QML live-reload

live_reload.hpp

#include <QFileSystemWatcher>
#include <QQmlApplicationEngine>
#include <QTimer>
#include <QUrl>
#include <QDirIterator>

void init_live_reload(QQmlApplicationEngine *engine, const QString &path) {
    auto *fs_watcher = new QFileSystemWatcher(engine);
    QDirIterator it(path, QDirIterator::Subdirectories);
    while (it.hasNext()) {
        it.next();
        auto i = it.fileInfo();
        if (i.isFile())
            fs_watcher->addPath(i.absoluteFilePath());
    }

    QObject::connect(fs_watcher, &QFileSystemWatcher::fileChanged, [=](const QString &file) {
        QTimer::singleShot(50, [=] {
            static QQuickItem *previousItem = nullptr;
            auto window = qobject_cast<QQuickWindow *>(engine->rootObjects().first());
            fs_watcher->addPath(file);

            auto loader = window->findChild<QObject*>("AppLoader");
            auto prev_source = loader->property("source");
            loader->setProperty("source", "");
            engine->clearComponentCache();
            loader->setProperty("source", prev_source);

        });
    });
}

main.cpp

#include <QtGui>
#include <QtQuick>
#include <filesystem>
#include <QQmlComponent>
#include "hotreload.hpp"


namespace fs = std::filesystem;

int main(int argc, char *argv[]) {
  QGuiApplication app(argc, argv);
  QQmlApplicationEngine engine;

  init_live_reload(&engine, "path/to/root/qml");
  QUrl url((fs::path(__FILE__).parent_path() / "qml" / "mainwindow.qml").c_str());
  engine.load(url);
  return QGuiApplication::exec();
}

Main.qml

// every time this file is edited the loader will refresh it.
import QtQuick
import QtQuick.Controls
import QtQuick.Controls.Material
import DogModule

Pane {
    anchors.fill: parent;
    Rectangle{
        width:234;
        height: 234;
    }

}

mainwindow.qml

import QtQuick
import QtQuick.Controls
import QtQuick.Controls.Material

ApplicationWindow {
    id: root
    width: 500
    height: 400
    visible: true
    Material.theme: Material.Dark
    Material.accent: Material.Orange
    Loader{objectName: "AppLoader"
        source: "./Main.qml"
    }
}

Hello!
I tried this out with Qt 6.5.3, as it looked very promissing, but unfortunatelly it does not work... The reload seems to happen as planned, but it seems to me as if the old item is still cached. No change is visible. Do you have any idea why this is happening?

Make sure the engine shows the qml files that you are editing (and the fs watcher is wathing) and not the build copies.

Wow thanks for the hint! That really was the problem...
I was not able to get your code passing a QUrl to work. I fixed it like this:

    const auto path = std::filesystem::path(__FILE__).parent_path().parent_path() / "resources" / "qml" / "main.qml";
    m_qmlEngine.load(QString::fromStdString(path.string()));

Yeah QML tooling's are awful...