Custom resource pre serialization handler

[AFE-GmdG]

Describe the project you are working on

I'm working on a RTS maze / labyrinth like game right now. It uses a 3D environment composed of a grid of cells, visible from above.

Describe the problem or limitation you are having in your project

I'm using custom resources which are inherited from ArrayMesh which is used to generate reusable meshes with additional properties like Custom Vector data:

	@warning_ignore("int_as_enum_without_match")
	var format: Mesh.ArrayFormat = (
		Mesh.ARRAY_FORMAT_VERTEX
		| Mesh.ARRAY_FORMAT_NORMAL
		| Mesh.ARRAY_FORMAT_TEX_UV
		| Mesh.ARRAY_FORMAT_TEX_UV2
		| Mesh.ARRAY_FORMAT_INDEX
		| Mesh.ARRAY_FORMAT_CUSTOM0
		| (Mesh.ARRAY_CUSTOM_RGBA_FLOAT << Mesh.ARRAY_FORMAT_CUSTOM0_SHIFT)
	) as Mesh.ArrayFormat

	add_surface_from_arrays(Mesh.PRIMITIVE_TRIANGLES, surface, [], {}, format)
	if (surfaceMaterial != null):
		surface_set_material(0, surfaceMaterial)

This works fine as i'm reaching my target with this soultion. Hovever ay using a resouce like this the Scene file become way to big, because the resource serialization store to many data:

[gd_scene load_steps=4 format=3 uid="uid://b8b8dqorg2d76"]

[ext_resource type="Material" uid="uid://dugjl37doaws1" path="res://Materials/FloorDisplaceTest.tres" id="1_qi5u1"]
[ext_resource type="Script" path="res://Meshes/FloorGrid/FloorGridMesh.gd" id="2_b6gr0"]

[sub_resource type="ArrayMesh" id="ArrayMesh_qp3q4"]
_surfaces = [{
"aabb": AABB(-8.1, 0, -8.1, 18.9, 1e-05, 18.9),
"attribute_data": PackedByteArray(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 253, 255, 255, 255, 253, 255, 255, 255, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 128, 63, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 253, 255, 255, 255, 253, 255, 255, 255, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 128, 63, 0, 0, 0, 0, 0, 0, 0, 0, 253, 255, 255, 255, 253, 255, 255, 255, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 128, 63, 0, 0, 128, 63, 0, 0, 0, 0, 0, 0, 0, 0, 253, 255, 255, 255, 253, 255, 255, 255, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 
...

[node name="CustomDataTestScene" type="Node3D"]
metadata/_edit_lock_ = true

[node name="FloorGridMesh_LOD_0" type="MeshInstance3D" parent="."]
mesh = SubResource("ArrayMesh_qp3q4")
metadata/_edit_lock_ = true

Since the mesh data is calculated anyway, it should not saved in the tscn file. By using multiple of these calculated mesh resources the tscn file gets really big (over multiple MB)

Describe the feature / enhancement and how it helps to overcome the problem or limitation

If I'm be able to intercept the serialization process of the resource I could remove unnecessary data. This virtual method should be called by the scene serialization while saving the scene file. It should somehow provide a method to change the stored data, remove some of it or add additional data

If this enhancement will not be used often, can it be worked around with a few lines of script?

I could inherit from Mesh instead of ArrayMesh, since the additional stored data come from the ArrayMesh internal implementation of _get_property_list(). Using _get_property_list() in my custom resource let me not override settings from ArrayMesh so I could not prevent the storing of the unnecessary data this way.
However inheriting from Mesh means a lot of additional work for me because I have to deal with the RenderingServer directly and have to overwrite a ton of functions that ArrayMesh already implement just fine. Additionally this method may not be usable for other resource types which may not have a common virtual ancestor like Mesh is.

I hope this all makes sense and my explanations are sufficient.

[dalexeev]

There is the _validate_property() method, but it is not yet available in the scripting API.