说明：  本书对 QWidget 的各个子类及其相关的框架结构作了详细的介绍，主要包 括，元对象系统，信号和槽， Qt 事件， Qt 主窗口，布局管理及焦点系统，对话 框，模型/视图框架，拖放和剪贴板， Qt 文本系统， Qt 界面外观， Qt 2D 绘图和 Qt 的输入输出。
(This book provides a detailed introduction to the various subclasses of QWidget and their related frameworks, the main packagesIncludes, meta-object system, signals and slots, Qt events, Qt main window, layout management and focus system, dialog Box, model/view frame, drag and drop and clipboard, Qt text system, Qt interface appearance, Qt 2D drawing and Qt input and output.)