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Making Quicksilver and friends play nice with wasm32.

Date published: 2019-06-12T20:23:00Z

Disclaimer:

This technology is indistinguishable from magic. I am merely recalling which spells and incantations worked for me. If you have anything to add, you can reach me on https://gitgud.io/yuvallanger/kaka.farm/ or https://gitlab.com/yuvallanger/kaka.farm/.

Several months ago, writing a Flappy Bird clone called Rectangly Rect, I have done a bunch of asking around and found exactly which parts don't work and how to replace them. Yesterday, trying to adapt YasamSim to the web, I have re-discovered those workarounds, and decided to write this down.

`println!`!

First thing, drop all of your println!s. For some esoteric reason, this function throws a wrench into the web's machinery. Same goes for std::time::Instance::now(). For now I just dropped all calls to now(), maybe I could ask the browser manually with whatever function Javascript has, or maybe there is a more standardized std alternative for the web - I don't know.

In order to replace println!, I had to add to Cargo.toml a general dependency for the crate log, an entry for every target that is not wasm32 for the crate env_logger, and an entry for the crate web_logger for the wasm32 target:

[dependencies]
log = "0.4"

[target.'cfg(not(wasm32))'.dependencies]
env_logger = "0.6"

[target.'cfg(wasm32)'.dependencies]
web_logger = "0.1"

In src/logging.rs, conditionally compile a different init_logger() function for each platform, wasm32 and not-wasm32:

#[cfg(target_arch = "wasm32")]
pub fn init_logger() {
    ::web_logger::init();
}

#[cfg(not(target_arch = "wasm32"))]
pub fn init_logger() {
    ::env_logger::init();
}

In src/main.rs, call the init_logger() defined in the logging.rs sub-module at the head of your main() function:

mod logging;

fn main() {
    logging::init_logger();
    …
}

Now you can call info!(), error!(), warn!(), etc., as described in https://docs.rs/log/0.4/log/.

If you also debug it in your native target, you can also provide the RUST_LOG environment variable, as per https://docs.rs/env_logger/0.6/env_logger/'s documentation, in your command line incantations:

$ RUST_LOG=DEBUG cargo run

Sequentialize SPECS.

For some more esoteric reasons, probably something to do with threads, I had to rewrite how I run my specs::Systems , and how specs::Systems written.

Dispatching `specs::Dispatcher`.

One normally builds a dependency graph of specs::Systems using something like:

fn make_specs_dispatcher() -> specs::Dispatcher<'static, 'static> {
    specs::DispatcherBuilder::new()
        .with(
            SystemFoo,
            "system_foo",
            &[],
        )
        .with(
            SystemBar,
            "system_bar",
            &["system_foo"],
        )
        .build()
}

struct OurGameState {
    specs_world: specs::World,
    specs_dispatcher: specs::Dispatcher,
}


impl State for OurGameState {
    fn new() -> Result<World> {
        let specs_world = make_specs_world_and_register_components(); // Implemented elsewhere…
        let specs_dispatcher = make_specs_dispatcher();

        Ok(
            OurGameState {
                specs_world,
                specs_dispatcher,
            }
        )
    }
            
    fn update(&mut self, window: &mut Window) -> Result<()> {
        let system_foo = SystemFoo;
        let system_bar = SystemBar;
        
        system_foo.run_now(&selfworld.res);
        system_bar.run_now(&world.res);
        
        world.maintain();
        
        Ok(())
    }

    [imagine the rest of the quicksilver::lifecycle::State methods implemented here…]
}

In this example SystemBar depends on the state of the specs::World left by SystemFoo after it does its thing.

Instead of using this Dispatcher as described in https://slide-rs.github.io/specs/03_dispatcher.html, you do this in your quicksilver::lifecycle::State::update()

struct OurGameState {
    specs_world: specs::World,
}

impl State for OurGameState {
    fn new() -> Result<World> {
        let specs_world = make_specs_world_and_register_components(); // Implemented elsewhere…

        Ok(
            OurGameState {
                specs_world,
            }
        )
    }

    fn update(&mut self, window: &mut Window) -> Result<()> {
        let system_foo = SystemFoo;
        let system_bar = SystemBar;
        
        system_foo.run_now(&selfworld.res);
        system_bar.run_now(&world.res);
        
        world.maintain();
        
        Ok(())
    }

    [imagine the rest of the quicksilver::lifecycle::State methods implemented here…]
}

But in order to sequentialize how you deal with specs, you'd need to change one more thing:

Lay off `specs::LazyUpdater`.

If you do anything with specs::LazyUpdate, you would have to convert it into another form, interacting with your Component Storages directly with WriteStorage or whatever.

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