Implementation of Technology Tree Using Traits or Lambdas

[coopergj]

My Bevy application includes a technology tree. Each technology uses the same code for determining when its requirements are met and tracking the research progress using a standard ECS approach. But, my difficulty arises when the technologies are researched because each one needs custom code for when that happens.

I admit I have done object-oriented programming for most of my development career so my thoughts immediately went to an object-oriented approach for this. I did then try to find an ECS approach but came up empty. And then I also ran into roadblocks with an object-oriented approach.

If anyone has a more ECS-style approach, I would be open to it. But, the problem is that I am going from generic system for all technologies to specific code for a given technology and so, at the very least, you would need to spawn a technology-specific Event class or attach a technology-specific Component class to some entity that I could then build a system to Query on and I think I need at least a little tiny bit of object-oriented code for this.

For that, I was thinking about a Resource with a HashMap of a technology id to an implementation of a Trait where the Trait would have nothing but one method to add a specific Component to an Entity. It seemed like a simple thing to do but does not seem possible.

#[derive(Component, Hash, Eq, PartialEq, Debug, Clone)]
pub struct PersistentId(pub String);

trait InnovationTypeTrait {
    fn start_innovation(&self, mut commands: Commands, entity: Entity) {
    }
}

#[derive(Resource)]
pub struct InnovationDatabase {
    map_innovation: HashMap<PersistentId, Box<dyn InnovationTypeTrait>>
}

This produced:

error[E0277]: (dyn InnovationTypeTrait + 'static) cannot be sent between threads safely

I attempted a few things like adding an unsafe Sync trait to InnovationTypeTrait but had no luck.

I then thought all I really needed was a single lambda function. So, I tried a HashMap pointing to a lambda:

#[derive(Resource)]
pub struct InnovationDatabase {
    map_innovation: HashMap<PersistentId, fn(mut commands: Commands, entity: Entity)>
}

But. for some reason I can't fathom, I can't use "mut" in a lambda:

Patterns aren't allowed in function pointer types [E0561]