Synchronously starting a new Thread?

[skerit]

As mentioned in the documentation and in a few discussions & issues: you shouldn't run a coroutine-method in a JavaScript callback. You would just get the Uncaught TypeError: can't access property "isResuming", thread is null error, since there's no thread to interrupt yet.

In a few issues, like #941, the suggested workaround for this is like the following:

button.onClick(e -> {
   var thread = new Thread(() -> doSomething());
   thread.start();
});

Pretty easy to do. But let's say you would want the doSomething method to also handle the event instance...
It will then not be able to call preventDefault() on it, since the thread won't actually start until after the original method has finished (and thus the event's default action has already happened)

So could there be a way to start a new thread immediately/synchronously, so it could at least handle the event before it does anything "asynchronously"?

The way I see it, in Platform.js new threads are basically always launched using:

teavm_globals.setTimeout(() => {
        $rt_threadStarter(teavm_javaMethod("org.teavm.platform.Platform",
            "launchThread(Lorg/teavm/platform/PlatformRunnable;)V"))(runnable);
    }, 0);

could there be a way to start it synchronously, in some cases?

[konsoletyper]

Hi. I'm not sure what are you trying to do. You can't cancel event in JavaScript asynchronously. Consider you have something like:

button.addEventListener("click", e => {
   setTimeout(() => e.preventDefault()
}, 1.0);

This also won't work. So the answer is: if you want to call preventDefault, avoid async calls.

[skerit]

Thanks for your response, I think my question may have been misunderstood.

I'm not asking about calling preventDefault() from an asynchronous context. I understand that's impossible in JavaScript's event model.
I'm specifically asking whether TeaVM could be modified to support a different thread creation mechanism that doesn't rely on setTimeout for initial execution.

So that it could behave similar to JavaScript's async/await:

button.addEventListener("click", async (e) => {
  // This part runs synchronously
  e.preventDefault();
  
  // Only after the first await does it become asynchronous
  await someAsyncOperation();
});

In this pattern, the function runs synchronously until the first await, allowing event manipulation, and only then becomes asynchronous.

The issue is that starting a new thread in TeaVM is always asynchronous from the start.

My question is: Could TeaVM implement thread creation in a way that mimics this behavior? Instead of always using setTimeout to schedule thread execution (making it fully asynchronous), could there be an option for threads to start executing synchronously (allowing event.preventDefault() to work).

[konsoletyper]

In you case the alternative would be:

button.onClick(e -> {
   e.preventDefault();
   var thread = new Thread(() -> someAsyncOperation());
   thread.start();
});

[konsoletyper]

Anyway, there's no easy way to do what you want. You can try to hack and generate bytecode that calls java.lang.Thread.runThread()V.

[skerit]

I didn't know how easy it was to get this to work using metaprogramming.
I managed to emit this custom bytecode rather quickly.
(It ultimately didn't work out, but that's ok)