Implement warning message when you change material diameter

The undo functionality currently gives an error though. Investigating...

Contributes to issue CURA-2822.

cura/Settings/MaterialManager.py

@@ -0,0 +1,56 @@
+# Copyright (c) 2017 Ultimaker B.V.
+# Cura is released under the terms of the AGPLv3 or higher.
+
+from PyQt5.QtCore import QObject, pyqtSlot #To expose data to QML.
+
+from UM.Logger import Logger
+from UM.Message import Message #To create a warning message about material diameter.
+from UM.i18n import i18nCatalog #Translated strings.
+from UM.Settings.ContainerRegistry import ContainerRegistry #To find the material containers we need.
+
+catalog = i18nCatalog("cura")
+
+##  Handles material-related data, processing requests to change them and
+#   providing data for the GUI.
+#
+#   TODO: Move material-related managing over from the machine manager to here.
+class MaterialManager(QObject):
+    ##  Creates the global values for the material manager to use.
+    def __init__(self, parent = None):
+        super().__init__(parent)
+
+        #Material diameter changed warning message.
+        self._material_diameter_warning_message = Message(catalog.i18nc("@info:status Has a cancel button next to it.",
+            "The selected material diameter causes the material to become incompatible with the current printer."))
+        self._material_diameter_warning_message.addAction("Undo", catalog.i18nc("@action:button", "Undo"), None, catalog.i18nc("@action", "Undo changing the material diameter."))
+        self._material_diameter_warning_message.actionTriggered = self._materialWarningMessageAction
+
+    ##  Creates an instance of the MaterialManager.
+    #
+    #   This should only be called by PyQt to create the singleton instance of
+    #   this class.
+    @staticmethod
+    def createMaterialManager(engine = None, script_engine = None):
+        return MaterialManager()
+
+    @pyqtSlot(str, str)
+    def showMaterialWarningMessage(self, material_id, previous_diameter):
+        self._material_diameter_warning_message.previous_diameter = previous_diameter #Make sure that the undo button can properly undo the action.
+        self._material_diameter_warning_message.material_id = material_id
+        self._material_diameter_warning_message.show()
+
+    ##  Called when clicking "undo" on the warning dialogue for disappeared
+    #   materials.
+    #
+    #   This executes the undo action, restoring the material diameter.
+    #
+    #   \param button The identifier of the button that was pressed.
+    def _materialWarningMessageAction(self, button):
+        if button == "Undo":
+            container_registry = ContainerRegistry.getInstance()
+            matches = container_registry.findInstanceContainers(type = "material", id = self._material_diameter_warning_message.material_id)
+            if matches:
+                matches[0].setMetaDataEntry("diameter", self._material_diameter_warning_message.previous_diameter)
+            #No matches? Then the material has been deleted in the meanwhile.
+        else:
+            Logger.log("w", "Unknown button action for material diameter warning message: {action}".format(action = button))