Is it possible to embed deno_core with a self-managed event loop?

[linrongbin16]

And the 2nd question is: does the API deno_core::JsRuntime::run_event_loop only run 1 tick? or it try to wait everything inside its internal queue to complete?

Because after reading some tutorials and source code of deno_core, I found there's already some queues inside deno_core, I guess they're for the queueMicrotask and built-in async/await functions.

For example, if we have below sample code:

async function readFile(filename) {
   // Read a file in async mode
}

async function sleepFor(seconds) {
  // Sleep for seconds
}

const content = await readFile("README.md");
console.log(`We have read: ${content}`);
await sleepFor(10);
console.log(`We have sleep for 10s`);

Once the deno_core::JsRuntime execute it, I guess the 1st console.log will not be printed, instead it's been put in the internal queue and wait for the async readFile function done. Correct?

Then we call the js_runtime.run_event_loop().await , once it's completed, is the only 1st console.log been printed? Or both two console.log are been printed? i.e. the run_event_loop API runs for only 1 tick, or wait for all the tasks done?

If it only runs for 1 tick, then I can embed it into my command line TUI application. Because the TUI application uses tokio runtime with the crossterm library (it handles terminal events and renders terminal). I want to use tokio::select! on both the crossterm's EventStream (it receives terminal keyboard/mouse events, such as key pressed, mouse clicked, etc) and the deno_core::JsRuntime::run_event_loop. Only 1 tick will never block the terminal events.

If it waits for all async tasks done, then it means it cannot work along with the tokio runtime managed by my TUI application. Because it can block the terminal.

Note: I also post this question in discord: https://discord.com/channels/684898665143206084/1287318426909544528

[linrongbin16]

Oh, I found this: https://discord.com/channels/684898665143206084/1243892880188248084

Seems the run_event_loop will wait for all things done, which doesn't fit my requirement.
But the trick in the discord post is worth a try.

[bartlomieju]

And the 2nd question is: does the API deno_core::JsRuntime::run_event_loop only run 1 tick? or it try to wait everything inside its internal queue to complete?

No, it runs for as long as event loop finishes all work. If you want to run a single "tick" you should use JsRuntime::poll_event_loop.

[linrongbin16]

hi, @bartlomieju thanks for reply!

Just one more question: I found the return value of poll_event_loop is a Poll.

I am a little confused how to wait for it done?

Or, it only returns Ready when done (just like run_event_loop is done), and for other cases it always returns Pending? correct?

[bartlomieju]

Or, it only returns Ready when done (just like run_event_loop is done), and for other cases it always returns Pending? correct?

Yes, that's correct - if it returns Poll::Pending you should call JsRuntime::poll_event_loop some time in the future - in your case inside the tokio::select! macro.

[linrongbin16]

thanks again for your teaching/sharing! quite helpful.

[congzhangzh]

@bartlomieju hi, maybe this pattern possible? https://www.electronjs.org/blog/electron-internals-node-integration