Some people mentioned that QT5.7 and later versions only come with a free keyboard plug-in.
About the QKeyEvent class in QT5.10: click to open the link
QT sendEvent and PostEvent, click to open the link
My god, I installed QT5.10 today. There are ready-made virtual keyboard examples in the help documentation. You can directly search (not index) Qt Virtual Keyboard Examples in the help interface.
oh, a dude on stackover had a similar question: click to open the link
Discovery process:
1) About using QT plugin, QT input method: click to open the link
2) Send out the message of the system key press in the program, use QKeyEvent, so that the control currently in the focus state can automatically receive the signal.