Add preference to get export filename from object

Add an addon preferences panel that contains a single preference to use
the active object's name as the initial filename instead of the blend
file's name.

The preference defaults to disabled, which maintains the existing
behaviour that matches default exporters - when first invoked the export
filepath is initialised to that of the blend file, with subsequent
exports remembering the previous filepath.

When the preference is enabled, the stem of the export filepath is set
to the active object's name each time the exporter is invoked - the rest
of the filepath behaves as before. Characters that are invalid in
Windows filenames are removed on all systems, with a placeholder being
used if the active object's name contains no valid characters.

Closes #2

.github/README.md

@@ -42,6 +42,20 @@ The exporter can be found at `File` > `Export` > `OMSI Mesh (.o3d)`. Like most
 exporters, it exports the current selection - hidden objects and non-mesh
 objects will be excluded.
 
+### Preferences
+
+Expanding the entry for the addon in the addons list will reveal not just the
+addon information, but also an addon preferences panel - this panel currently
+contains one preference:
+
+#### Get export filename from active object
+
+By default the exporter acts like default Blender exporters - when first invoked
+the export filepath is initialised to that of the blend file, with subsequent
+exports remembering the previous filepath. With this preference enabled the stem
+is set to the active object's name each time the exporter is invoked - the rest
+of the filepath behaves as before.
+
 ### Settings
 
 `Merge Materials`

io_export_o3d/__init__.py

@@ -24,10 +24,18 @@
 
 # <pep8-80 compliant>
 
+import re
 from pathlib import Path
 # PYTHON39: builtins.set supports []
 from typing import AbstractSet
-from bpy.types import Operator, Panel, Context, Event, TOPBAR_MT_file_export
+from bpy.types import (
+    Operator,
+    Panel,
+    AddonPreferences,
+    Context,
+    Event,
+    TOPBAR_MT_file_export,
+)
 from bpy.utils import register_class, unregister_class
 from bpy.props import BoolProperty, EnumProperty, StringProperty
 from .exporter import Exporter
@@ -127,16 +135,32 @@ def poll(cls, context: Context) -> bool:
         return bool(Exporter.filter_objects(context.selected_objects))
 
     def invoke(self, context: Context, _: Event) -> AbstractSet[str]:
-        # if a file has not yet been exported, this will be empty
-        if not self.filepath:
-            if context.blend_data.is_saved:
-                # if the file is saved we use the save location
-                path = Path(context.blend_data.filepath)
-            else:
-                # otherwise the file is untitled
-                path = Path("untitled")
-            # replace/add suffix and set filepath
-            self.filepath = str(path.with_suffix(self.filename_ext))
+        if self.filepath:
+            # use previous export path if this is not the first export
+            path = Path(self.filepath)
+        elif context.blend_data.filepath:
+            # use blend file path if this is the first export
+            path = Path(context.blend_data.filepath)
+        else:
+            # blend file is untitled if it hasn't been saved
+            path = Path("untitled.blend")
+        # ensure suffix
+        path = path.with_suffix(self.filename_ext)
+
+        if context.preferences.addons[__package__].preferences.use_object_name:
+            stem: str = context.active_object.name
+            # object names are unrestricted UTF-8 strings, which are
+            # unsafe for use as a filename - invalid characters are
+            # removed and leading/trailing spaces/periods are stripped
+            stem = re.sub(r"[\0-\x1F<>:\"\\/|?*]", "", stem).strip(". ")
+            # if nothing remains, a placeholder is used instead
+            if not stem:
+                stem = "mesh"
+            # change stem
+            path = path.with_name(stem + self.filename_ext)
+
+        # set filepath
+        self.filepath = str(path)
         # show the file selector
         context.window_manager.fileselect_add(self)
         # prevent the operator terminating while in the file selector
@@ -234,6 +258,23 @@ def draw(self, context: Context) -> None:
         col.prop(operator, 'material_output_node_target', text="Render Target")
 
 
+class OMSI_Exporter_Preferences(AddonPreferences):
+    bl_idname = __package__
+
+    use_object_name: BoolProperty(
+        name="Get export filename from active object",
+        description=(
+            "Use the active object's name as the export filename"
+            " instead of the blend file's name"
+        ),
+        default=False,
+    )
+
+    def draw(self, _: Context) -> None:
+        layout = self.layout
+        layout.prop(self, 'use_object_name')
+
+
 def menu_func_export(self, _: Context) -> None:
     self.layout.operator(ExportO3D.bl_idname, text="OMSI Mesh (.o3d)")
 
@@ -242,6 +283,7 @@ def register() -> None:
     register_class(ExportO3D)
     register_class(O3D_PT_export_basic)
     register_class(O3D_PT_export_advanced)
+    register_class(OMSI_Exporter_Preferences)
     TOPBAR_MT_file_export.append(menu_func_export)
 
 
@@ -250,6 +292,7 @@ def unregister() -> None:
     unregister_class(ExportO3D)
     unregister_class(O3D_PT_export_basic)
     unregister_class(O3D_PT_export_advanced)
+    unregister_class(OMSI_Exporter_Preferences)
 
 
 if __name__ == "__main__":