", "Aborting process layers job...")
            self._process_layers_job.abort()
            self._process_layers_job = None

        if self._error_message:
            self._error_message.hide()

    @pyqtSlot()
    def forceSlice(self) -> None:
        """Manually triggers a reslice"""

        self.markSliceAll()
        self.slice()

    @call_on_qt_thread  # Must be called from the main thread because of OpenGL
    def _createSnapshot(self) -> None:
        self._snapshot = None
        if not CuraApplication.getInstance().isVisible:
            Logger.log("w", "Can't create snapshot when renderer not initialized.")
            return
        Logger.log("i", "Creating thumbnail image (just before slice)...")
        try:
            self._snapshot = Snapshot.snapshot(width = 300, height = 300)
        except Exception:
            Logger.logException("w", "Failed to create snapshot image")
            self._snapshot = None  # Failing to create thumbnail should not fail creation of UFP

    def getLatestSnapshot(self) -> Optional[QImage]:
        return self._snapshot

    def slice(self) -> None:
        """Perform a slice of the scene."""

        self._createSnapshot()

        self.startPlugins()

        Logger.log("i", "Starting to slice...")
        self._time_start_process = time()
        if not self._build_plates_to_be_sliced:
            self.processingProgress.emit(1.0)
            Logger.log("w", "Slice unnecessary, nothing has changed that needs reslicing.")
            self.setState(BackendState.Done)
            return

        if self._process_layers_job:
            Logger.log("d", "Process layers job still busy, trying later.")
            return

        if not hasattr(self._scene, "gcode_dict"):
            self._scene.gcode_dict = {}  # type: ignore
            # We need to ignore type because we are creating the missing attribute here.

        # see if we really have to slice
        application = CuraApplication.getInstance()
        active_build_plate = application.getMultiBuildPlateModel().activeBuildPlate
        build_plate_to_be_sliced = self._build_plates_to_be_sliced.pop(0)
        Logger.log("d", "Going to slice build plate [%s]!" % build_plate_to_be_sliced)
        num_objects = self._numObjectsPerBuildPlate()

        self._stored_layer_data = []

        if build_plate_to_be_sliced not in num_objects or num_objects[build_plate_to_be_sliced] == 0:
            self._scene.gcode_dict[build_plate_to_be_sliced] = []   # type: ignore
            # We need to ignore the type because we created this attribute above.
            Logger.log("d", "Build plate %s has no objects to be sliced, skipping", build_plate_to_be_sliced)
            if self._build_plates_to_be_sliced:
                self.slice()
            return
        self._stored_optimized_layer_data[build_plate_to_be_sliced] = []
        if application.getPrintInformation() and build_plate_to_be_sliced == active_build_plate:
            application.getPrintInformation().setToZeroPrintInformation(build_plate_to_be_sliced)

        if self._process is None:  # type: ignore
            self._createSocket()
        self.stopSlicing()
        self._engine_is_fresh = False  # Yes we're going to use the engine

        self.processingProgress.emit(0.0)
        self.backendStateChange.emit(BackendState.NotStarted)

        self._scene.gcode_dict[build_plate_to_be_sliced] = []  # type: ignore #[] indexed by build plate number
        self._slicing = True
        self.slicingStarted.emit()

        self.determineAutoSlicing()  # Switch timer on or off if appropriate

        slice_message = self._socket.createMessage("cura.proto.Slice")
        self._start_slice_job = StartSliceJob(slice_message)
        self._start_slice_job_build_plate = build_plate_to_be_sliced
        self._start_slice_job.setBuildPlate(self._start_slice_job_build_plate)
        self._start_slice_job.start()
        self._start_slice_job.finished.connect(self._onStartSliceCompleted)

    def _terminate(self) -> None:
        """Terminate the engine process.

        Start the engine process by calling _createSocket()
        """
        self._slicing = False
        self._stored_layer_data = []
        if self._start_slice_job_build_plate in self._stored_optimized_layer_data:
            del self._stored_optimized_layer_data[self._start_slice_job_build_plate]
        if self._start_slice_job is not None:
            self._start_slice_job.cancel()

        self.stopPlugins()

        self.slicingCancelled.emit()
        self.processingProgress.emit(0)
        Logger.log("d", "Attempting to kill the engine process")

        if CuraApplication.getInstance().getUseExternalBackend():
            return

        if self._process is not None:  # type: ignore
            Logger.log("d", "Killing engine process")
            try:
                self._process.terminate()  # type: ignore
                Logger.log("d", "Engine process is killed. Received return code %s", self._process.wait())  # type: ignore
                self._process = None  # type: ignore

            except Exception as e:
                # Terminating a process that is already terminating causes an exception, silently ignore this.
                Logger.log("d", "Exception occurred while trying to kill the engine %s", str(e))

    def _onStartSliceCompleted(self, job: StartSliceJob) -> None:
        """Event handler to call when the job to initiate the slicing process is
        completed.

        When the start slice job is successfully completed, it will be happily
        slicing. This function handles any errors that may occur during the
        bootstrapping of a slice job.

        :param job: The start slice job that was just finished.
        """
        if self._error_message:
            self._error_message.hide()

        # Note that cancelled slice jobs can still call this method.
        if self._start_slice_job is job:
            self._start_slice_job = None

        if job.isCancelled() or job.getError() or job.getResult() == StartJobResult.Error:
            self.setState(BackendState.Error)
            self.backendError.emit(job)
            return

        application = CuraApplication.getInstance()
        if job.getResult() == StartJobResult.MaterialIncompatible:
            if application.platformActivity:
                self._error_message = Message(catalog.i18nc("@info:status",
                                                            "Unable to slice with the current material as it is incompatible with the selected machine or configuration."),
                                              title = catalog.i18nc("@info:title", "Unable to slice"),
                                              message_type = Message.MessageType.WARNING)
                self._error_message.show()
                self.setState(BackendState.Error)
                self.backendError.emit(job)
            else:
                self.setState(BackendState.NotStarted)
            return

        if job.getResult() == StartJobResult.SettingError:
            if application.platformActivity:
                if not self._global_container_stack:
                    Logger.log("w", "Global container stack not assigned to CuraEngineBackend!")
                    return
                extruders = ExtruderManager.getInstance().getActiveExtruderStacks()
                error_keys: List[str] = []
                for extruder in extruders:
                    error_keys.extend(extruder.getErrorKeys())
                if not extruders:
                    error_keys = self._global_container_stack.getErrorKeys()
                error_labels = set()
                for key in error_keys:
                    for stack in [self._global_container_stack] + extruders:  #Search all container stacks for the definition of this setting. Some are only in an extruder stack.
                        definitions = cast(DefinitionContainerInterface, stack.getBottom()).findDefinitions(key = key)
                        if definitions:
                            break #Found it! No need to continue search.
                    else: #No stack has a definition for this setting.
                        Logger.log("w", "When checking settings for errors, unable to find definition for key: {key}".format(key = key))
                        continue
                    error_labels.add(definitions[0].label)

                self._error_message = Message(catalog.i18nc("@info:status",
                                                            "Unable to slice with the current settings. The following settings have errors: {0}").format(", ".join(error_labels)),
                                              title = catalog.i18nc("@info:title", "Unable to slice"),
                                              message_type = Message.MessageType.WARNING)
                Logger.warning(f"Unable to slice with the current settings. The following settings have errors: {', '.join(error_labels)}")
                self._error_message.show()
                self.setState(BackendState.Error)
                self.backendError.emit(job)
            else:
                self.setState(BackendState.NotStarted)
            return

        elif job.getResult() == StartJobResult.ObjectSettingError:
            errors = {}
            for node in DepthFirstIterator(application.getController().getScene().getRoot()):
                stack = node.callDecoration("getStack")
                if not stack:
                    continue
                for key in stack.getErrorKeys():
                    if not self._global_container_stack:
                        Logger.log("e", "CuraEngineBackend does not have global_container_stack assigned.")
                        continue
                    definition = cast(DefinitionContainerInterface, self._global_container_stack.getBottom()).findDefinitions(key = key)
                    if not definition:
                        Logger.log("e", "When checking settings for errors, unable to find definition for key {key} in per-object stack.".format(key = key))
                        continue
                    errors[key] = definition[0].label
            self._error_message = Message(catalog.i18nc("@info:status",
                                                        "Unable to slice due to some per-model settings. The following settings have errors on one or more models: {error_labels}").format(error_labels = ", ".join(errors.values())),
                                          title = catalog.i18nc("@info:title", "Unable to slice"),
                                          message_type = Message.MessageType.WARNING)
            Logger.warning(f"Unable to slice due to per-object settings. The following settings have errors on one or more models: {', '.join(errors.values())}")
            self._error_message.show()
            self.setState(BackendState.Error)
            self.backendError.emit(job)
            return

        if job.getResult() == StartJobResult.BuildPlateError:
            if application.platformActivity:
                self._error_message = Message(catalog.i18nc("@info:status",
                                                            "Unable to slice because the prime tower or prime position(s) are invalid."),
                                              title = catalog.i18nc("@info:title", "Unable to slice"),
                                              message_type = Message.MessageType.WARNING)
                self._error_message.show()
                self.setState(BackendState.Error)
                self.backendError.emit(job)
                return
            else:
                self.setState(BackendState.NotStarted)

        if job.getResult() == StartJobResult.ObjectsWithDisabledExtruder:
            self._error_message = Message(catalog.i18nc("@info:status",
                                                        "Unable to slice because there are objects associated with disabled Extruder %s.") % job.getMessage(),
                                          title = catalog.i18nc("@info:title", "Unable to slice"),
                                          message_type = Message.MessageType.WARNING)
            self._error_message.show()
            self.setState(BackendState.Error)
            self.backendError.emit(job)
            return

        if job.getResult() == StartJobResult.NothingToSlice:
            if application.platformActivity:
                self._error_message = Message(catalog.i18nc("@info:status", "Please review settings and check if your models:"
                                                                            "\n- Fit within the build volume"
                                                                            "\n- Are assigned to an enabled extruder"
                                                                            "\n- Are not all set as modifier meshes"),
                                              title = catalog.i18nc("@info:title", "Unable to slice"),
                                              message_type = Message.MessageType.WARNING)
                self._error_message.show()
                self.setState(BackendState.Error)
                self.backendError.emit(job)
            else:
                self.setState(BackendState.NotStarted)
            self._invokeSlice()
            return

        # Preparation completed, send it to the backend.
        self._socket.sendMessage(job.getSliceMessage())

        # Notify the user that it's now up to the backend to do its job
        self.setState(BackendState.Processing)

        # Handle time reporting.
        self._time_send_message = time()
        if self._time_start_process:
            Logger.log("d", "Sending slice message took %s seconds", self._time_send_message - self._time_start_process)

    def determineAutoSlicing(self) -> bool:
        """Determine enable or disable auto slicing. Return True for enable timer and False otherwise.

        It disables when:
            - preference auto slice is off
            - decorator isBlockSlicing is found (used in g-code reader)
        """
        enable_timer = True
        self._is_disabled = False

        if not CuraApplication.getInstance().getPreferences().getValue("general/auto_slice"):
            enable_timer = False
        for node in DepthFirstIterator(self._scene.getRoot()):
            if node.callDecoration("isBlockSlicing"):
                enable_timer = False
                self.setState(BackendState.Disabled)
                self._is_disabled = True
            gcode_list = node.callDecoration("getGCodeList")
            if gcode_list is not None:
                self._scene.gcode_dict[node.callDecoration("getBuildPlateNumber")] = gcode_list  # type: ignore
                # We need to ignore type because we generate this attribute dynamically.

        if self._use_timer == enable_timer:
            return self._use_timer
        if enable_timer:
            self.setState(BackendState.NotStarted)
            self.enableTimer()
            return True
        else:
            self.disableTimer()
            return False

    def _numObjectsPerBuildPlate(self) -> Dict[int, int]:
        """Return a dict with number of objects per build plate"""

        num_objects: Dict[int, int] = defaultdict(int)
        for node in DepthFirstIterator(self._scene.getRoot()):
            # Only count sliceable objects
            if node.callDecoration("isSliceable"):
                build_plate_number = node.callDecoration("getBuildPlateNumber")
                if build_plate_number is not None:
                    num_objects[build_plate_number] += 1
        return num_objects

    def _onSceneChanged(self, source: SceneNode) -> None:
        """Listener for when the scene has changed.

        This should start a slice if the scene is now ready to slice.

        :param source: The scene node that was changed.
        """

        if not source.callDecoration("isSliceable") and source != self._scene.getRoot():
            return

        # This case checks if the source node is a node that contains GCode. In this case the
        # current layer data is removed so the previous data is not rendered - CURA-4821
        if source.callDecoration("isBlockSlicing") and source.callDecoration("getLayerData"):
            self._stored_optimized_layer_data = {}

        build_plate_changed = set()
        source_build_plate_number = source.callDecoration("getBuildPlateNumber")
        if source == self._scene.getRoot():
            # we got the root node
            num_objects = self._numObjectsPerBuildPlate()
            for build_plate_number in list(self._last_num_objects.keys()) + list(num_objects.keys()):
                if build_plate_number not in self._last_num_objects or num_objects[build_plate_number] != self._last_num_objects[build_plate_number]:
                    self._last_num_objects[build_plate_number] = num_objects[build_plate_number]
                    build_plate_changed.add(build_plate_number)
        else:
            # we got a single scenenode
            if not source.callDecoration("isGroup"):
                mesh_data = source.getMeshData()
                if mesh_data is None or mesh_data.getVertices() is None:
                    return

            # There are some SceneNodes that do not have any build plate associated, then do not add to the list.
            if source_build_plate_number is not None:
                build_plate_changed.add(source_build_plate_number)

        if not build_plate_changed:
            return

        if self._tool_active:
            # do it later, each source only has to be done once
            if source not in self._postponed_scene_change_sources:
                self._postponed_scene_change_sources.append(source)
            return

        self.stopSlicing()
        for build_plate_number in build_plate_changed:
            if build_plate_number not in self._build_plates_to_be_sliced:
                self._build_plates_to_be_sliced.append(build_plate_number)
            self.printDurationMessage.emit(source_build_plate_number, {}, [])
        self.processingProgress.emit(0.0)
        self._clearLayerData(build_plate_changed)

        self._invokeSlice()

    def _onSocketError(self, error: Arcus.Error) -> None:
        """Called when an error occurs in the socket connection towards the engine.

        :param error: The exception that occurred.
        """

        if CuraApplication.getInstance().isShuttingDown():
            return

        super()._onSocketError(error)
        if error.getErrorCode() == Arcus.ErrorCode.Debug:
            return

        self._terminate()
        self._createSocket()

        if error.getErrorCode() not in [Arcus.ErrorCode.BindFailedError,
                                        Arcus.ErrorCode.ConnectionResetError,
                                        Arcus.ErrorCode.Debug]:
            Logger.log("w", "A socket error caused the connection to be reset")

        # _terminate()' function sets the job status to 'cancel', after reconnecting to another Port the job status
        # needs to be updated. Otherwise, backendState is "Unable To Slice"
        if error.getErrorCode() == Arcus.ErrorCode.BindFailedError and self._start_slice_job is not None:
            self._start_slice_job.setIsCancelled(False)

    # Check if there's any slicable object in the scene.
    def hasSlicableObject(self) -> bool:
        has_slicable = False
        for node in DepthFirstIterator(self._scene.getRoot()):
            if node.callDecoration("isSliceable"):
                has_slicable = True
                break
        return has_slicable

    def _clearLayerData(self, build_plate_numbers: Set = None) -> None:
        """Remove old layer data (if any)"""

        # Clear out any old gcode
        self._scene.gcode_dict = {}  # type: ignore

        for node in DepthFirstIterator(self._scene.getRoot()):
            if node.callDecoration("getLayerData"):
                if not build_plate_numbers or node.callDecoration("getBuildPlateNumber") in build_plate_numbers:
                    # We can assume that all nodes have a parent as we're looping through the scene and filter out root
                    cast(SceneNode, node.getParent()).removeChild(node)

    def markSliceAll(self) -> None:
        for build_plate_number in range(CuraApplication.getInstance().getMultiBuildPlateModel().maxBuildPlate + 1):
            if build_plate_number not in self._build_plates_to_be_sliced:
                self._build_plates_to_be_sliced.append(build_plate_number)

    def needsSlicing(self) -> None:
        """Convenient function: mark everything to slice, emit state and clear layer data"""

        # CURA-6604: If there's no slicable object, do not (try to) trigger slice, which will clear all the current
        # gcode. This can break Gcode file loading if it tries to remove it afterwards.
        if not self.hasSlicableObject():
            return
        self.determineAutoSlicing()
        self.stopSlicing()
        self.markSliceAll()
        self.processingProgress.emit(0.0)
        if not self._use_timer:
            # With manually having to slice, we want to clear the old invalid layer data.
            self._clearLayerData()

    def _onSettingChanged(self, instance: SettingInstance, property: str) -> None:
        """A setting has changed, so check if we must reslice.

        :param instance: The setting instance that has changed.
        :param property: The property of the setting instance that has changed.
        """
        if property == "value":  # Only re-slice if the value has changed.
            self.needsSlicing()
            self._onChanged()

        elif property == "validationState":
            if self._use_timer:
                self._change_timer.stop()

    def _onStackErrorCheckFinished(self) -> None:
        self.determineAutoSlicing()
        if self._is_disabled:
            return

        if not self._slicing and self._build_plates_to_be_sliced:
            self.needsSlicing()
            self._onChanged()

    def _onLayerMessage(self, message: Arcus.PythonMessage) -> None:
        """Called when a sliced layer data message is received from the engine.

        :param message: The protobuf message containing sliced layer data.
        """

        self._stored_layer_data.append(message)

    def _onOptimizedLayerMessage(self, message: Arcus.PythonMessage) -> None:
        """Called when an optimized sliced layer data message is received from the engine.

        :param message: The protobuf message containing sliced layer data.
        """

        if self._start_slice_job_build_plate is not None:
            if self._start_slice_job_build_plate not in self._stored_optimized_layer_data:
                self._stored_optimized_layer_data[self._start_slice_job_build_plate] = []
            self._stored_optimized_layer_data[self._start_slice_job_build_plate].append(message)

    def _onProgressMessage(self, message: Arcus.PythonMessage) -> None:
        """Called when a progress message is received from the engine.

        :param message: The protobuf message containing the slicing progress.
        """

        self.processingProgress.emit(message.amount)
        self.setState(BackendState.Processing)

    def _invokeSlice(self) -> None:
        if self._use_timer:
            # if the error check is scheduled, wait for the error check finish signal to trigger auto-slice,
            # otherwise business as usual
            if self._machine_error_checker is None:
                self._change_timer.stop()
                return

            if self._machine_error_checker.needToWaitForResult:
                self._change_timer.stop()
            else:
                self._change_timer.start()

    def _onSlicingFinishedMessage(self, message: Arcus.PythonMessage) -> None:
        """Called when the engine sends a message that slicing is finished.

        :param message: The protobuf message signalling that slicing is finished.
        """

        self.stopPlugins()

        self.setState(BackendState.Done)
        self.processingProgress.emit(1.0)
        self._time_end_slice = time()

        try:
            gcode_list = self._scene.gcode_dict[self._start_slice_job_build_plate] #type: ignore
            # We need to ignore the type because it was generated dynamically.
        except KeyError:
            # Can occur if the g-code has been cleared while a slice message is still arriving from the other end.
            gcode_list = []
        application = CuraApplication.getInstance()
        for index, line in enumerate(gcode_list):
            replaced = line.replace("{print_time}", str(application.getPrintInformation().currentPrintTime.getDisplayString(DurationFormat.Format.ISO8601)))
            replaced = replaced.replace("{filament_amount}", str(application.getPrintInformation().materialLengths))
            replaced = replaced.replace("{filament_weight}", str(application.getPrintInformation().materialWeights))
            replaced = replaced.replace("{filament_cost}", str(application.getPrintInformation().materialCosts))
            replaced = replaced.replace("{jobname}", str(application.getPrintInformation().jobName))

            gcode_list[index] = replaced

        self._slicing = False
        if self._time_start_process:
            Logger.log("d", "Slicing took %s seconds", time() - self._time_start_process)
        Logger.log("d", "Number of models per buildplate: %s", dict(self._numObjectsPerBuildPlate()))

        # See if we need to process the sliced layers job.
        active_build_plate = application.getMultiBuildPlateModel().activeBuildPlate
        if (
            self._layer_view_active and
            (self._process_layers_job is None or not self._process_layers_job.isRunning()) and
            active_build_plate == self._start_slice_job_build_plate and
            active_build_plate not in self._build_plates_to_be_sliced):

            self._startProcessSlicedLayersJob(active_build_plate)
        # self._onActiveViewChanged()
        self._start_slice_job_build_plate = None

        Logger.log("d", "See if there is more to slice...")
        # Somehow this results in an Arcus Error
        # self.slice()
        # Call slice again using the timer, allowing the backend to restart
        if self._build_plates_to_be_sliced:
            self.enableTimer()  # manually enable timer to be able to invoke slice, also when in manual slice mode
            self._invokeSlice()

    def _onGCodeLayerMessage(self, message: Arcus.PythonMessage) -> None:
        """Called when a g-code message is received from the engine.

        :param message: The protobuf message containing g-code, encoded as UTF-8.
        """

        try:
            self._scene.gcode_dict[self._start_slice_job_build_plate].append(message.data.decode("utf-8", "replace")) #type: ignore #Because we generate this attribute dynamically.
        except KeyError:
            # Can occur if the g-code has been cleared while a slice message is still arriving from the other end.
            pass  # Throw the message away.

    def _onGCodePrefixMessage(self, message: Arcus.PythonMessage) -> None:
        """Called when a g-code prefix message is received from the engine.

        :param message: The protobuf message containing the g-code prefix,
        encoded as UTF-8.
        """

        try:
            self._scene.gcode_dict[self._start_slice_job_build_plate].insert(0, message.data.decode("utf-8", "replace")) #type: ignore #Because we generate this attribute dynamically.
        except KeyError:
            # Can occur if the g-code has been cleared while a slice message is still arriving from the other end.
            pass  # Throw the message away.

    def _onSliceUUIDMessage(self, message: Arcus.PythonMessage) -> None:
        application = CuraApplication.getInstance()
        application.getPrintInformation().slice_uuid = message.slice_uuid

    def _createSocket(self, protocol_file: str = None) -> None:
        """Creates a new socket connection."""

        if not protocol_file:
            if not self.getPluginId():
                Logger.error("Can't create socket before CuraEngineBackend plug-in is registered.")
                return
            plugin_path = PluginRegistry.getInstance().getPluginPath(self.getPluginId())
            if not plugin_path:
                Logger.error("Could not get plugin path!", self.getPluginId())
                return
            protocol_file = os.path.abspath(os.path.join(plugin_path, "Cura.proto"))
        super()._createSocket(protocol_file)
        self._engine_is_fresh = True

    def _onChanged(self, *args: Any, **kwargs: Any) -> None:
        """Called when anything has changed to the stuff that needs to be sliced.

        This indicates that we should probably re-slice soon.
        """

        self.needsSlicing()
        if self._use_timer:
            # if the error check is scheduled, wait for the error check finish signal to trigger auto-slice,
            # otherwise business as usual
            if self._machine_error_checker is None:
                self._change_timer.stop()
                return

            if self._machine_error_checker.needToWaitForResult:
                self._change_timer.stop()
            else:
                self._change_timer.start()

    def _onPrintTimeMaterialEstimates(self, message: Arcus.PythonMessage) -> None:
        """Called when a print time message is received from the engine.

        :param message: The protobuf message containing the print time per feature and
            material amount per extruder
        """

        material_amounts = []
        for index in range(message.repeatedMessageCount("materialEstimates")):
            material_amounts.append(message.getRepeatedMessage("materialEstimates", index).material_amount)

        times = self._parseMessagePrintTimes(message)
        self.printDurationMessage.emit(self._start_slice_job_build_plate, times, material_amounts)

    def _parseMessagePrintTimes(self, message: Arcus.PythonMessage) -> Dict[str, float]:
        """Called for parsing message to retrieve estimated time per feature

        :param message: The protobuf message containing the print time per feature
        """

        result = {
            "inset_0": message.time_inset_0,
            "inset_x": message.time_inset_x,
            "skin": message.time_skin,
            "infill": message.time_infill,
            "support_infill": message.time_support_infill,
            "support_interface": message.time_support_interface,
            "support": message.time_support,
            "skirt": message.time_skirt,
            "prime_tower": message.time_prime_tower,
            "travel": message.time_travel,
            "retract": message.time_retract,
            "none": message.time_none
        }
        return result

    def _onBackendConnected(self) -> None:
        """Called when the back-end connects to the front-end."""

        if self._restart:
            self._restart = False
            self._onChanged()

    def _onToolOperationStarted(self, tool: Tool) -> None:
        """Called when the user starts using some tool.

        When the user starts using a tool, we should pause slicing to prevent
        continuously slicing while the user is dragging some tool handle.

        :param tool: The tool that the user is using.
        """

        self._tool_active = True  # Do not react on scene change
        self.disableTimer()
        # Restart engine as soon as possible, we know we want to slice afterwards
        if not self._engine_is_fresh:
            self._terminate()
            self._createSocket()

    def _onToolOperationStopped(self, tool: Tool) -> None:
        """Called when the user stops using some tool.

        This indicates that we can safely start slicing again.

        :param tool: The tool that the user was using.
        """

        self._tool_active = False  # React on scene change again
        self.determineAutoSlicing()  # Switch timer on if appropriate
        # Process all the postponed scene changes
        while self._postponed_scene_change_sources:
            source = self._postponed_scene_change_sources.pop(0)
            self._onSceneChanged(source)

    def _startProcessSlicedLayersJob(self, build_plate_number: int) -> None:
        self._process_layers_job = ProcessSlicedLayersJob(self._stored_optimized_layer_data[build_plate_number])
        self._process_layers_job.setBuildPlate(build_plate_number)
        self._process_layers_job.finished.connect(self._onProcessLayersFinished)
        self._process_layers_job.start()

    def _onActiveViewChanged(self) -> None:
        """Called when the user changes the active view mode."""

        view = CuraApplication.getInstance().getController().getActiveView()
        if view:
            active_build_plate = CuraApplication.getInstance().getMultiBuildPlateModel().activeBuildPlate
            if view.getPluginId() == "SimulationView":
                # If switching to layer view, we should process the layers if that hasn't been done yet.
                self._layer_view_active = True
                # There is data and we're not slicing at the moment
                # if we are slicing, there is no need to re-calculate the data as it will be invalid in a moment.
                # TODO: what build plate I am slicing
                if (active_build_plate in self._stored_optimized_layer_data and
                    not self._slicing and
                    not self._process_layers_job and
                    active_build_plate not in self._build_plates_to_be_sliced):

                    self._startProcessSlicedLayersJob(active_build_plate)
            else:
                self._layer_view_active = False

    def _onBackendQuit(self) -> None:
        """Called when the back-end self-terminates.

        We should reset our state and start listening for new connections.
        """
        if not self._restart:
            if self._process: # type: ignore
                return_code = self._process.wait()
                if return_code != 0:
                    Logger.log("e", f"Backend exited abnormally with return code {return_code}!")
                    self._slicing_error_message.show()
                    self.setState(BackendState.Error)
                    self.stopSlicing()
                else:
                    Logger.log("d", "Backend finished slicing. Resetting process and socket.")
                    self.stopPlugins()
                self._process = None # type: ignore

    def _reportBackendError(self, _message_id: str, _action_id: str) -> None:
        """
        Triggered when the user wants to report an error in the back-end.
        """
        QDesktopServices.openUrl(QUrl("https://github.com/Ultimaker/Cura/issues/new/choose"))

    def _onGlobalStackChanged(self) -> None:
        """Called when the global container stack changes"""

        if self._global_container_stack:
            self._global_container_stack.propertyChanged.disconnect(self._onSettingChanged)
            self._global_container_stack.containersChanged.disconnect(self._onChanged)

            for extruder in self._global_container_stack.extruderList:
                extruder.propertyChanged.disconnect(self._onSettingChanged)
                extruder.containersChanged.disconnect(self._onChanged)

        self._global_container_stack = CuraApplication.getInstance().getMachineManager().activeMachine

        if self._global_container_stack:
            # Note: Only starts slicing when the value changed.
            self._global_container_stack.propertyChanged.connect(self._onSettingChanged)
            self._global_container_stack.containersChanged.connect(self._onChanged)

            for extruder in self._global_container_stack.extruderList:
                extruder.propertyChanged.connect(self._onSettingChanged)
                extruder.containersChanged.connect(self._onChanged)
            self._onChanged()

    def _onProcessLayersFinished(self, job: ProcessSlicedLayersJob) -> None:
        if job.getBuildPlate() in self._stored_optimized_layer_data:
            del self._stored_optimized_layer_data[job.getBuildPlate()]
        else:
            Logger.log("w", "The optimized layer data was already deleted for buildplate %s", job.getBuildPlate())
        self._process_layers_job = None
        Logger.log("d", "See if there is more to slice(2)...")
        self._invokeSlice()

    def enableTimer(self) -> None:
        """Connect slice function to timer."""

        if not self._use_timer:
            self._change_timer.timeout.connect(self.slice)
            self._use_timer = True

    def disableTimer(self) -> None:
        """Disconnect slice function from timer.

        This means that slicing will not be triggered automatically
        """
        if self._use_timer:
            self._use_timer = False
            self._change_timer.timeout.disconnect(self.slice)

    def _onPreferencesChanged(self, preference: str) -> None:
        if preference != "general/auto_slice":
            return
        auto_slice = self.determineAutoSlicing()
        if auto_slice:
            self._change_timer.start()

    def tickle(self) -> None:
        """Tickle the backend so in case of auto slicing, it starts the timer."""

        if self._use_timer:
            self._change_timer.start()

    def _extruderChanged(self) -> None:
        if not self._multi_build_plate_model:
            Logger.log("w", "CuraEngineBackend does not have multi_build_plate_model assigned!")
            return
        for build_plate_number in range(self._multi_build_plate_model.maxBuildPlate + 1):
            if build_plate_number not in self._build_plates_to_be_sliced:
                self._build_plates_to_be_sliced.append(build_plate_number)
        self._invokeSlice()