Event handlers never consume events in Asyncify

[devappd]

Per html5.h, event handlers can return TRUE to consume the event, i.e., event.preventDefault() is called. This does not work in Asyncify.

Here's a simple test case for mouse events:

• Normal build
• Asyncify build

The expected result is that the default handler is never invoked on the canvas, e.g., mouse clicks do not cause the canvas to gain focus.

I saw that the event handlers check the result of dynCall_iiii() to determine whether to call e.preventDefault(). This result is never passed in Asyncify.

Therefore, I get the desired result if I modify the compiled Module as below:

function registerMouseEventCallback(target, userData, useCapture, callbackfunc, eventTypeId, eventTypeString, targetThread) {
  /* ... */
  
  var mouseEventHandlerFunc = function(ev) {
    var e = ev || event;

    // TODO: Make this access thread safe, or this could update live while app is reading it.
    fillMouseEventData(JSEvents.mouseEvent, e, target);

-    if ((function(a1, a2, a3) { dynCall_iiii.apply(null, [callbackfunc, a1, a2, a3]); })(eventTypeId, JSEvents.mouseEvent, userData)) e.preventDefault();
+    if ((function(a1, a2, a3) { return dynCall_iiii.apply(null, [callbackfunc, a1, a2, a3]); })(eventTypeId, JSEvents.mouseEvent, userData)) e.preventDefault();
  };

  /* ... */
}

See Asyncify build with the fix applied, which passes the test.

Is this fix appropriate? If so, I can use guidance on patching the codebase as I'm unfamiliar with Asyncify modifications.

[kripken]

Good catch! I think the fix is correct.

I believe that mouseEventHandlerFunc uses the makeDynCall method, so fixing that is the right thing. A PR would be great!

[devappd]

OK, the fix looks fairly simple, I should get to this in the next day or two. I'll also try to add the test case above if you believe I should.

[kripken]

@devappd Great! And yes, the testcase is good to include there.