As mentioned in the previous article the next task is to implement our first proper System, the FpsCounter.

A relatively easy, yet important, component is some sort of FPS counter. Ideally there’ll be a bit of text in the corner showing the number of poll()s per second and the average duration. That way we can get a better feel for our simulator’s performance characteristics.

An FpsCounter has two responsibilities,

  • Keep track of the last n poll times
  • Calculate some statistics on the poll times and display it in the browser

Creating The System Link to heading

Instead of making the FpsCounter part of our top-level sim application, it should be given its own crate. This means the FpsCounter won’t need to care about JavaScript, and lets us maintain distinct layers of abstraction (see Top-Level Infrastructure - Layers for more).

First up, lets create the crate.

cargo new --lib fps-counter
cd fps-counter
cargo add ../hal

We’ll also make simplest system that compiles.

// fps-counter/src/


use aimc_hal::{Clock, System};

#[derive(Debug, Clone, PartialEq)]
pub struct FpsCounter;

impl<In, Out> System<In, Out> for FpsCounter {
    fn poll(&mut self, inputs: &In, outputs: &mut Out) {

Next, we want to make sure the FpsCounter tracks the time of the last tick.

use aimc_hal::clock::DummyClock;

fn track_time_of_last_tick() {
    let mut fps = FpsCounter::default();
    let should_be = Duration::new(1, 23);
    let time = DummyClock(should_be);

    fps.poll(&time, &mut ());

    assert_eq!(fps.last_tick, should_be);

The aimc_hal::clock::DummyClock type is a helper Clock that always returns the same Duration.

To make this test pass, we’ll need to update our FpsCounter to track the last tick. It’ll also need a source of time, we constrain the In type using aimc_hal::clock::HasClock. This means input will have a getter that yields a Clock.

#[derive(Debug, Clone, Default, PartialEq)]
pub struct FpsCounter {
    last_tick: Duration,

impl<In: HasClock, Out> System<In, Out> for FpsCounter {
    fn poll(&mut self, inputs: &In, outputs: &mut Out) {
        self.last_tick = inputs.clock().elapsed();

The next step is to calculate the corresponding FPS and send the result somewhere.

We’ll wrap the calculated result in its own Fps struct, then make sure the outputs can handle it.

#[derive(Debug, Copy, Clone, Default, PartialEq)]
pub struct Fps {
    pub frequency: f32,

pub trait FpsSink {
    fn emit_fps(&mut self, fps: Fps);

And the corresponding test:

extern crate std;

use std::prelude::v1::*;

#[derive(Debug, Default)]
pub struct Sink(Vec<Fps>);

impl FpsSink for Sink {
    fn emit_fps(&mut self, fps: Fps) { self.0.push(fps); }

fn record_fps() {
    let start = Duration::new(42, 0);
    let period = Duration::from_millis(20);
    let now = start + period;
    let mut fps = FpsCounter::with_start_time(start);
    let mut sink = Sink::default();
    let time = DummyClock(now);

    fps.poll(&time, &mut sink);

    assert_eq!(sink.0.len(), 1);
        Fps {
            frequency: 1000.0 / 20.0,
    assert_eq!(fps.last_tick, now);

Updating FpsCounter::poll() to make the test pass is left as an exercise for the reader.

A further extension would be to add an average_poll_duration field to Fps. That way sim::App can record when App::poll() starts and poll the FpsCounter as the last thing before App::poll() exits. Embedded systems won’t generally have access to an allocator, so you may want to checkout arraydeque as a #[no_std] alternative.

Wiring up to the Application Link to heading

Now we’ve got an FPS counter it’s time to show the FPS in our window.

First, let’s give the FPS counter somewhere to be displayed. This means adding a dummy element to frontend/index.html.

   <noscript>This page contains webassembly and javascript content, please enable javascript in your browser.</noscript>

-  <div>
-    <span id="fps-counter"></span>
-  </div>
   <script src="./bootstrap.js"></script>

The Browser will also need updating so it takes a reference to the #fps-counter span in its constructor.

// sim/src/

use fps_counter::{Fps, FpsSink};
use wasm_bindgen::JsValue;
use web_sys::Element;

#[derive(Debug, Clone)]
pub struct Browser {
    fps_div: Element,

impl Browser {
    pub fn from_element(fps_selector: &str) -> Result<Browser, &'static str> {
        let document = web_sys::window()
            .ok_or("Can't get a reference to the window")?
            .ok_or("Can't get a reference to the document")?;

        let element = document
            .map_err(|_| "Invalid selector")?
            .ok_or("Can't find the FPS element")?;

        Ok(Browser { fps_div: element })

While we’re at it, we should probably implement fps_counter::FpsSink for Browser

// sim/src/

impl FpsSink for Browser {
    fn emit_fps(&mut self, fps: Fps) {
        let mut buffer = ArrayString::<[u8; 128]>::default();

        let result = write!(buffer, "FPS: {:.1}Hz", fps.frequency);

        if result.is_ok() {
        } else {
            self.fps_div.set_inner_html("FPS: ? Hz");

We’ll need to propagate possible errors now that constructing a Browser may fail.

The idiomatic way to do this is by returning a Result<T, JsValue> from a #[wasm_bindgen] function. That way, the shims generated by wasm-bindgen will be notified of failure and raise the JsValue as an exception.

// sim/src/

pub fn setup_world(fps_div: &str) -> Result<App, JsValue> {
    let browser = Browser::from_element(fps_div)?;
    let inputs = Inputs::default();

    Ok(App::new(inputs, browser))

And of course setup_world()’s signature changed, so we’ll need to pass in the #fps-counter selector from our JavaScript.

// frontend/index.js

function init() {
    console.log("Initializing the world");
    world = wasm.setup_world("#fps-counter");

With any luck, this should be everything required to wire the FpsCounter up to the UI.

Reloading the window shows some rapidly changing text in the top-left corner.

FPS: 55.56Hz

The label itself isn’t overly pleasing to look at, but it gets the job done. If the text itself seems to flicker, remember that we’re updating it every time the App gets polled (about 60 times per second).

As an exercise for the reader, try implementing a moving average to smooth that flicker out.

The Next Step Link to heading

Now we’ve got a better feel for the work required to add new systems to the application and wire them up to the UI, the next step is probably going to be the Communications system.

The real world tends to be messy with lots of places where errors can enter the Communications system, but lots of people have solved the problem in the past so we should be okay.