QEventDispatcherWasm: handle EventLoopExec

Call emscripten_set_main_loop like the old GUI event
dispatcher did, with one difference that requestAnimationFrame
updates are now no longer handled by the event dispatcher.

Change-Id: If02d90ae9c45d7b38999567d733a237af842cded
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/corelib/kernel/qeventdispatcher_wasm.cpp

@@ -208,6 +208,8 @@ bool QEventDispatcherWasm::processEvents(QEventLoop::ProcessEventsFlags flags)
 
     if (flags & QEventLoop::DialogExec)
         handleDialogExec();
+    else if (flags & QEventLoop::EventLoopExec)
+        handleEventLoopExec();
 
     if (!(flags & QEventLoop::ExcludeUserInputEvents))
         pollForNativeEvents();
@@ -364,6 +366,22 @@ void QEventDispatcherWasm::wakeUp()
     emscripten_async_call(&QEventDispatcherWasm::callProcessEvents, this, 0);
 }
 
+void QEventDispatcherWasm::handleEventLoopExec()
+{
+    // Start the main loop, and then stop it on the first callback. This
+    // is done for the "simulateInfiniteLoop" functionality where
+    // emscripten_set_main_loop() throws a JS exception which returns
+    // control to the browser while preserving the C++ stack.
+    //
+    // Note that we don't use asyncify here: Emscripten supports one level of
+    // asyncify only and we want to reserve that for dialog exec() instead of
+    // using it for the one qApp exec().
+    const bool simulateInfiniteLoop = true;
+    emscripten_set_main_loop([](){
+        emscripten_pause_main_loop();
+    }, 0, simulateInfiniteLoop);
+}
+
 void QEventDispatcherWasm::handleDialogExec()
 {
 #if !QT_HAVE_EMSCRIPTEN_ASYNCIFY

src/corelib/kernel/qeventdispatcher_wasm_p.h

@@ -90,6 +90,7 @@ private:
     bool isMainThreadEventDispatcher();
     bool isSecondaryThreadEventDispatcher();
 
+    void handleEventLoopExec();
     void handleDialogExec();
     void pollForNativeEvents();
     bool waitForForEvents();