Correct variants after all metadata is loaded

As explained in the block of documentation, we can't find the variants because there is no guarantee that the variants have been loaded by the time that the metadata of materials is deserialised and we don't know their IDs either (so no lazy loading). This registers a function to be called upon completely loading the metadata, which runs a query on the metadata to find the variant by its definition and name. A bit hacky but I see no better solution.

Contributes to issue CURA-4243.

plugins/XmlMaterialProfile/XmlMaterialProfile.py

@@ -17,7 +17,6 @@ import UM.Dictionary
 from UM.Settings.InstanceContainer import InstanceContainer
 from UM.Settings.ContainerRegistry import ContainerRegistry
 
-
 ##  Handles serializing and deserializing material containers from an XML file
 class XmlMaterialProfile(InstanceContainer):
     CurrentFdmMaterialVersion = "1.3"
@@ -761,8 +760,6 @@ class XmlMaterialProfile(InstanceContainer):
                     if not variant_containers:
                         # It is not really properly defined what "ID" is so also search for variants by name.
                         variant_containers = ContainerRegistry.getInstance().findInstanceContainersMetadata(definition = machine_id, name = hotend_id)
-                    if not variant_containers:
-                        continue
 
                     hotend_compatibility = machine_compatibility
                     for entry in hotend.iterfind("./um:setting", cls.__namespaces):
@@ -781,7 +778,11 @@ class XmlMaterialProfile(InstanceContainer):
                         new_hotend_material_metadata = {}
 
                     new_hotend_material_metadata.update(base_metadata)
+                    if variant_containers:
                         new_hotend_material_metadata["variant"] = variant_containers[0]["id"]
+                    else:
+                        new_hotend_material_metadata["variant"] = hotend_id
+                        _with_missing_variants.append(new_hotend_material_metadata)
                     new_hotend_material_metadata["compatible"] = hotend_compatibility
                     new_hotend_material_metadata["machine_manufacturer"] = machine_manufacturer
                     new_hotend_material_metadata["id"] = new_hotend_id
@@ -877,3 +878,21 @@ def _indent(elem, level = 0):
 # before the last }
 def _tag_without_namespace(element):
     return element.tag[element.tag.rfind("}") + 1:]
+
+#While loading XML profiles, some of these profiles don't know what variant
+#they belong to. We'd like to search by the machine ID and the variant's
+#name, but we don't know the variant's ID. Not all variants have been loaded
+#yet so we can't run a filter on the name and machine. The ID is unknown
+#so we can't lazily load the variant either. So we have to wait until all
+#the rest is loaded properly and then assign the correct variant to the
+#material files that were missing it.
+_with_missing_variants = []
+def _fillMissingVariants():
+    registry = ContainerRegistry.getInstance()
+    for variant_metadata in _with_missing_variants:
+        variants = registry.findContainersMetadata(definition = variant_metadata["definition"], name = variant_metadata["variant"])
+        if not variants:
+            Logger.log("w", "Could not find variant for variant-specific material {material_id}.".format(material_id = variant_metadata["id"]))
+            continue
+        variant_metadata["variant"] = variants[0]["id"]
+ContainerRegistry.allMetadataLoaded.connect(_fillMissingVariants)