Printer output device can now have a QML item

This contributes to CURA-3826. This will allow printer output devices to
add custom UI elements to the monitor tab in the future.

cura/PrinterOutputDevice.py

@@ -3,13 +3,18 @@
 
 from UM.i18n import i18nCatalog
 from UM.OutputDevice.OutputDevice import OutputDevice
-from PyQt5.QtCore import pyqtProperty, pyqtSignal, pyqtSlot, QObject, QTimer
+from PyQt5.QtCore import pyqtProperty, pyqtSignal, pyqtSlot, QObject, QTimer, pyqtSignal, QUrl
+from PyQt5.QtQml import QQmlComponent, QQmlContext
 from PyQt5.QtWidgets import QMessageBox
 from enum import IntEnum  # For the connection state tracking.
 
 from UM.Settings.ContainerRegistry import ContainerRegistry
 from UM.Logger import Logger
 from UM.Signal import signalemitter
+from UM.PluginRegistry import PluginRegistry
+from UM.Application import Application
+
+import os
 
 i18n_catalog = i18nCatalog("cura")
 
@@ -57,6 +62,11 @@ class PrinterOutputDevice(QObject, OutputDevice):
 
         self._camera_active = False
 
+        self._monitor_view_qml_path = ""
+        self._monitor_component = None
+        self._monitor_item = None
+        self._qml_context = None
+
     def requestWrite(self, nodes, file_name = None, filter_by_machine = False, file_handler = None):
         raise NotImplementedError("requestWrite needs to be implemented")
 
@@ -111,6 +121,32 @@ class PrinterOutputDevice(QObject, OutputDevice):
     # Signal to be emitted when some drastic change occurs in the remaining time (not when the time just passes on normally).
     preheatBedRemainingTimeChanged = pyqtSignal()
 
+    @pyqtProperty(QObject, constant=True)
+    def monitorItem(self):
+        # Note that we specifically only check if the monitor component is created.
+        # It could be that it failed to actually create the qml item! If we check if the item was created, it will try to
+        # create the item (and fail) every time.
+        if not self._monitor_component:
+            self._createMonitorViewFromQML()
+
+        return self._monitor_item
+
+    def _createMonitorViewFromQML(self):
+        path = QUrl.fromLocalFile(os.path.join(PluginRegistry.getInstance().getPluginPath(self.getPluginId()), self._monitor_view_qml_path))
+
+        # Because of garbage collection we need to keep this referenced by python.
+        self._monitor_component = QQmlComponent(Application.getInstance()._engine, path)
+
+        # Check if the context was already requested before (Printer output device might have multiple items in the future)
+        if self._qml_context is None:
+            self._qml_context = QQmlContext(Application.getInstance()._engine.rootContext())
+            self._qml_context.setContextProperty("OutputDevice", self)
+
+        self._monitor_item = self._monitor_component.create(self._qml_context)
+        if self._monitor_item is None:
+            Logger.log("e", "QQmlComponent status %s", self._monitor_component.status())
+            Logger.log("e", "QQmlComponent error string %s", self._monitor_component.errorString())
+
     @pyqtProperty(str, notify=printerTypeChanged)
     def printerType(self):
         return self._printer_type