System data problem

[kvark]

Anything more than a simple demo will probably have some soft of a System trait, with objects implementing it being boxed and stored in a list to be executed automatically. It may look like this:

trait System {
    fn process(&mut self, &mut secs::Planner, TimeDelta);
}

Unfortunately, using secs under such abstraction is not elegant:

1. The system execution is now split into 3 different chunks, executed at different moments:
   • process() entry, executed upon queuing the system
   • fetch phase, executed at the start of the system on another thread
   • iter phase, executed after all the storage locks are acquired
2. Any data that system has (and wants to access in the execution closure) needs to be Clone + Sync. Imaging a mutable vector there - now you'd need to wrap it into Arc<Mutex<X>> , even though it should not be needed. This is inconvenient and bloats the code.

Optimally, I'd like to base ECS onto the system interface like this (edit: moved to #40):

trait System<C: Clone>: Send { //`C` stands for "Context"
    type Data<'a>; // some sort of HKT, which is not supported in current Rust
    fn fetch<'a>(&self, &'a secs::World) -> Self::Data<'a>;
    fn run(&mut self, Data<'a>, C);
}

Why this would be nice:

• this system is required to implement Send: each time it needs to be executed, it's moved onto the thread, run, and then moved back (or not, if we can track this).
• it allows any sort of data to be kept within the system without harsh restrictions or bloated wrappers
• it clearly separates executable parts and their contexts. This enforces a fetch at compile time.
• it allows the users to start using it right away (without higher abstractions) in complex systems, one just need to decide what C is (in the original example, C = TimeDelta)