use std::cell::RefCell;

mod web_sys {
    pub struct Window;

    pub struct SomeEventListener;

    pub struct CanvasRenderingContext2d;

    impl CanvasRenderingContext2d {
        pub fn fill_rect(&self, x: f64, y: f64, w: f64, h: f64) {
            unimplemented!()
        }
    }
}

thread_local! {
    // If you don't need mutability in `Ctx`, you don't need to use `RefCell`
    static CTX: RefCell<Option<Ctx>> = RefCell::new(None);
}

struct Ctx {
    pub window: web_sys::Window,
    pub events: Vec<web_sys::SomeEventListener>,
    pub render: web_sys::CanvasRenderingContext2d,
}

impl Ctx {
    fn new() -> Ctx {
        Ctx {
            window: web_sys::Window,
            events: vec![],
            render: web_sys::CanvasRenderingContext2d,
        }
    }
}

fn main() {
    let ctx = Ctx::new();
    draw_player(&ctx, 0., 0.);
}

// #[wasm_bindgen]
fn call_draw_player(x: f64, y: f64) {
    // Can also call `borrow_mut` with `RefCell`
    CTX.with(|ctx| draw_player(ctx.borrow().as_ref().unwrap(), x, y))
}

fn draw_player(ctx: &Ctx, x: f64, y: f64) {
    ctx.render.fill_rect(x, y, 100., 100.);
}


/*

NOTE: We could use this:

```
impl Ctx {
    fn draw_player(&self, ...) { ... }
}

// ...later...

ctx.draw_player(...);
```

However, this can get dirty really quickly since you often need to use `Ctx` on
an impl for another item. Imagine:

```
struct GameManager;

impl GameManager {
    fn draw_player(&self, ctx: &Ctx) {
        ctx.render.fill_rect(...);
    }
}
```

For really really common functions, then yeah, put that in an `impl Ctx`, but
other less common or domain-specific functionality should be either a global
function or implemented on its appropriate struct.

*/