UI for assigning controls to flow elements #3981

Example - https://imgur.com/a/CZQIoHp

src/blenderbim/test/bim/feature/system.feature

@@ -101,3 +101,28 @@ Scenario: Select system products
     And I press "bim.assign_system(system={system})"
     When I press "bim.select_system_products(system={system})"
     Then nothing happens
+
+Scenario: Assign flow controls to flow element_type
+    Given an empty IFC project
+    And I add an empty
+    And the object "Empty" is selected
+    And I set "scene.BIMRootProperties.ifc_product" to "IfcElement"
+    And I set "scene.BIMRootProperties.ifc_class" to "IfcActuator"
+    And I press "bim.assign_class"
+    And I add an empty
+    And the object "Empty" is selected
+    And I set "scene.BIMRootProperties.ifc_product" to "IfcElement"
+    And I set "scene.BIMRootProperties.ifc_class" to "IfcActuator"
+    And I press "bim.assign_class"
+    And I add an empty
+    And the object "Empty" is selected
+    And I set "scene.BIMRootProperties.ifc_product" to "IfcElement"
+    And I set "scene.BIMRootProperties.ifc_class" to "IfcFlowSegment"
+    And I press "bim.assign_class"
+    And the object "IfcActuator/Empty" is selected
+    And additionally the object "IfcActuator/Empty.001" is selected
+    And additionally the object "IfcFlowSegment/Empty" is selected
+    When I press "bim.assign_unassign_flow_control(assign=True)"
+    And the variable "assigned_controls" is "set(tool.System.get_flow_element_controls({ifc}.by_type('IfcFlowSegment')[0]))"
+    Then the variable "assigned_controls" is "set[{ifc}.by_type('IfcActuator')]"
+

src/blenderbim/test/tool/test_system.py

@@ -277,3 +277,30 @@ class TestSetActiveSystem(NewFile):
         system = ifcopenshell.api.run("system.add_system", ifc, ifc_class="IfcSystem")
         subject.set_active_edited_system(system)
         assert bpy.context.scene.BIMSystemProperties.edited_system_id == system.id()
+
+
+class TestFlowElementAndControls(NewFile):
+    def test_run(self):
+        ifc = ifcopenshell.file()
+        flow_element = ifc.createIfcFlowSegment()
+        flow_control = ifc.createIfcController()
+        flow_control1 = ifc.createIfcController()
+
+        assert len(subject.get_flow_element_controls(flow_element)) == 0
+        assert subject.get_flow_control_flow_element(flow_control) == None
+
+        ifcopenshell.api.run(
+            "system.assign_flow_control",
+            ifc,
+            related_flow_control=flow_control,
+            relating_flow_element=flow_element,
+        )
+        ifcopenshell.api.run(
+            "system.assign_flow_control",
+            ifc,
+            related_flow_control=flow_control1,
+            relating_flow_element=flow_element,
+        )
+        controls = subject.get_flow_element_controls(flow_element)
+        assert set(controls) == set((flow_control, flow_control1))
+        assert subject.get_flow_control_flow_element(flow_control) == flow_element