QT / QML主GUI窗口的加载会减慢启动画面的渲染

Question

I am using the C++/Qt/QML stack to develop a GUI application.

On application startup, I have a splash screen that displays while the main window is loading. I want the splash screen to provides updates (let's say, every 200ms) to the loading status of the main window.

However, the loading of the main window causes the 200ms "this is the update" rendering of the splash screen to get interrupted. The code which slows down the splash screen is:

this->engine = new QQmlApplicationEngine;
engine->load(QUrl("qrc:/mainWindow.qml")); // This line causes splash screen rendering to lag!

So I thought, ok, I'll put the splash screen process into a higher priority thread, so that it's render/update process won't get "as" interrupted.

But...I have found out that all QQmlApplicationEngine objects must be created/used in the same thread, namely the one which QApplication (a singleton) is created in.

I get that if I had heavily computational operations I could put them into a worker thread, but this scenario is a little different, because the thing that is causing the lag is a GUI method call, which Qt doesn't allow to be in a separate thread.

How can I achieve what I want with this limitation?

Answer 1

Just an idea (after reading the source code and docs), not a tested solution:

QQmlApplicationEngine combines a QQmlEngine and QQmlComponent to provide a convenient way to load a single QML file.

QQmlComponent provides progressChanged() signal which you can use to update your splash screen.

I hope you can achieve what you want if you can separate QQmlApplicationEngine into QQmlComponent to load a QML first (use the progressChanged() or a timer to update the splash screen) and then chuck it into QQmlEngine (mimicking the way of QQmlApplicationEngine C:\\Qt\\5.4\\Src\\qtdeclarative\\src\\qml\\qml\\qqmlapplicationengine.cpp ).

PS When you do all this, your main loop should be running already in order to make signals work.

Answer 2

Instead of using QQmlApplicationEngine 's engine, you can use your own. You can certainly load everything up in another thread, then ship the whole engine back to the main thread and start rendering there. Just make sure that none of the windows are visible, if you have Window objects there.