QT 事件和信号

仔细来看，事件与信号其实并无多大差别，从我们对其需求上来说，都只要能注册事件或信号响应函数，在事件或信号产生时能够被通知到即可。但有一项区别在于，事件处理函数的返回值是有意义的，我们要根据这个返回值来确定是否还要继续事件的处理，比如在QT中，事件处理函数如果返回true，则这个事件处理已完成，QApplication会接着处理下一个事件，而如果返回false，那么事件分派函数会继续向上寻找下一个可以处理该事件的注册方法。信号处理函数的返回值对信号分派器来说是无意义的。

另外还有一个需要我们关注的问题是事件和信号处理时的优先级问题。在QT中，事件因为都是与窗口相关的，所以事件回调时都是从当前窗口开始，一级一级向上派发，直到有一个窗口返回true，截断了事件的处理为止。对于信号的处理则比较简单，默认是没有顺序的，如果需要明确的顺序，可以在信号注册时显示地指明槽的位置。

在QT中，事件使用了一个事件队列来维护，如果事件的处理中又产生了新的事件，那么新的事件会加入到队列尾，直到当前事件处理完毕后， QApplication再去队列头取下一个事件来处理。而信号的处理方式有些不同，信号处理是立即回调的，也就是一个信号产生后，他上面所注册的所有槽都会立即被回调。这样就会产生一个递归调用的问题，比如某个信号处理器中又产生了一个信号，会使得信号的处理像一棵树一样的展开。

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::None0Not an event.
QEvent::AccessibilityDescription130Used to query accessibility description texts (QAccessibleEvent).
QEvent::AccessibilityHelp119Used to query accessibility help texts (QAccessibleEvent).
QEvent::AccessibilityPrepare86Accessibility information is requested.
QEvent::ActionAdded114A new action has been added (QActionEvent).
QEvent::ActionChanged113An action has been changed (QActionEvent).
QEvent::ActionRemoved115An action has been removed (QActionEvent).
QEvent::ActivationChange99A widget's top-level window activation state has changed.
QEvent::ApplicationActivate121The application has been made available to the user.
QEvent::ApplicationActivatedApplicationActivateThis enum has been deprecated. Use

ApplicationActivate instead.
QEvent::ApplicationDeactivate122The application has been suspended, and is unavailable to the user.
QEvent::ApplicationFontChange36The default application font has changed.
QEvent::ApplicationLayoutDirectionChange37The default application layout direction has changed.
QEvent::ApplicationPaletteChange38The default application palette has changed.
QEvent::ApplicationWindowIconChange35The application's icon has changed.
QEvent::ChildAdded68An object gets a child (QChildEvent).
QEvent::ChildInserted70An object gets a child (QChildEvent). Qt3Support only, use ChildAdded

instead.
QEvent::ChildPolished69A widget child gets polished (QChildEvent).
QEvent::ChildRemoved71An object loses a child (QChildEvent).
QEvent::Clipboard40The clipboard contents have changed (QClipboardEvent).
QEvent::Close19Widget was closed (QCloseEvent).
QEvent::CloseSoftwareInputPanel200A widget wants to close the software input panel (SIP).
QEvent::ContentsRectChange178The margins of the widget's content rect changed.
QEvent::ContextMenu82Context popup menu (QContextMenuEvent).
QEvent::CursorChange183The widget's cursor has changed.
QEvent::DeferredDelete52The object will be deleted after it has cleaned up.
QEvent::DragEnter60The cursor enters a widget during a drag and drop operation

(QDragEnterEvent).
QEvent::DragLeave62The cursor leaves a widget during a drag and drop operation

(QDragLeaveEvent).
QEvent::DragMove61A drag and drop operation is in progress (QDragMoveEvent).
QEvent::Drop63A drag and drop operation is completed (QDropEvent).
QEvent::EnabledChange98Widget's enabled state has changed.
QEvent::Enter10Mouse enters widget's boundaries.
QEvent::EnterEditFocus150An editor widget gains focus for editing.
QEvent::EnterWhatsThisMode124Send to toplevel widgets when the application enters "What's This?" mode.
QEvent::FileOpen116File open request (QFileOpenEvent).
QEvent::FocusIn8Widget gains keyboard focus (QFocusEvent).
QEvent::FocusOut9Widget loses keyboard focus (QFocusEvent).
QEvent::FontChange97Widget's font has changed.
QEvent::GrabKeyboard188Item gains keyboard grab (QGraphicsItem only).
QEvent::GrabMouse186Item gains mouse grab (QGraphicsItem only).
QEvent::GraphicsSceneContextMenu159Context popup menu over a graphics scene (QGraphicsSceneContextMenuEvent).
QEvent::GraphicsSceneDragEnter164The cursor enters a graphics scene during a drag and drop operation

(QGraphicsSceneDragDropEvent).
QEvent::GraphicsSceneDragLeave166The cursor leaves a graphics scene during a drag and drop operation

(QGraphicsSceneDragDropEvent).
QEvent::GraphicsSceneDragMove165A drag and drop operation is in progress over a scene

(QGraphicsSceneDragDropEvent).
QEvent::GraphicsSceneDrop167A drag and drop operation is completed over a scene

(QGraphicsSceneDragDropEvent).
QEvent::GraphicsSceneHelp163The user requests help for a graphics scene (QHelpEvent).
QEvent::GraphicsSceneHoverEnter160The mouse cursor enters a hover item in a graphics scene

(QGraphicsSceneHoverEvent).
QEvent::GraphicsSceneHoverLeave162The mouse cursor leaves a hover item in a graphics scene

(QGraphicsSceneHoverEvent).
QEvent::GraphicsSceneHoverMove161The mouse cursor moves inside a hover item in a graphics scene

(QGraphicsSceneHoverEvent).
QEvent::GraphicsSceneMouseDoubleClick158Mouse press again (double click) in a graphics scene

(QGraphicsSceneMouseEvent).
QEvent::GraphicsSceneMouseMove155Move mouse in a graphics scene (QGraphicsSceneMouseEvent).
QEvent::GraphicsSceneMousePress156Mouse press in a graphics scene (QGraphicsSceneMouseEvent).
QEvent::GraphicsSceneMouseRelease157Mouse release in a graphics scene (QGraphicsSceneMouseEvent).
QEvent::GraphicsSceneMove182Widget was moved (QGraphicsSceneMoveEvent).
QEvent::GraphicsSceneResize181Widget was resized (QGraphicsSceneResizeEvent).
QEvent::GraphicsSceneWheel168Mouse wheel rolled in a graphics scene (QGraphicsSceneWheelEvent).
QEvent::Hide18Widget was hidden (QHideEvent).
QEvent::HideToParent27A child widget has been hidden.
QEvent::HoverEnter127The mouse cursor enters a hover widget (QHoverEvent).
QEvent::HoverLeave128The mouse cursor leaves a hover widget (QHoverEvent).
QEvent::HoverMove129The mouse cursor moves inside a hover widget (QHoverEvent).
QEvent::IconDrag96The main icon of a window has been dragged away (QIconDragEvent).
QEvent::IconTextChange101Widget's icon text has been changed.
QEvent::InputMethod83An input method is being used (QInputMethodEvent).
QEvent::KeyPress6Key press (QKeyEvent).
QEvent::KeyRelease7Key release (QKeyEvent).
QEvent::LanguageChange89The application translation changed.
QEvent::LayoutDirectionChange90The direction of layouts changed.
QEvent::LayoutRequest76Widget layout needs to be redone.
QEvent::Leave11Mouse leaves widget's boundaries.
QEvent::LeaveEditFocus151An editor widget loses focus for editing.
QEvent::LeaveWhatsThisMode125Send to toplevel widgets when the application leaves "What's This?" mode.
QEvent::LocaleChange88The system locale has changed.
QEvent::NonClientAreaMouseButtonDblClick176A mouse double click occurred outside the client area.
QEvent::NonClientAreaMouseButtonPress174A mouse button press occurred outside the client area.
QEvent::NonClientAreaMouseButtonRelease175A mouse button release occurred outside the client area.
QEvent::NonClientAreaMouseMove173A mouse move occurred outside the client area.
QEvent::MacSizeChange177The user changed his widget sizes (Mac OS X only).
QEvent::MenubarUpdated153The window's menu bar has been updated.
QEvent::MetaCall43An asynchronous method invocation via QMetaObject::invokeMethod().
QEvent::ModifiedChange102Widgets modification state has been changed.
QEvent::MouseButtonDblClick4Mouse press again (QMouseEvent).
QEvent::MouseButtonPress2Mouse press (QMouseEvent).
QEvent::MouseButtonRelease3Mouse release (QMouseEvent).
QEvent::MouseMove5Mouse move (QMouseEvent).
QEvent::MouseTrackingChange109The mouse tracking state has changed.
QEvent::Move13Widget's position changed (QMoveEvent).
QEvent::Paint12Screen update necessary (QPaintEvent).
QEvent::PaletteChange39Palette of the widget changed.
QEvent::ParentAboutToChange131The widget parent is about to change.
QEvent::ParentChange21The widget parent has changed.
QEvent::Polish75The widget is polished.
QEvent::PolishRequest74The widget should be polished.
QEvent::QueryWhatsThis123The widget should accept the event if it has "What's This?" help.
QEvent::RequestSoftwareInputPanel199A widget wants to open a software input panel (SIP).
QEvent::Resize14Widget's size changed (QResizeEvent).
QEvent::Shortcut117Key press in child for shortcut key handling (QShortcutEvent).
QEvent::ShortcutOverride51Key press in child, for overriding shortcut key handling (QKeyEvent).
QEvent::Show17Widget was shown on screen (QShowEvent).
QEvent::ShowToParent26A child widget has been shown.
QEvent::SockAct50Socket activated, used to implement QSocketNotifier.
QEvent::StateMachineSignal192A signal delivered to a state machine (QStateMachine::SignalEvent).
QEvent::StateMachineWrapped193The event is a wrapper for, i.e., contains, another event

(QStateMachine::WrappedEvent).
QEvent::StatusTip112A status tip is requested (QStatusTipEvent).
QEvent::StyleChange100Widget's style has been changed.
QEvent::TabletMove87Wacom tablet move (QTabletEvent).
QEvent::TabletPress92Wacom tablet press (QTabletEvent).
QEvent::TabletRelease93Wacom tablet release (QTabletEvent).
QEvent::OkRequest94Ok button in decoration pressed. Supported only for Windows CE.
QEvent::TabletEnterProximity171Wacom tablet enter proximity event (QTabletEvent), sent to QApplication.
QEvent::TabletLeaveProximity172Wacom tablet leave proximity event (QTabletEvent), sent to QApplication.
QEvent::Timer1Regular timer events (QTimerEvent).
QEvent::ToolBarChange120The toolbar button is toggled on Mac OS X.
QEvent::ToolTip110A tooltip was requested (QHelpEvent).
QEvent::ToolTipChange184The widget's tooltip has changed.
QEvent::UngrabKeyboard189Item loses keyboard grab (QGraphicsItem only).
QEvent::UngrabMouse187Item loses mouse grab (QGraphicsItem only).
QEvent::UpdateLater78The widget should be queued to be repainted at a later time.
QEvent::UpdateRequest77The widget should be repainted.
QEvent::WhatsThis111The widget should reveal "What's This?" help (QHelpEvent).
QEvent::WhatsThisClicked118A link in a widget's "What's This?" help was clicked.
QEvent::Wheel31Mouse wheel rolled (QWheelEvent).
QEvent::WinEventAct132A Windows-specific activation event has occurred.
QEvent::WindowActivate24Window was activated.
QEvent::WindowBlocked103The window is blocked by a modal dialog.
QEvent::WindowDeactivate25Window was deactivated.
QEvent::WindowIconChange34The window's icon has changed.
QEvent::WindowStateChange105The window's state (minimized, maximized or full-screen) has changed

(QWindowStateChangeEvent).
QEvent::WindowTitleChange33The window title has changed.
QEvent::WindowUnblocked104The window is unblocked after a modal dialog exited.
QEvent::ZOrderChange126The widget's z-order has changed. This event is never sent to top level

windows.
QEvent::KeyboardLayoutChange169The keyboard layout has changed.
QEvent::DynamicPropertyChange170A dynamic property was added, changed or removed from the object.
QEvent::TouchBegin194Beginning of a sequence of touch-screen and/or track-pad events

(QTouchEvent)
QEvent::TouchUpdate195Touch-screen event (QTouchEvent)
QEvent::TouchEnd196End of touch-event sequence (QTouchEvent)
QEvent::WinIdChange203The window system identifer for this native widget has changed
QEvent::Gesture198A gesture was triggered (QGestureEvent)
QEvent::GestureOverride202A gesture override was triggered (QGestureEvent)