事件(event)是由窗口系统或者Qt自身产生的，用以响应所发生的各类事情。当用户按下或者松开键盘或者鼠标上的按键时，就可以产生一个键
盘或者鼠标事件；当某个窗口第一次显示的时候，就会产生一个绘制事件，用来告诉窗口需要重新绘制它本身，从而使得该窗口可见。大多数事件是
作为用户动作的响应而产生的，但是也有一些例外，比如像定时器事件，则是由系统独立产生的。
一般情况下，在使用窗口部件的时候，Qt的窗口部件往往都会发送信号，而这时信号也是十分有用的，但是当我们需要编写自己的自定义窗口部
件的时候，又或者是我们想改变已经存在的Qt的窗口部件的行为的时候，事件就变的非常有用了。例如，当我们使用QPushButton时，我们对于它的
clicked()信号往往更为关注，而很少关心促成发射该信号的底层鼠标事件或者键盘事件。但是我们要实现的是一个类似于QPushButton，或者说我们
要实现一个QPushButton的话，就要编写一定的处理鼠标和键盘的事件的代码，而且在必要的时候，还需要发射clicked()信号。可以这么想信号是命
令而事件就是消息，一个战场上的军官只要在等到准确的消息之后，才会根据所传来的消息然后下达命令。
信号和事件的区别就在与，事件比信号更加底层，而且如果一个信号对应多个槽的话，信号的传递是无序的，而事件的传递是有序的。其实在多
线程的实现也依赖与Qt的事件处理机制。以上我们对于事件有了初步的了解了。下面我们继续对事件进行更细致的了解。
在QT中，事件被封装成一个个对象，所有的事件均继承自抽象类QEvent.接下来依次谈谈QT中有谁来产生，分发，接受和处理事件：
1.谁来产生事件:最容易想到的是我们的输入设备，比如键盘，鼠标产生的事件keyPressEvent,keyReleaseEvent,mousePressEvent事件（他
们被封装成QMouseEvent和QKeyEvent），这些事件来自于底层的操作系统，他们可以异步的形式通知Qt事件处理系统，后文会仔细道来。当然QT自己
也会产生很多事件，比如QObject::startTimer()会触发QTimerEvent。用户的程序还可以自己定制事件。
2.谁来接受和处理事件:答案是QObject。在Qt的内省机制剖析一文中已经介绍QObject类是整个QT对象模型的心脏，事件处理机制是QObject
三大职责(内存管理，内省(intropection)与事件处理)之一。任何一个想要接受并处理事件的对象均须继承自QObject，可以选择重QObject::event
()函数或事件的处理权交给父类。
3.谁来负责分发事件:对于non-GUI的QT程序，是由QCoreApplication负责将QEvent分发给QObject的子类-Receiver.对于Qt GUI程序，由
QApplication来负责。
在QT中，事件就是QEvent子类的一个实例。Qt处理的事件类型有一百多种
QEvent::None 0 Not an event.
QEvent::AccessibilityDescription 130 Used to query accessibility description texts (QAccessibleEvent).
QEvent::AccessibilityHelp 119 Used to query accessibility help texts (QAccessibleEvent).
QEvent::AccessibilityPrepare 86 Accessibility information is requested.
QEvent::ActionAdded 114 A new action has been added (QActionEvent).
QEvent::ActionChanged 113 An action has been changed (QActionEvent).
QEvent::ActionRemoved 115 An action has been removed (QActionEvent).
QEvent::ActivationChange 99 A widget's top-level window activation state has changed.
QEvent::ApplicationActivate 121 The application has been made available to the user.
QEvent::ApplicationActivated ApplicationActivate This enum has been deprecated. Use
ApplicationActivate instead.
QEvent::ApplicationDeactivate 122 The application has been suspended, and is unavailable to the user.
QEvent::ApplicationFontChange 36 The default application font has changed.
QEvent::ApplicationLayoutDirectionChange 37 The default application layout direction has changed.
QEvent::ApplicationPaletteChange 38 The default application palette has changed.
QEvent::ApplicationWindowIconChange 35 The application's icon has changed.
QEvent::ChildAdded 68 An object gets a child (QChildEvent).
QEvent::ChildInserted 70 An object gets a child (QChildEvent). Qt3Support only, use ChildAdded
instead.
QEvent::ChildPolished 69 A widget child gets polished (QChildEvent).
QEvent::ChildRemoved 71 An object loses a child (QChildEvent).
QEvent::Clipboard 40 The clipboard contents have changed (QClipboardEvent).
QEvent::Close 19 Widget was closed (QCloseEvent).
QEvent::CloseSoftwareInputPanel 200 A widget wants to close the software input panel (SIP).
QEvent::ContentsRectChange 178 The margins of the widget's content rect changed.
QEvent::ContextMenu 82 Context popup menu (QContextMenuEvent).
QEvent::CursorChange 183 The widget's cursor has changed.
QEvent::DeferredDelete 52 The object will be deleted after it has cleaned up.
QEvent::DragEnter 60 The cursor enters a widget during a drag and drop operation
(QDragEnterEvent).
QEvent::DragLeave 62 The cursor leaves a widget during a drag and drop operation
(QDragLeaveEvent).
QEvent::DragMove 61 A drag and drop operation is in progress (QDragMoveEvent).
QEvent::Drop 63 A drag and drop operation is completed (QDropEvent).
QEvent::EnabledChange 98 Widget's enabled state has changed.
QEvent::Enter 10 Mouse enters widget's boundaries.
QEvent::EnterEditFocus 150 An editor widget gains focus for editing.
QEvent::EnterWhatsThisMode 124 Send to toplevel widgets when the application enters "What's This?" mode.
QEvent::FileOpen 116 File open request (QFileOpenEvent).
QEvent::FocusIn 8 Widget gains keyboard focus (QFocusEvent).
QEvent::FocusOut 9 Widget loses keyboard focus (QFocusEvent).
QEvent::FontChange 97 Widget's font has changed.
QEvent::GrabKeyboard 188 Item gains keyboard grab (QGraphicsItem only).
QEvent::GrabMouse 186 Item gains mouse grab (QGraphicsItem only).
QEvent::GraphicsSceneContextMenu 159 Context popup menu over a graphics scene (QGraphicsSceneContextMenuEvent).
QEvent::GraphicsSceneDragEnter 164 The cursor enters a graphics scene during a drag and drop operation
(QGraphicsSceneDragDropEvent).
QEvent::GraphicsSceneDragLeave 166 The cursor leaves a graphics scene during a drag and drop operation
(QGraphicsSceneDragDropEvent).
QEvent::GraphicsSceneDragMove 165 A drag and drop operation is in progress over a scene
(QGraphicsSceneDragDropEvent).
QEvent::GraphicsSceneDrop 167 A drag and drop operation is completed over a scene
(QGraphicsSceneDragDropEvent).
QEvent::GraphicsSceneHelp 163 The user requests help for a graphics scene (QHelpEvent).
QEvent::GraphicsSceneHoverEnter 160 The mouse cursor enters a hover item in a graphics scene
(QGraphicsSceneHoverEvent).
QEvent::GraphicsSceneHoverLeave 162 The mouse cursor leaves a hover item in a graphics scene
(QGraphicsSceneHoverEvent).
QEvent::GraphicsSceneHoverMove 161 The mouse cursor moves inside a hover item in a graphics scene
(QGraphicsSceneHoverEvent).
QEvent::GraphicsSceneMouseDoubleClick 158 Mouse press again (double click) in a graphics scene
(QGraphicsSceneMouseEvent).
QEvent::GraphicsSceneMouseMove 155 Move mouse in a graphics scene (QGraphicsSceneMouseEvent).
QEvent::GraphicsSceneMousePress 156 Mouse press in a graphics scene (QGraphicsSceneMouseEvent).
QEvent::GraphicsSceneMouseRelease 157 Mouse release in a graphics scene (QGraphicsSceneMouseEvent).
QEvent::GraphicsSceneMove 182 Widget was moved (QGraphicsSceneMoveEvent).
QEvent::GraphicsSceneResize 181 Widget was resized (QGraphicsSceneResizeEvent).
QEvent::GraphicsSceneWheel 168 Mouse wheel rolled in a graphics scene (QGraphicsSceneWheelEvent).
QEvent::Hide 18 Widget was hidden (QHideEvent).
QEvent::HideToParent 27 A child widget has been hidden.
QEvent::HoverEnter 127 The mouse cursor enters a hover widget (QHoverEvent).
QEvent::HoverLeave 128 The mouse cursor leaves a hover widget (QHoverEvent).
QEvent::HoverMove 129 The mouse cursor moves inside a hover widget (QHoverEvent).
QEvent::IconDrag 96 The main icon of a window has been dragged away (QIconDragEvent).
QEvent::IconTextChange 101 Widget's icon text has been changed.
QEvent::InputMethod 83 An input method is being used (QInputMethodEvent).
QEvent::KeyPress 6 Key press (QKeyEvent).
QEvent::KeyRelease 7 Key release (QKeyEvent).
QEvent::LanguageChange 89 The application translation changed.
QEvent::LayoutDirectionChange 90 The direction of layouts changed.
QEvent::LayoutRequest 76 Widget layout needs to be redone.
QEvent::Leave 11 Mouse leaves widget's boundaries.
QEvent::LeaveEditFocus 151 An editor widget loses focus for editing.
QEvent::LeaveWhatsThisMode 125 Send to toplevel widgets when the application leaves "What's This?" mode.
QEvent::LocaleChange 88 The system locale has changed.
QEvent::NonClientAreaMouseButtonDblClick 176 A mouse double click occurred outside the client area.
QEvent::NonClientAreaMouseButtonPress 174 A mouse button press occurred outside the client area.
QEvent::NonClientAreaMouseButtonRelease 175 A mouse button release occurred outside the client area.
QEvent::NonClientAreaMouseMove 173 A mouse move occurred outside the client area.
QEvent::MacSizeChange 177 The user changed his widget sizes (Mac OS X only).
QEvent::MenubarUpdated 153 The window's menu bar has been updated.
QEvent::MetaCall 43 An asynchronous method invocation via QMetaObject::invokeMethod().
QEvent::ModifiedChange 102 Widgets modification state has been changed.
QEvent::MouseButtonDblClick 4 Mouse press again (QMouseEvent).
QEvent::MouseButtonPress 2 Mouse press (QMouseEvent).
QEvent::MouseButtonRelease 3 Mouse release (QMouseEvent).
QEvent::MouseMove 5 Mouse move (QMouseEvent).
QEvent::MouseTrackingChange 109 The mouse tracking state has changed.
QEvent::Move 13 Widget's position changed (QMoveEvent).
QEvent::Paint 12 Screen update necessary (QPaintEvent).
QEvent::PaletteChange 39 Palette of the widget changed.
QEvent::ParentAboutToChange 131 The widget parent is about to change.
QEvent::ParentChange 21 The widget parent has changed.
QEvent::Polish 75 The widget is polished.
QEvent::PolishRequest 74 The widget should be polished.
QEvent::QueryWhatsThis 123 The widget should accept the event if it has "What's This?" help.
QEvent::RequestSoftwareInputPanel 199 A widget wants to open a software input panel (SIP).
QEvent::Resize 14 Widget's size changed (QResizeEvent).
QEvent::Shortcut 117 Key press in child for shortcut key handling (QShortcutEvent).
QEvent::ShortcutOverride 51 Key press in child, for overriding shortcut key handling (QKeyEvent).
QEvent::Show 17 Widget was shown on screen (QShowEvent).
QEvent::ShowToParent 26 A child widget has been shown.
QEvent::SockAct
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