System Initialization

Systems may need to access resources from the World in order to be instantiated. For example, obtaining a ReaderId to an EventChannel that exists in the World. When there is an existing event channel in the World, a System should register itself as a reader of that channel instead of replacing it, as that invalidates all other readers.

In Amethyst, the World that the application begins with is populated with a number of default resources -- event channels, a thread pool, a frame limiter, and so on.

Given the default resources begin with special limits, we need a way to pass the System initialization logic through to the application, including parameters to the System's constructor. This is information the SystemDesc trait captures.

For each System, an implementation of the SystemDesc trait specifies the logic to instantiate the System. For Systems that do not require special initialization logic, the SystemDesc derive automatically implements the SystemDesc trait on the system type itself:

# extern crate amethyst;
use amethyst::{
    core::SystemDesc,
    derive::SystemDesc,
    ecs::{System, SystemData, World},
};

#[derive(SystemDesc)]
struct SystemName;

impl<'a> System<'a> for SystemName {
    type SystemData = ();

    fn run(&mut self, data: Self::SystemData) {
        println!("Hello!");
    }
}

The SystemDesc derive page demonstrates the use cases supported by the SystemDesc derive. For more complex cases, the Implementing the SystemDesc Trait page explains how to implement the SystemDesc trait.