Qt中事件過濾器一般是用在繼承QObject的類中，但是更多的是用在繼承QWidget的界面類中。在界面類中，一般用的最多的是鼠標和鍵盤事件，包括鼠標摁下、移入、移除、拖拽等。一般在鼠標做出相應動作時，系統就會自動發送對應事件，然後被事件過濾器捕獲，從而執行邏輯業務代碼。
注意：事件過濾器作用域只在該界面內有效；出了該界面，在該界面重寫的事件過濾器無效！！！
也即是如果我在QPushbutton內重寫的事件過濾器，只能捕獲該QPushbutton的各類事件，不能捕獲該按鈕以外的事件！
下面代碼是Qt事件過濾器中所有事件枚舉類型QEvent::type定義：
enum Type {
/*
If you get a strange compiler error on the line with None,
it's probably because you're also including X11 headers,
which #define the symbol None. Put the X11 includes after
the Qt includes to solve this problem.
*/
None = 0, // invalid event
Timer = 1, // timer event
MouseButtonPress = 2, // mouse button pressed
MouseButtonRelease = 3, // mouse button released
MouseButtonDblClick = 4, // mouse button double click
MouseMove = 5, // mouse move
KeyPress = 6, // key pressed
KeyRelease = 7, // key released
FocusIn = 8, // keyboard focus received
FocusOut = 9, // keyboard focus lost
FocusAboutToChange = 23, // keyboard focus is about to be lost
Enter = 10, // mouse enters widget
Leave = 11, // mouse leaves widget
Paint = 12, // paint widget
Move = 13, // move widget
Resize = 14, // resize widget
Create = 15, // after widget creation
Destroy = 16, // during widget destruction
Show = 17, // widget is shown
Hide = 18, // widget is hidden
Close = 19, // request to close widget
Quit = 20, // request to quit application
ParentChange = 21, // widget has been reparented
ParentAboutToChange = 131, // sent just before the parent change is done
ThreadChange = 22, // object has changed threads
WindowActivate = 24, // window was activated
WindowDeactivate = 25, // window was deactivated
ShowToParent = 26, // widget is shown to parent
HideToParent = 27, // widget is hidden to parent
Wheel = 31, // wheel event
WindowTitleChange = 33, // window title changed
WindowIconChange = 34, // icon changed
ApplicationWindowIconChange = 35, // application icon changed
ApplicationFontChange = 36, // application font changed
ApplicationLayoutDirectionChange = 37, // application layout direction changed
ApplicationPaletteChange = 38, // application palette changed
PaletteChange = 39, // widget palette changed
Clipboard = 40, // internal clipboard event
Speech = 42, // reserved for speech input
MetaCall = 43, // meta call event
SockAct = 50, // socket activation
WinEventAct = 132, // win event activation
DeferredDelete = 52, // deferred delete event
DragEnter = 60, // drag moves into widget
DragMove = 61, // drag moves in widget
DragLeave = 62, // drag leaves or is cancelled
Drop = 63, // actual drop
DragResponse = 64, // drag accepted/rejected
ChildAdded = 68, // new child widget
ChildPolished = 69, // polished child widget
ChildRemoved = 71, // deleted child widget
ShowWindowRequest = 73, // widget's window should be mapped
PolishRequest = 74, // widget should be polished
Polish = 75, // widget is polished
LayoutRequest = 76, // widget should be relayouted
UpdateRequest = 77, // widget should be repainted
UpdateLater = 78, // request update() later
EmbeddingControl = 79, // ActiveX embedding
ActivateControl = 80, // ActiveX activation
DeactivateControl = 81, // ActiveX deactivation
ContextMenu = 82, // context popup menu
InputMethod = 83, // input method
TabletMove = 87, // Wacom tablet event
LocaleChange = 88, // the system locale changed
LanguageChange = 89, // the application language changed
LayoutDirectionChange = 90, // the layout direction changed
Style = 91, // internal style event
TabletPress = 92, // tablet press
TabletRelease = 93, // tablet release
OkRequest = 94, // CE (Ok) button pressed
HelpRequest = 95, // CE (?) button pressed
IconDrag = 96, // proxy icon dragged
FontChange = 97, // font has changed
EnabledChange = 98, // enabled state has changed
ActivationChange = 99, // window activation has changed
StyleChange = 100, // style has changed
IconTextChange = 101, // icon text has changed. Deprecated.
ModifiedChange = 102, // modified state has changed
MouseTrackingChange = 109, // mouse tracking state has changed
WindowBlocked = 103, // window is about to be blocked modally
WindowUnblocked = 104, // windows modal blocking has ended
WindowStateChange = 105,
ReadOnlyChange = 106, // readonly state has changed
ToolTip = 110,
WhatsThis = 111,
StatusTip = 112,
ActionChanged = 113,
ActionAdded = 114,
ActionRemoved = 115,
FileOpen = 116, // file open request
Shortcut = 117, // shortcut triggered
ShortcutOverride = 51, // shortcut override request
WhatsThisClicked = 118,
ToolBarChange = 120, // toolbar visibility toggled
ApplicationActivate = 121, // deprecated. Use ApplicationStateChange instead.
ApplicationActivated = ApplicationActivate, // deprecated
ApplicationDeactivate = 122, // deprecated. Use ApplicationStateChange instead.
ApplicationDeactivated = ApplicationDeactivate, // deprecated
QueryWhatsThis = 123, // query what's this widget help
EnterWhatsThisMode = 124,
LeaveWhatsThisMode = 125,
ZOrderChange = 126, // child widget has had its z-order changed
HoverEnter = 127, // mouse cursor enters a hover widget
HoverLeave = 128, // mouse cursor leaves a hover widget
HoverMove = 129, // mouse cursor move inside a hover widget
// last event id used = 132
#ifdef QT_KEYPAD_NAVIGATION
EnterEditFocus = 150, // enter edit mode in keypad navigation
LeaveEditFocus = 151, // enter edit mode in keypad navigation
#endif
AcceptDropsChange = 152,
ZeroTimerEvent = 154, // Used for Windows Zero timer events
GraphicsSceneMouseMove = 155, // GraphicsView
GraphicsSceneMousePress = 156,
GraphicsSceneMouseRelease = 157,
GraphicsSceneMouseDoubleClick = 158,
GraphicsSceneContextMenu = 159,
GraphicsSceneHoverEnter = 160,
GraphicsSceneHoverMove = 161,
GraphicsSceneHoverLeave = 162,
GraphicsSceneHelp = 163,
GraphicsSceneDragEnter = 164,
GraphicsSceneDragMove = 165,
GraphicsSceneDragLeave = 166,
GraphicsSceneDrop = 167,
GraphicsSceneWheel = 168,
KeyboardLayoutChange = 169, // keyboard layout changed
DynamicPropertyChange = 170, // A dynamic property was changed through setProperty/property
TabletEnterProximity = 171,
TabletLeaveProximity = 172,
NonClientAreaMouseMove = 173,
NonClientAreaMouseButtonPress = 174,
NonClientAreaMouseButtonRelease = 175,
NonClientAreaMouseButtonDblClick = 176,
MacSizeChange = 177, // when the Qt::WA_Mac{Normal,Small,Mini}Size changes
ContentsRectChange = 178, // sent by QWidget::setContentsMargins (internal)
MacGLWindowChange = 179, // Internal! the window of the GLWidget has changed
FutureCallOut = 180,
GraphicsSceneResize = 181,
GraphicsSceneMove = 182,
CursorChange = 183,
ToolTipChange = 184,
NetworkReplyUpdated = 185, // Internal for QNetworkReply
GrabMouse = 186,
UngrabMouse = 187,
GrabKeyboard = 188,
UngrabKeyboard = 189,
MacGLClearDrawable = 191, // Internal Cocoa, the window has changed, so we must clear
StateMachineSignal = 192,
StateMachineWrapped = 193,
TouchBegin = 194,
TouchUpdate = 195,
TouchEnd = 196,
#ifndef QT_NO_GESTURES
NativeGesture = 197, // QtGui native gesture
#endif
RequestSoftwareInputPanel = 199,
CloseSoftwareInputPanel = 200,
WinIdChange = 203,
#ifndef QT_NO_GESTURES
Gesture = 198,
GestureOverride = 202,
#endif
ScrollPrepare = 204,
Scroll = 205,
Expose = 206,
InputMethodQuery = 207,
OrientationChange = 208, // Screen orientation has changed
TouchCancel = 209,
ThemeChange = 210,
SockClose = 211, // socket closed
PlatformPanel = 212,
StyleAnimationUpdate = 213, // style animation target should be updated
ApplicationStateChange = 214,
WindowChangeInternal = 215, // internal for QQuickWidget
ScreenChangeInternal = 216,
PlatformSurface = 217, // Platform surface created or about to be destroyed
Pointer = 218, // QQuickPointerEvent; ### Qt 6: QPointerEvent
TabletTrackingChange = 219, // tablet tracking state has changed
// 512 reserved for Qt Jambi's MetaCall event
// 513 reserved for Qt Jambi's DeleteOnMainThread event
User = 1000, // first user event id
MaxUser = 65535 // last user event id
};
參考內容：
Qt幫助文檔