Bonsai: implement Add Derived Unit in the units UI (#2554)

The IfcDerivedUnit branch of the Add Unit panel was a literal
`pass  # TODO`, so derived units (velocity, thermal conductance, etc.)
could not be created from the UI even though
ifcopenshell.api.unit.add_derived_unit already supports them.

Add the missing flow, mirroring the module's existing add-unit patterns:
- UnitsData caches derived_unit_types (IfcDerivedUnitEnum from the schema,
  USERDEFINED excluded since it additionally requires a UserDefinedType)
  and named_units (the project's existing IfcNamedUnits, labelled with
  their full unit name and unit type).
- A DerivedUnitElement property group (unit enum + integer exponent) and a
  derived_unit_elements collection on BIMUnitProperties.
- Operators to add/remove element rows and AddDerivedUnit, which validates
  (at least one element per the schema's SET [1:?], no unselected unit, no
  duplicate units), builds the {named unit: exponent} mapping, and calls
  core.add_derived_unit -> unit.add_derived_unit; rows are cleared on
  success.
- The panel draws the type dropdown, per-row unit/exponent/remove controls,
  and an Add Element button.

Verified live in headless Blender with the real operators: metre^1 x
second^-1 with LINEARVELOCITYUNIT creates
IfcDerivedUnit((IfcDerivedUnitElement(metre,1), IfcDerivedUnitElement(
second,-1)), .LINEARVELOCITYUNIT.), assignable via bim.assign_unit; the
empty-rows and duplicate-unit error paths report cleanly; existing SI and
monetary unit flows are unaffected. Core test_unit.py: 19 passed
(includes a new TestAddDerivedUnit).

Maintainer context: Moult invited an implementation on the issue
("Indeed I couldn't think of a nice UI for it, go for it!").

Generated with the assistance of an AI coding tool.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/bonsai/bonsai/bim/module/unit/__init__.py

@@ -23,6 +23,8 @@ from . import operator, prop, ui
 classes = (
     operator.AddContextDependentUnit,
     operator.AddConversionBasedUnit,
+    operator.AddDerivedUnit,
+    operator.AddDerivedUnitElement,
     operator.AddMonetaryUnit,
     operator.AddSIUnit,
     operator.AssignSceneUnits,
@@ -32,9 +34,11 @@ classes = (
     operator.EditUnit,
     operator.EnableEditingUnit,
     operator.LoadUnits,
+    operator.RemoveDerivedUnitElement,
     operator.RemoveUnit,
     operator.UnassignUnit,
     prop.Unit,
+    prop.DerivedUnitElement,
     prop.BIMUnitProperties,
     ui.BIM_PT_units,
     ui.BIM_UL_units,

src/bonsai/bonsai/bim/module/unit/data.py

@@ -41,6 +41,8 @@ class UnitsData:
             "unit_classes": cls.unit_classes(),
             "named_unit_types": cls.named_unit_types(),
             "conversion_unit_types": cls.conversion_unit_types(),
+            "derived_unit_types": cls.derived_unit_types(),
+            "named_units": cls.named_units(),
             "total_units": cls.get_total_units(),
         }
         cls.is_loaded = True
@@ -88,6 +90,21 @@ class UnitsData:
     def conversion_unit_types(cls):
         return [(u, u, "") for u in ifcopenshell.util.unit.si_conversions.keys()]
 
+    @classmethod
+    def derived_unit_types(cls):
+        assert (entity := tool.Ifc.schema().declaration_by_name("IfcDerivedUnit").as_entity())
+        values = ifcopenshell.util.attribute.get_enum_items(entity.all_attributes()[1])
+        # USERDEFINED is skipped as it additionally requires a UserDefinedType.
+        return [(c, c, "") for c in sorted(values) if c != "USERDEFINED"]
+
+    @classmethod
+    def named_units(cls):
+        results = []
+        for unit in sorted(tool.Ifc.get().by_type("IfcNamedUnit"), key=lambda u: u.id()):
+            name = ifcopenshell.util.unit.get_full_unit_name(unit)
+            results.append((str(unit.id()), f"{name} ({unit.UnitType})", ""))
+        return results
+
     @classmethod
     def get_total_units(cls):
         ifc = tool.Ifc.get()

src/bonsai/bonsai/bim/module/unit/operator.py

@@ -132,6 +132,56 @@ class AddContextDependentUnit(bpy.types.Operator, tool.Ifc.Operator):
         core.add_context_dependent_unit(tool.Ifc, tool.Unit, unit_type=self.unit_type, name=self.name)
 
 
+class AddDerivedUnitElement(bpy.types.Operator):
+    bl_idname = "bim.add_derived_unit_element"
+    bl_label = "Add Derived Unit Element"
+    bl_options = {"REGISTER", "UNDO"}
+    bl_description = "Add a new named unit and exponent to the derived unit definition"
+
+    def execute(self, context) -> set["rna_enums.OperatorReturnItems"]:
+        props = tool.Unit.get_unit_props()
+        props.derived_unit_elements.add()
+        return {"FINISHED"}
+
+
+class RemoveDerivedUnitElement(bpy.types.Operator):
+    bl_idname = "bim.remove_derived_unit_element"
+    bl_label = "Remove Derived Unit Element"
+    bl_options = {"REGISTER", "UNDO"}
+    bl_description = "Remove this named unit and exponent from the derived unit definition"
+    index: bpy.props.IntProperty()
+
+    def execute(self, context) -> set["rna_enums.OperatorReturnItems"]:
+        props = tool.Unit.get_unit_props()
+        props.derived_unit_elements.remove(self.index)
+        return {"FINISHED"}
+
+
+class AddDerivedUnit(bpy.types.Operator, tool.Ifc.Operator):
+    bl_idname = "bim.add_derived_unit"
+    bl_label = "Add Derived Unit"
+    bl_options = {"REGISTER", "UNDO"}
+    bl_description = "Add a derived unit built from the defined named units and exponents"
+
+    def _execute(self, context):
+        props = tool.Unit.get_unit_props()
+        elements = {}
+        for element in props.derived_unit_elements:
+            if not element.unit:
+                self.report({"ERROR"}, "No named unit selected. Add a named unit (e.g. an SI unit) first.")
+                return {"CANCELLED"}
+            unit = tool.Ifc.get().by_id(int(element.unit))
+            if unit in elements:
+                self.report({"ERROR"}, f"Unit '{unit.Name}' is used in more than one element.")
+                return {"CANCELLED"}
+            elements[unit] = element.exponent
+        if not elements:
+            self.report({"ERROR"}, "A derived unit requires at least one element. Click 'Add Element' first.")
+            return {"CANCELLED"}
+        core.add_derived_unit(tool.Ifc, tool.Unit, unit_type=props.derived_unit_types, elements=elements)
+        props.derived_unit_elements.clear()
+
+
 class EnableEditingUnit(bpy.types.Operator):
     bl_idname = "bim.enable_editing_unit"
     bl_label = "Enable Editing Unit"

src/bonsai/bonsai/bim/module/unit/prop.py

@@ -50,6 +50,27 @@ def get_named_unit_types(self: "BIMUnitProperties", context: bpy.types.Context)
     return UnitsData.data["named_unit_types"]
 
 
+def get_derived_unit_types(self: "BIMUnitProperties", context: bpy.types.Context) -> list[tuple[str, str, str]]:
+    if not UnitsData.is_loaded:
+        UnitsData.load()
+    return UnitsData.data["derived_unit_types"]
+
+
+def get_named_units(self: "DerivedUnitElement", context: bpy.types.Context) -> list[tuple[str, str, str]]:
+    if not UnitsData.is_loaded:
+        UnitsData.load()
+    return UnitsData.data["named_units"]
+
+
+class DerivedUnitElement(PropertyGroup):
+    unit: EnumProperty(items=get_named_units, name="Unit")
+    exponent: IntProperty(name="Exponent", default=1)
+
+    if TYPE_CHECKING:
+        unit: str
+        exponent: int
+
+
 class Unit(PropertyGroup):
     unit_type: StringProperty(name="Unit Type")
     is_assigned: BoolProperty(name="Is Assigned")
@@ -71,6 +92,8 @@ class BIMUnitProperties(PropertyGroup):
     unit_classes: EnumProperty(items=get_unit_classes, name="Unit Classes")
     conversion_unit_types: EnumProperty(items=get_conversion_unit_types, name="Conversion Unit Types")
     named_unit_types: EnumProperty(items=get_named_unit_types, name="Named Unit Types")
+    derived_unit_types: EnumProperty(items=get_derived_unit_types, name="Derived Unit Types")
+    derived_unit_elements: CollectionProperty(name="Derived Unit Elements", type=DerivedUnitElement)
     unit_attributes: CollectionProperty(name="Unit Attributes", type=Attribute)
 
     if TYPE_CHECKING:
@@ -81,4 +104,6 @@ class BIMUnitProperties(PropertyGroup):
         unit_classes: str
         conversion_unit_types: str
         named_unit_types: str
+        derived_unit_types: str
+        derived_unit_elements: bpy.types.bpy_prop_collection_idprop[DerivedUnitElement]
         unit_attributes: bpy.types.bpy_prop_collection_idprop[Attribute]

src/bonsai/bonsai/bim/module/unit/ui.py

@@ -85,7 +85,16 @@ class BIM_PT_units(Panel):
             op = row.operator("bim.add_conversion_based_unit", text="", icon="ADD")
             op.name = self.props.conversion_unit_types
         elif self.props.unit_classes == "IfcDerivedUnit":
-            pass  # TODO
+            prop_with_search(row, self.props, "derived_unit_types", text="")
+            row.operator("bim.add_derived_unit", text="", icon="ADD")
+            box = self.layout.box()
+            for i, element in enumerate(self.props.derived_unit_elements):
+                row = box.row(align=True)
+                prop_with_search(row, element, "unit", text="")
+                row.prop(element, "exponent", text="")
+                row.operator("bim.remove_derived_unit_element", text="", icon="X").index = i
+            row = box.row()
+            row.operator("bim.add_derived_unit_element", text="Add Element", icon="ADD")
         elif self.props.unit_classes == "IfcSIUnit":
             prop_with_search(row, self.props, "named_unit_types", text="")
             op = row.operator("bim.add_si_unit", text="", icon="ADD")

src/bonsai/bonsai/core/unit.py

@@ -91,6 +91,17 @@ def add_conversion_based_unit(ifc: type[tool.Ifc], unit: type[tool.Unit], name:
     return result
 
 
+def add_derived_unit(
+    ifc: type[tool.Ifc],
+    unit: type[tool.Unit],
+    unit_type: str,
+    elements: dict[ifcopenshell.entity_instance, int],
+) -> ifcopenshell.entity_instance:
+    result = ifc.run("unit.add_derived_unit", unit_type=unit_type, userdefinedtype=None, attributes=elements)
+    unit.import_units()
+    return result
+
+
 def enable_editing_unit(unit_tool: type[tool.Unit], unit: ifcopenshell.entity_instance) -> None:
     unit_tool.set_active_unit(unit)
     unit_tool.import_unit_attributes(unit)

src/bonsai/test/core/test_unit.py

@@ -174,6 +174,15 @@ class TestAddConversionBasedUnit:
         assert subject.add_conversion_based_unit(ifc, unit, name="name") == "unit"
 
 
+class TestAddDerivedUnit:
+    def test_run(self, ifc, unit):
+        ifc.run(
+            "unit.add_derived_unit", unit_type="unit_type", userdefinedtype=None, attributes="elements"
+        ).should_be_called().will_return("unit")
+        unit.import_units().should_be_called()
+        assert subject.add_derived_unit(ifc, unit, unit_type="unit_type", elements="elements") == "unit"
+
+
 class TestEnableEditingUnit:
     def test_run(self, unit):
         unit.set_active_unit("unit").should_be_called()