uuids, what are they?

[lukors]

I'm looking at this example: https://github.com/bevyengine/bevy/blob/v0.6.1/examples/shader/shader_material.rs

At lines 47 through 51:

#[derive(Debug, Clone, TypeUuid)]
#[uuid = "4ee9c363-1124-4113-890e-199d81b00281"]
pub struct CustomMaterial {
    color: Color,
}

I haven't been able to find any documentation about this for Bevy, so I'm wondering:

1. What's a uuid?
2. How do I know which uuid I should write?
3. Why is a uuid needed there in that example?

[bjorn3]

1. A uuid is a universally unique id. See https://en.wikipedia.org/wiki/Universally_unique_identifier for more info.
2. You should generate a random version 4 uuid. For example using https://www.uuidgenerator.net/ or by searching uuid on duckduckgo.
3. Materials are a kind of asset. Every asset type needs a unique uuid as asset handles consist of either a path (not applicable for runtime generated assets like this one) or the combination of a uuid corresponding to the asset type and a random 64bit integer. See

   bevy/crates/bevy_asset/src/handle.rs

   Lines 53 to 55 in 9a7852d

   	pub fn random<T: Asset>() -> Self {
   	HandleId::Id(T::TYPE_UUID, rand::random())
   	}

[lukors]

Great explanation, thanks! :)

[Sorseg]

For anyone who, like me, still finds mentions of this in older documentation:
TypeUuid was replaced with TypePath in bevy 0.13