QT之QEvent机制

1.QEvent简介

       QEvent是QT的事件机制。QT的事件机制，肯定是借鉴了Windows的消息机制。Windows把所有的鼠标键盘以及内部事件都当作消息，并放进消息处理机制处理。QT本身，是把各个操作系统的API进行调用。Windows的消息机制，是一个非常简单有效的机制，QT作为借鉴，不为过。

      QEvent是QT的事件机制的基础；QEvent可以被QOject灵活运动，使事件得到接收和响应。

       Qt的主事件循环（QCoreApplication :: exec（））从事件队列中获取本机窗口系统事件，将其转换为QEvents，然后将转换后的事件发送给QObject。

       通常，事件来自底层窗口系统（spontaneous()返回true），但是也可以使用QCoreApplication :: sendEvent（）和QCoreApplication :: postEvent（）手动发送事件（spontaneous()返回false）。

       QObject通过调用其QObject :: event（）函数来接收事件。可以在子类中重新实现该函数，以自定义事件处理并添加其他事件类型。QWidget :: event（）是一个著名的例子。默认情况下，事件被调度到事件处理程序，例如QObject :: timerEvent（）和QWidget :: mouseMoveEvent（）。QObject :: installEventFilter（）允许一个对象拦截发往另一个对象的事件。

       基本的QEvent仅包含事件类型参数和“ accept”标志。接受标志设置为accept（），并使用ignore（）清除。它是默认设置的，但是不要依赖它，因为子类可以选择在其构造函数中清除它。

          QEvent的子类包含描述特定事件的其他参数。

2.QEvent分类

       QEvent的分类如下：

     

Constant	Value	Description
QEvent::None	0	Not an event.
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	This enum has been deprecated. Use ApplicationStateChange instead.
QEvent::ApplicationActivated	ApplicationActivate	This enum has been deprecated. Use ApplicationStateChange instead.
QEvent::ApplicationDeactivate	122	This enum has been deprecated. Use ApplicationStateChange instead.
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::ApplicationStateChange	214	The state of the application has changed.
QEvent::ApplicationWindowIconChange	35	The application's icon has changed.
QEvent::ChildAdded	68	An object gets a child (QChildEvent).
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.
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 (QDeferredDeleteEvent)
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::DynamicPropertyChange	170	A dynamic property was added, changed, or removed from the object.
QEvent::EnabledChange	98	Widget's enabled state has changed.
QEvent::Enter	10	Mouse enters widget's boundaries (QEnterEvent).
QEvent::EnterEditFocus	150	An editor widget gains focus for editing. QT_KEYPAD_NAVIGATION must be defined.
QEvent::EnterWhatsThisMode	124	Send to toplevel widgets when the application enters "What's This?" mode.
QEvent::Expose	206	Sent to a window when its on-screen contents are invalidated and need to be flushed from the backing store.
QEvent::FileOpen	116	File open request (QFileOpenEvent).
QEvent::FocusIn	8	Widget or Window gains keyboard focus (QFocusEvent).
QEvent::FocusOut	9	Widget or Window loses keyboard focus (QFocusEvent).
QEvent::FocusAboutToChange	23	Widget or Window focus is about to change (QFocusEvent)
QEvent::FontChange	97	Widget's font has changed.
QEvent::Gesture	198	A gesture was triggered (QGestureEvent).
QEvent::GestureOverride	202	A gesture override was triggered (QGestureEvent).
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. (Deprecated)
QEvent::InputMethod	83	An input method is being used (QInputMethodEvent).
QEvent::InputMethodQuery	207	A input method query event (QInputMethodQueryEvent)
QEvent::KeyboardLayoutChange	169	The keyboard layout has changed.
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. QT_KEYPAD_NAVIGATION must be defined.
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 (QMouseEvent).
QEvent::NonClientAreaMouseButtonPress	174	A mouse button press occurred outside the client area (QMouseEvent).
QEvent::NonClientAreaMouseButtonRelease	175	A mouse button release occurred outside the client area (QMouseEvent).
QEvent::NonClientAreaMouseMove	173	A mouse move occurred outside the client area (QMouseEvent).
QEvent::MacSizeChange	177	The user changed his widget sizes (macOS only).
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::NativeGesture	197	The system has detected a gesture (QNativeGestureEvent).
QEvent::OrientationChange	208	The screens orientation has changes (QScreenOrientationChangeEvent).
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::PlatformPanel	212	A platform specific panel has been requested.
QEvent::PlatformSurface	217	A native platform surface has been created or is about to be destroyed (QPlatformSurfaceEvent).
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 (QHelpEvent).
QEvent::ReadOnlyChange	106	Widget's read-only state has changed (since Qt 5.4).
QEvent::RequestSoftwareInputPanel	199	A widget wants to open a software input panel (SIP).
QEvent::Resize	14	Widget's size changed (QResizeEvent).
QEvent::ScrollPrepare	204	The object needs to fill in its geometry information (QScrollPrepareEvent).
QEvent::Scroll	205	The object needs to scroll to the supplied position (QScrollEvent).
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). When a shortcut is about to trigger, ShortcutOverride is sent to the active window. This allows clients (e.g. widgets) to signal that they will handle the shortcut themselves, by accepting the event. If the shortcut override is accepted, the event is delivered as a normal key press to the focus widget. Otherwise, it triggers the shortcut action, if one exists.
QEvent::Show	17	Widget was shown on screen (QShowEvent).
QEvent::ShowToParent	26	A child widget has been shown.
QEvent::SockAct	50	Socket activated, used to implement QSocketNotifier.
QEvent::StateMachineSignal	192	A signal delivered to a state machine (QStateMachine::SignalEvent).
QEvent::StateMachineWrapped	193	The event is a wrapper for, i.e., contains, another event (QStateMachine::WrappedEvent).
QEvent::StatusTip	112	A status tip is requested (QStatusTipEvent).
QEvent::StyleChange	100	Widget's style has been changed.
QEvent::TabletMove	87	Wacom tablet move (QTabletEvent).
QEvent::TabletPress	92	Wacom tablet press (QTabletEvent).
QEvent::TabletRelease	93	Wacom tablet release (QTabletEvent).
QEvent::TabletEnterProximity	171	Wacom tablet enter proximity event (QTabletEvent), sent to QApplication.
QEvent::TabletLeaveProximity	172	Wacom tablet leave proximity event (QTabletEvent), sent to QApplication.
QEvent::TabletTrackingChange	219	The Wacom tablet tracking state has changed (since Qt 5.9).
QEvent::ThreadChange	22	The object is moved to another thread. This is the last event sent to this object in the previous thread. See QObject::moveToThread().
QEvent::Timer	1	Regular timer events (QTimerEvent).
QEvent::ToolBarChange	120	The toolbar button is toggled on macOS.
QEvent::ToolTip	110	A tooltip was requested (QHelpEvent).
QEvent::ToolTipChange	184	The widget's tooltip has changed.
QEvent::TouchBegin	194	Beginning of a sequence of touch-screen or track-pad events (QTouchEvent).
QEvent::TouchCancel	209	Cancellation of touch-event sequence (QTouchEvent).
QEvent::TouchEnd	196	End of touch-event sequence (QTouchEvent).
QEvent::TouchUpdate	195	Touch-screen event (QTouchEvent).
QEvent::UngrabKeyboard	189	Item loses keyboard grab (QGraphicsItem only).
QEvent::UngrabMouse	187	Item loses mouse grab (QGraphicsItem, QQuickItem).
QEvent::UpdateLater	78	The widget should be queued to be repainted at a later time.
QEvent::UpdateRequest	77	The widget should be repainted.
QEvent::WhatsThis	111	The widget should reveal "What's This?" help (QHelpEvent).
QEvent::WhatsThisClicked	118	A link in a widget's "What's This?" help was clicked.
QEvent::Wheel	31	Mouse wheel rolled (QWheelEvent).
QEvent::WinEventAct	132	A Windows-specific activation event has occurred.
QEvent::WindowActivate	24	Window was activated.
QEvent::WindowBlocked	103	The window is blocked by a modal dialog.
QEvent::WindowDeactivate	25	Window was deactivated.
QEvent::WindowIconChange	34	The window's icon has changed.
QEvent::WindowStateChange	105	The window's state (minimized, maximized or full-screen) has changed (QWindowStateChangeEvent).
QEvent::WindowTitleChange	33	The window title has changed.
QEvent::WindowUnblocked	104	The window is unblocked after a modal dialog exited.
QEvent::WinIdChange	203	The window system identifer for this native widget has changed.
QEvent::ZOrderChange	126	The widget's z-order has changed. This event is never sent to top level windows.

3.QEvetnt使用和捕捉

     QObject的类（和继承类），可以使用QObject::event(), QObject::installEventFilter(),QCoreApplication::sendEvent(), QCoreApplication::postEvent(), 和 QCoreApplication::processEvents()等机制发送事件和和捕捉事件。

     上面2所属事件均可被使用和捕捉。

     提供样例如下：

     头文件：

   

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QEvent>


namespace Ui
{
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

    virtual bool eventFilter(QObject *object, QEvent *event) override;

private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

    源文件：

  

#include <QDebug>

#include "mainwindow.h"
#include "ui_mainwindow.h"


MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    installEventFilter(this);
}

MainWindow::~MainWindow()
{
    delete ui;
}

bool MainWindow::eventFilter(QObject *object, QEvent *event)
{
    switch (event->type())
    {
    case QEvent::KeyPress:
        event->ignore();
        return true;

    default:
        qDebug() << event->type();
        break;
    }

    return QObject::eventFilter(object, event);
}

      可以打印出相关事件。

      发送事件源文件如下：

     

void MainWindow::on_pushButton_clicked()
{
    int key = Qt::Key_Delete;

     QKeyEvent EventPress(QEvent::KeyPress, key, Qt::NoModifier);
     QApplication::sendEvent(ui->textEdit, &EventPress);   //发送键盘按下事件

     QKeyEvent EventRelease(QEvent::KeyRelease, key, Qt::NoModifier);
     QApplication::sendEvent(ui->textEdit, &EventRelease); //发送键盘松开事件
}

   具体代码见GitHub：

   https://github.com/diziqian/QEvnetTest