mainthread: add helper function to run code in main Qt thread

qt6/mainthread/mainthread.cpp

@@ -0,0 +1,13 @@
+#include <QMetaObject>
+#include <QCoreApplication>
+
+#ifndef _Bool
+#define _Bool bool
+#endif
+#include "_cgo_export.h"
+
+void mainthread_exec(intptr_t cb) {
+    QMetaObject::invokeMethod(qApp, [=]{
+        mainthread_exec_handle(cb);
+    }, Qt::QueuedConnection);
+}

qt6/mainthread/mainthread.go

@@ -0,0 +1,69 @@
+package mainthread
+
+import (
+	"sync"
+	"runtime/cgo"
+)
+
+/*
+#cgo pkg-config: Qt6Core
+
+#include "mainthread.h"
+*/
+import "C"
+
+// Start runs the callback in the main Qt thread. You should use this whenever
+// accessing the main Qt GUI from inside a goroutine.
+// This function is non-blocking.
+func Start(gofunc func()) {
+	h := cgo.NewHandle(gofunc)
+
+	C.mainthread_exec(C.intptr_t(h))
+}
+
+// Wait runs the callback in the main Qt thread. You should use this whenever
+// accessing the main Qt GUI from inside a goroutine.
+// The call blocks until the callback is executed in the main thread's eventloop.
+func Wait(gofunc func()) {	
+	// It's possible to use Qt::BlockingQueuedConnection to implement the
+	// blocking, but it has a deadlock risk
+	var wg sync.WaitGroup
+	wg.Add(1)
+	outerfunc := func() {
+		gofunc()
+		wg.Done()
+	}
+	Start(outerfunc)
+	wg.Wait()
+}
+
+func Wait2[T any](gofunc func() T) (ret T) {
+	outerfunc := func() {
+		ret = gofunc()
+	}
+	Wait(outerfunc)
+	return ret
+}
+
+func Wait3[T any](gofunc func() (T, error)) (ret T, err error) {
+	outerfunc := func() {
+		ret, err = gofunc()
+	}
+	Wait(outerfunc)
+	return ret, err
+}
+
+//export mainthread_exec_handle
+func mainthread_exec_handle(u uintptr) {
+	h := cgo.Handle(u)
+
+	gofunc, ok := h.Value().(func())
+	if !ok {
+		panic("miqt: callback of non-callback type (heap corruption?)")
+	}
+
+	gofunc()
+
+	// Free handle after use
+	h.Delete()
+}

qt6/mainthread/mainthread.h

@@ -0,0 +1,18 @@
+#pragma once
+
+#ifndef QT_MAINTHREAD_H
+#define QT_MAINTHREAD_H
+
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+void mainthread_exec(intptr_t cb);
+
+#ifdef __cplusplus
+}
+#endif 
+
+#endif