Qt 事件處理機制 （上篇）
作者：佚名
在Qt中，事件被封裝成一個個對象，所有的事件均繼承自抽象類QEvent. 接下來依次談?wù)凲t中有誰來產(chǎn)生、分發(fā)、接受和處理事件。
本篇來介紹Qt 事件處理機制 。深入了解事件處理系統(tǒng)對于每個學(xué)習(xí)Qt人來說非常重要，可以說，Qt是以事件驅(qū)動的UI工具集。 大家熟知Signals/Slots在多線程的實現(xiàn)也依賴于Qt的事件處理機制。
在Qt中，事件被封裝成一個個對象，所有的事件均繼承自抽象類QEvent. 接下來依次談?wù)?strong>Qt中有誰來產(chǎn)生、分發(fā)、接受和處理事件：
1、誰來產(chǎn)生事件： 最容易想到的是我們的輸入設(shè)備，比如鍵盤、鼠標(biāo)產(chǎn)生的
keyPressEvent，keyReleaseEvent，mousePressEvent，mouseReleaseEvent事件（他們被封裝成QMouseEvent和QKeyEvent），這些事件來自于底層的操作系統(tǒng)，它們以異步的形式通知Qt事件處理系統(tǒng)，后文會仔細道來。當(dāng)然Qt自己也會產(chǎn)生很多事件，比如QObject::startTimer()會觸發(fā)QTimerEvent. 用戶的程序可還以自己定制事件。
2、誰來接受和處理事件：答案是QObject。在Qt的內(nèi)省機制剖析一文已經(jīng)介紹QObject 類是整個Qt對象模型的心臟，事件處理機制是QObject三大職責(zé)（內(nèi)存管理、內(nèi)省(intropection)與事件處理制）之一。任何一個想要接受并處理事件的對象均須繼承自QObject，可以選擇重載QObject::event()函數(shù)或事件的處理權(quán)轉(zhuǎn)給父類。
3、誰來負責(zé)分發(fā)事件：對于non-GUI的Qt程序，是由QCoreApplication負責(zé)將QEvent分發(fā)給QObject的子類-Receiver. 對于Qt GUI程序，由QApplication來負責(zé)。
接下來，將通過對代碼的解析來看看QT是利用event loop從事件隊列中獲取用戶輸入事件，又是如何將事件轉(zhuǎn)義成QEvents，并分發(fā)給相應(yīng)的QObject處理。
- #include <QApplication>
- #include "widget.h"
- //Section 1
- int main(int argc, char *argv[])
- {
- QApplication app(argc, argv);
- Widget window; // Widget 繼承自QWidget
- window.show();
- return app.exec(); // 進入Qpplication事件循環(huán)，見section 2
- }
- // Section 2:
- int QApplication::exec()
- {
- //skip codes
- //簡單的交給QCoreApplication來處理事件循環(huán)=〉section 3
- return QCoreApplication::exec();
- }
- // Section 3
- int QCoreApplication::exec()
- {
- //得到當(dāng)前Thread數(shù)據(jù)
- QThreadData *threadData = self->d_func()->threadData;
- if (threadData != QThreadData::current()) {
- qWarning("%s::exec: Must be called from the main thread", self->metaObject()->className());
- return -1;
- }
- //檢查event loop是否已經(jīng)創(chuàng)建
- if (!threadData->eventLoops.isEmpty()) {
- qWarning("QCoreApplication::exec: The event loop is already running");
- return -1;
- }
- ...
- QEventLoop eventLoop;
- self->d_func()->in_exec = true;
- self->d_func()->aboutToQuitEmitted = false;
- //委任QEventLoop 處理事件隊列循環(huán) ==> Section 4
- int returnCode = eventLoop.exec();
- ....
- }
- return returnCode;
- }
- // Section 4
- int QEventLoop::exec(ProcessEventsFlags flags)
- {
- //這里的實現(xiàn)代碼不少，最為重要的是以下幾行
- Q_D(QEventLoop); // 訪問QEventloop私有類實例d
- try {
- //只要沒有遇見exit，循環(huán)派發(fā)事件
- while (!d->exit)
- processEvents(flags | WaitForMoreEvents | EventLoopExec);
- } catch (...) {}
- }
- // Section 5
- bool QEventLoop::processEvents(ProcessEventsFlags flags)
- {
- Q_D(QEventLoop);
- if (!d->threadData->eventDispatcher)
- return false;
- if (flags & DeferredDeletion)
- QCoreApplication::sendPostedEvents(0, QEvent::DeferredDelete);
- //將事件派發(fā)給與平臺相關(guān)的QAbstractEventDispatcher子類 =>Section 6
- return d->threadData->eventDispatcher->processEvents(flags);
- }
- #include <QApplication>
- #include "widget.h"
- //Section 1
- int main(int argc, char *argv[])
- {
- QApplication app(argc, argv);
- Widget window; // Widget 繼承自QWidget
- window.show();
- return app.exec(); // 進入Qpplication事件循環(huán)，見section 2
- }
- // Section 2:
- int QApplication::exec()
- {
- //skip codes
- //簡單的交給QCoreApplication來處理事件循環(huán)=〉section 3
- return QCoreApplication::exec();
- }
- // Section 3
- int QCoreApplication::exec()
- {
- //得到當(dāng)前Thread數(shù)據(jù)
- QThreadData *threadData = self->d_func()->threadData;
- if (threadData != QThreadData::current()) {
- qWarning("%s::exec: Must be called from the main thread", self->metaObject()->className());
- return -1;
- }
- //檢查event loop是否已經(jīng)創(chuàng)建
- if (!threadData->eventLoops.isEmpty()) {
- qWarning("QCoreApplication::exec: The event loop is already running");
- return -1;
- }
- ...
- QEventLoop eventLoop;
- self->d_func()->in_exec = true;
- self->d_func()->aboutToQuitEmitted = false;
- //委任QEventLoop 處理事件隊列循環(huán) ==> Section 4
- int returnCode = eventLoop.exec();
- ....
- }
- return returnCode;
- }
- // Section 4
- int QEventLoop::exec(ProcessEventsFlags flags)
- {
- //這里的實現(xiàn)代碼不少，最為重要的是以下幾行
- Q_D(QEventLoop); // 訪問QEventloop私有類實例d
- try {
- //只要沒有遇見exit，循環(huán)派發(fā)事件
- while (!d->exit)
- processEvents(flags | WaitForMoreEvents | EventLoopExec);
- } catch (...) {}
- }
- // Section 5
- bool QEventLoop::processEvents(ProcessEventsFlags flags)
- {
- Q_D(QEventLoop);
- if (!d->threadData->eventDispatcher)
- return false;
- if (flags & DeferredDeletion)
- QCoreApplication::sendPostedEvents(0, QEvent::DeferredDelete);
- //將事件派發(fā)給與平臺相關(guān)的QAbstractEventDispatcher子類 =>Section 6
- return d->threadData->eventDispatcher->processEvents(flags);
- }
- // Section 6，QTDIR\src\corelib\kernel\qeventdispatcher_win.cpp
- // 這段代碼是完成與windows平臺相關(guān)的windows c++。 以跨平臺著稱的Qt同時也提供了對Symiban,Unix等平臺的消息派發(fā)支持
- // 其事現(xiàn)分別封裝在QEventDispatcherSymbian和QEventDispatcherUNIX
- // QEventDispatcherWin32派生自QAbstractEventDispatcher.
- bool QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags flags)
- {
- Q_D(QEventDispatcherWin32);
- if (!d->internalHwnd)
- createInternalHwnd();
- d->interrupt = false;
- emit awake();
- bool canWait;
- bool retVal = false;
- bool seenWM_QT_SENDPOSTEDEVENTS = false;
- bool needWM_QT_SENDPOSTEDEVENTS = false;
- do {
- DWORD waitRet = 0;
- HANDLE pHandles[MAXIMUM_WAIT_OBJECTS - 1];
- QVarLengthArray<MSG> processedTimers;
- while (!d->interrupt) {
- DWORD nCount = d->winEventNotifierList.count();
- Q_ASSERT(nCount < MAXIMUM_WAIT_OBJECTS - 1);
- MSG msg;
- bool haveMessage;
- if (!(flags & QEventLoop::ExcludeUserInputEvents) && !d->queuedUserInputEvents.isEmpty()) {
- // process queued user input events
- haveMessage = true;
- //從處理用戶輸入隊列中取出一條事件
- msg = d->queuedUserInputEvents.takeFirst();
- } else if(!(flags & QEventLoop::ExcludeSocketNotifiers) && !d->queuedSocketEvents.isEmpty()) {
- // 從處理socket隊列中取出一條事件
- haveMessage = true;
- msg = d->queuedSocketEvents.takeFirst();
- } else {
- haveMessage = PeekMessage(&msg, 0, 0, 0, PM_REMOVE);
- if (haveMessage && (flags & QEventLoop::ExcludeUserInputEvents)
- && ((msg.message >= WM_KEYFIRST
- && msg.message <= WM_KEYLAST)
- || (msg.message >= WM_MOUSEFIRST
- && msg.message <= WM_MOUSELAST)
- || msg.message == WM_MOUSEWHEEL
- || msg.message == WM_MOUSEHWHEEL
- || msg.message == WM_TOUCH
- #ifndef QT_NO_GESTURES
- || msg.message == WM_GESTURE
- || msg.message == WM_GESTURENOTIFY
- #endif
- || msg.message == WM_CLOSE)) {
- // 用戶輸入事件入隊列，待以后處理
- haveMessage = false;
- d->queuedUserInputEvents.append(msg);
- }
- if (haveMessage && (flags & QEventLoop::ExcludeSocketNotifiers)
- && (msg.message == WM_QT_SOCKETNOTIFIER && msg.hwnd == d->internalHwnd)) {
- // socket 事件入隊列，待以后處理
- haveMessage = false;
- d->queuedSocketEvents.append(msg);
- }
- }
- ....
- if (!filterEvent(&msg)) {
- TranslateMessage(&msg);
- //將事件打包成message調(diào)用Windows API派發(fā)出去
- //分發(fā)一個消息給窗口程序。消息被分發(fā)到回調(diào)函數(shù)，將消息傳遞給windows系統(tǒng)，windows處理完畢，會調(diào)用回調(diào)函數(shù) => section 7
- DispatchMessage(&msg);
- }
- }
- }
- } while (canWait);
- ...
- return retVal;
- }
- // Section 6，QTDIR\src\corelib\kernel\qeventdispatcher_win.cpp
- // 這段代碼是完成與windows平臺相關(guān)的windows c++。 以跨平臺著稱的Qt同時也提供了對Symiban,Unix等平臺的消息派發(fā)支持
- // 其事現(xiàn)分別封裝在QEventDispatcherSymbian和QEventDispatcherUNIX
- // QEventDispatcherWin32派生自QAbstractEventDispatcher.
- bool QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags flags)
- {
- Q_D(QEventDispatcherWin32);
- if (!d->internalHwnd)
- createInternalHwnd();
- d->interrupt = false;
- emit awake();
- bool canWait;
- bool retVal = false;
- bool seenWM_QT_SENDPOSTEDEVENTS = false;
- bool needWM_QT_SENDPOSTEDEVENTS = false;
- do {
- DWORD waitRet = 0;
- HANDLE pHandles[MAXIMUM_WAIT_OBJECTS - 1];
- QVarLengthArray<MSG> processedTimers;
- while (!d->interrupt) {
- DWORD nCount = d->winEventNotifierList.count();
- Q_ASSERT(nCount < MAXIMUM_WAIT_OBJECTS - 1);
- MSG msg;
- bool haveMessage;
- if (!(flags & QEventLoop::ExcludeUserInputEvents) && !d->queuedUserInputEvents.isEmpty()) {
- // process queued user input events
- haveMessage = true;
- //從處理用戶輸入隊列中取出一條事件
- msg = d->queuedUserInputEvents.takeFirst();
- } else if(!(flags & QEventLoop::ExcludeSocketNotifiers) && !d->queuedSocketEvents.isEmpty()) {
- // 從處理socket隊列中取出一條事件
- haveMessage = true;
- msg = d->queuedSocketEvents.takeFirst();
- } else {
- haveMessage = PeekMessage(&msg, 0, 0, 0, PM_REMOVE);
- if (haveMessage && (flags & QEventLoop::ExcludeUserInputEvents)
- && ((msg.message >= WM_KEYFIRST
- && msg.message <= WM_KEYLAST)
- || (msg.message >= WM_MOUSEFIRST
- && msg.message <= WM_MOUSELAST)
- || msg.message == WM_MOUSEWHEEL
- || msg.message == WM_MOUSEHWHEEL
- || msg.message == WM_TOUCH
- #ifndef QT_NO_GESTURES
- || msg.message == WM_GESTURE
- || msg.message == WM_GESTURENOTIFY
- #endif
- || msg.message == WM_CLOSE)) {
- // 用戶輸入事件入隊列，待以后處理
- haveMessage = false;
- d->queuedUserInputEvents.append(msg);
- }
- if (haveMessage && (flags & QEventLoop::ExcludeSocketNotifiers)
- && (msg.message == WM_QT_SOCKETNOTIFIER && msg.hwnd == d->internalHwnd)) {
- // socket 事件入隊列，待以后處理
- haveMessage = false;
- d->queuedSocketEvents.append(msg);
- }
- }
- ....
- if (!filterEvent(&msg)) {
- TranslateMessage(&msg);
- //將事件打包成message調(diào)用Windows API派發(fā)出去
- //分發(fā)一個消息給窗口程序。消息被分發(fā)到回調(diào)函數(shù)，將消息傳遞給windows系統(tǒng)，windows處理完畢，會調(diào)用回調(diào)函數(shù) => section 7
- DispatchMessage(&msg);
- }
- }
- }
- } while (canWait);
- ...
- return retVal;
- }
- // Section 7 windows窗口回調(diào)函數(shù) 定義在QTDIR\src\gui\kernel\qapplication_win.cpp
- extern "C" LRESULT QT_WIN_CALLBACK QtWndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
- {
- ...
- //將消息重新封裝成QEvent的子類QMouseEvent ==> Section 8
- result = widget->translateMouseEvent(msg);
- ...
- }
- // Section 7 windows窗口回調(diào)函數(shù) 定義在QTDIR\src\gui\kernel\qapplication_win.cpp
- extern "C" LRESULT QT_WIN_CALLBACK QtWndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
- {
- ...
- //將消息重新封裝成QEvent的子類QMouseEvent ==> Section 8
- result = widget->translateMouseEvent(msg);
- ...
- }
從Section 1~Section7， Qt進入QApplication的event loop，經(jīng)過層層委任，最終QEventloop的processEvent將通過與平臺相關(guān)的QAbstractEventDispatcher的子類QEventDispatcherWin32獲得用戶的用戶輸入事件，并將其打包成message后，通過標(biāo)準(zhǔn)Windows API ，把消息傳遞給了Windows OS，Windows OS得到通知后回調(diào)QtWndProc, 至此事件的分發(fā)與處理完成了一半的路程。
小結(jié)：Qt 事件處理機制 （上篇）的內(nèi)容介紹完了，在下文中，我們將進一步討論當(dāng)我們收到來在Windows的回調(diào)后，事件又是怎么一步步打包成QEvent并通過QApplication分發(fā)給最終事件的接受和處理者QObject::event.請繼續(xù)看Qt 事件處理機制 （下篇）。***希望本文能幫你解決問題！
責(zé)任編輯：zhaolei
來源：
互聯(lián)網(wǎng)
