Simple Event Loop

event_loop.js

const events = [];
const tasks = [];

function registerInterest(event, task) {
  events.push({ event, task });
}

function waitForEvents() {
  if (events.length === 0) {
    return;
  }
  console.log("waiting for events for %s seconds", 1);
  blockFor(1);

  let entry = events.pop();
  while (entry) {
    tasks.push(entry.task);
    entry = events.pop();
  }
}

function blockFor(seconds) {
  const now = new Date().getTime();
  const goal = seconds * 1000 + now;
  while (new Date().getTime() < goal) {}
}

const files = {
  1: { fd: 1, contents: "contents of file 1" },
  2: { fd: 2, contents: "contents of file 2" },
};

function readFile(fd) {
  const isReady = Math.random() >= 0.5;

  if (isReady) {
    return files[fd].contents;
  }

  return "would_block";
}

class Task {
  constructor(fd) {
    this.fileDescriptor = fd;
    this.state = { type: "init" };
  }

  poll() {
    // Loop until we return either ready or pending:
    for (;;) {
      switch (this.state.type) {
        case "init": {
          console.log({ task: this.fileDescriptor, state: "init" });

          // Ask to be notified when something has happened to our file:
          registerInterest({ type: "ReadFile", fileDescriptor: this.fileDescriptor }, this);
          this.state = { type: "waiting_for_file", fileDescriptor: this.fileDescriptor };

          return "pending";
        }
        case "waiting_for_file": {
          console.log({ task: this.fileDescriptor, state: "waiting_for_file" });

          // Check in on the actual state of our file:
          const fileContents = readFile(this.state.fileDescriptor);

          // If it would still block, we just say "too bad, wake me up next time", and go back to
          // sleep by retunring "pending"
          if (fileContents === "would_block") {
            registerInterest({ type: "ReadFile", fileDescriptor: this.fileDescriptor }, this);
            return "pending";
          }

          // Otherwise, we successfully got some bytes out of our file, so we're done
          this.state = { type: "done", fileContents };
          break;
        }
        case "done": {
          // Here, we don't register interest in anything, so the event system will never schedule
          // us again after this.
          console.log({ task: this.fileDescriptor, state: "done" });
          console.log(
            "Task %d read file contents: %s",
            this.fileDescriptor,
            this.state.fileContents,
          );
          return "ready";
        }
      }
    }
  }
}

tasks.push(new Task(1));
tasks.push(new Task(2));

for (;;) {
  console.log("event loop tick");
  let currentTask = tasks.shift();
  while (currentTask) {
    currentTask.poll();
    currentTask = tasks.shift();
  }

  waitForEvents();

  if (tasks.length === 0) {
    break;
  }
}