How to port existing code from Game to GameIterator

[bvssvni]

The GameIterator is strictly better than the Game trait and it scales much better. I think we should mark Game as deprecated and port all code over to GameIterator.

For example, if you are writing a game consisting of higher level building blocks, then it is very nice to redirect e from the iterator to a single method:

for e in Gameiterator::new(...) {
    // If event is handled, continue to next event.
    if ui.event(&e) { continue; }
    if world.event(&e) { continue; }
}

If you are using the Game trait you need to deal with each event specifically and call the methods. When Piston adds a new type of event, for example gamepad input, the higher level building blocks will not receive those events. All the parents blocks need to update their code. This adds a linear complexity on the number of blocks.

[bvssvni]

Porting code is easy because you can implement the methods directly on the structure. Considering the following example:

impl<W: GameWindow> Game<W> for App {
    fn load(&mut self, _window: &mut W) {
        let asset_store = AssetStore::from_folder("../bin/assets");
        let image = asset_store.path("rust-logo.png").unwrap();
        self.image = Some(Texture::from_path(&image).unwrap());
    }

    fn render(&mut self, _window: &mut W, args: &RenderArgs) {
        let ref mut gl = self.gl;
        gl.viewport(0, 0, args.width as i32, args.height as i32);
        let ref c = Context::abs(args.width as f64, args.height as f64);
        c.rgb(1.0, 1.0, 1.0).draw(gl);

        // Draw image.
        c.image(self.image.as_ref().unwrap()).draw(gl);
    }
}

This can be changed to:

impl App {
    fn load<W: GameWindow>(&mut self, _window: &mut W) {
        let asset_store = AssetStore::from_folder("../bin/assets");
        let image = asset_store.path("rust-logo.png").unwrap();
        self.image = Some(Texture::from_path(&image).unwrap());
    }

    fn render<W: GameWindow>(&mut self, _window: &mut W, args: &RenderArgs) {
        let ref mut gl = self.gl;
        gl.viewport(0, 0, args.width as i32, args.height as i32);
        let ref c = Context::abs(args.width as f64, args.height as f64);
        c.rgb(1.0, 1.0, 1.0).draw(gl);

        // Draw image.
        c.image(self.image.as_ref().unwrap()).draw(gl);
    }
}