Setcentralwidget can set Qwidget as the main window of Qmainwindow

The word return app.exec () is used to get the program into the event loop, except for the events that are submitted directly, all events are distributed in this loop, and finally the corresponding handler function is found to handle the event.

Top-level windows and top-level windows exist in two different event loops, such as Qmainwindow and Qwidget, to allow events in a top-level window to respond in another top-level window, you must merge two top-level windows, For example, Setcentralwidget can set Qwidget to Qmainwindow's main window.

http://blog.csdn.net/ftworld21/article/details/12528519

Setcentralwidget can set Qwidget as the main window of Qmainwindow