Rendering is often a little bit tricky when you're dealing with a multi-threaded ECS. That's why we have something called "thread-local systems".

There are two things to keep in mind about thread-local systems:

1. They're always executed at the end of dispatch
2. They cannot have dependencies, you just add them in the order you want them to run

Adding one is a simple line added to the builder code:

DispatcherBuilder::new()
    .add_thread_local(RenderSys);

As for Amethyst, it's very easy because Specs is already integrated. So there's no special effort required, just look at the current examples.

Piston has an event loop which looks like this:

while let Some(event) = window.poll_event() {
    // Handle event
}

Now, we'd like to do as much as possible in the ECS, so we feed in input as a resource. This is what your code could look like:

struct ResizeEvents(Vec<(u32, u32)>);

world.add_resource(ResizeEvents(Vec::new()));

while let Some(event) = window.poll_event() {
    match event {
        Input::Resize(x, y) => world.write_resource::<ResizeEvents>().0.push((x, y)),
        // ...
    }
}

The actual dispatching should happen every time the Input::Update event occurs.

If you want a section for your game engine added, feel free to submit a PR!