QT 源码之 Qt 事件机制原理是本文要介绍的内容，在用Qt写Gui程序的时候，在main函数里面最后依据都是app.exec();很多书上对这句的解释是，使 Qt 程序进入消息循环。下面我们就到exec()函数内部，来看一下他的实现原理。

Let's go!

首先来到QTDIR\src\corelib\kernel\qcoreapplication.cpp

int QCoreApplication::exec()  {      if (!QCoreApplicationPrivate::checkInstance("exec"))          return -1;      //获取线程数据      QThreadData *threadData = self->d_func()->threadData;      //判断是否在主线程创建      if (threadData != QThreadData::current()) {          qWarning("%s::exec: Must be called from the main thread", self->metaObject()->className());          return -1;      }      //判断eventLoop是否已经创建      if (!threadData->eventLoops.isEmpty()) {          qWarning("QCoreApplication::exec: The event loop is already running");          return -1;      }      threadData->quitNow = false;      QEventLoop eventLoop;      self->d_func()->in_exec = true;      //创建eventLoop      int returnCode = eventLoop.exec();      threadData->quitNow = false;      if (self) {          self->d_func()->in_exec = false;          //退出程序          emit self->aboutToQuit();          sendPostedEvents(0, QEvent::DeferredDelete);      }      return returnCode;  }  再来到qeventloop.cpp中。  int QEventLoop::exec(ProcessEventsFlags flags)  {      Q_D(QEventLoop);      if (d->threadData->quitNow)          return -1;      //已经调用过exec了。      if (d->inExec) {          qWarning("QEventLoop::exec: instance %p has already called exec()", this);          return -1;      }      d->inExec = true;      d->exit = false;      ++d->threadData->loopLevel;      //将事件类对象压入线程结构体中      d->threadData->eventLoops.push(this);      // remove posted quit events when entering a new event loop      // 这句不用翻译了把！      if (qApp->thread() == thread())          QCoreApplication::removePostedEvents(qApp, QEvent::Quit);  #if defined(QT_NO_EXCEPTIONS)      while (!d->exit)          //这里才是关键,我们还要继续跟踪进去。          processEvents(flags | WaitForMoreEvents);  #else      try {          while (!d->exit)              processEvents(flags | WaitForMoreEvents);      } catch (...) {          //如果使用了EXCEPTION，则继续对下一条时间进行处理。          qWarning("Qt has caught an exception thrown from an event handler. Throwing\n"                   "exceptions from an event handler is not supported in Qt. You must\n"                   "reimplement QApplication::notify() and catch all exceptions there.\n");          throw;      }  #endif      //退出eventloop前，将时间对象从线程结构中取出。      QEventLoop *eventLoop = d->threadData->eventLoops.pop();      Q_ASSERT_X(eventLoop == this, "QEventLoop::exec()", "internal error");      Q_UNUSED(eventLoop); // --release warning       d->inExec = false;      --d->threadData->loopLevel;      //退出事件循环。      return d->returnCode;  }   来到了processEvents函数：  bool QEventLoop::processEvents(ProcessEventsFlags flags)  {      Q_D(QEventLoop);      //判断事件分派器是否为空。      if (!d->threadData->eventDispatcher)          return false;      if (flags & DeferredDeletion)          QCoreApplication::sendPostedEvents(0, QEvent::DeferredDelete);      //调用不同平台下的事件分派器来处理事件。      return d->threadData->eventDispatcher->processEvents(flags);  }  processEvents是在QAbstractEventDispatcher类中定义的纯虚方法。在QEventDispatcherWin32类有processEvents的实现。  bool QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags flags)  {      Q_D(QEventDispatcherWin32);      //内部数据创建。registerClass注册窗口类，createwindow创建窗体。      //注册socket notifiers，启动所有的normal timers      if (!d->internalHwnd)          createInternalHwnd();      d->interrupt = false;      emit awake();       bool canWait;      bool retVal = false;      do {          QCoreApplicationPrivate::sendPostedEvents(0, 0, d->threadData);          DWORD waitRet = 0;          HANDLE pHandles[MAXIMUM_WAIT_OBJECTS - 1];          QVarLengthArray
     
       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处理用户输入事件，放入队列中。                  haveMessage = true;                  msg = d->queuedUserInputEvents.takeFirst();              } else if(!(flags & QEventLoop::ExcludeSocketNotifiers) && !d->queuedSocketEvents.isEmpty()) {                  // process queued socket events  处理socket事件，放入队列中。                  haveMessage = true;                  msg = d->queuedSocketEvents.takeFirst();              } else {                  //从消息队列中取消息，同PeekMessage                  haveMessage = winPeekMessage(&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)) {                      // queue user input events for later processing                      haveMessage = false;                      d->queuedUserInputEvents.append(msg);                  }                  if (haveMessage && (flags & QEventLoop::ExcludeSocketNotifiers)                      && (msg.message == WM_USER && msg.hwnd == d->internalHwnd)) {                      // queue socket events for later processing                      haveMessage = false;                      d->queuedSocketEvents.append(msg);                  }              }              if (!haveMessage) {                  // no message - check for signalled objects                  for (int i=0; i<(int)nCount; i++)                      pHandles[i] = d->winEventNotifierList.at(i)->handle();                  //注册signal--slot。                  waitRet = MsgWaitForMultipleObjectsEx(nCount, pHandles, 0, QS_ALLINPUT, MWMO_ALERTABLE);                  if ((haveMessage = (waitRet == WAIT_OBJECT_0 + nCount))) {                      // a new message has arrived, process it                      continue;                  }              }              //事件队列中有事件需要处理。              if (haveMessage) {                   //处理timer事件                  if (msg.message == WM_TIMER) {                      // avoid live-lock by keeping track of the timers we've already sent                      bool found = false;                      for (int i = 0; !found && i < processedTimers.count(); ++i) {                          const MSG processed = processedTimers.constData()[i];                          found = (processed.wParam == msg.wParam && processed.hwnd == msg.hwnd && processed.lParam == msg.lParam);                      }                      if (found)                          continue;                      processedTimers.append(msg);                  } else if (msg.message == WM_QUIT) {                      if (QCoreApplication::instance())                          QCoreApplication::instance()->quit();                      return false;                  }                  //消息分发处理。                  if (!filterEvent(&msg)) {                      TranslateMessage(&msg);                      QT_WA({                          DispatchMessage(&msg);                      } , {                          DispatchMessageA(&msg);                      });                  }              } else if (waitRet >= WAIT_OBJECT_0 && waitRet < WAIT_OBJECT_0 + nCount) {                  //处理signal--slot                  d->activateEventNotifier(d->winEventNotifierList.at(waitRet - WAIT_OBJECT_0));              } else {                  // nothing todo so break                  break;              }              retVal = true;          }          // still nothing - wait for message or signalled objects          QThreadData *ddata = d->threadData;          canWait = (!retVal                     && data->canWait                     && !d->interrupt                     && (flags & QEventLoop::WaitForMoreEvents));          if (canWait) {              DWORD nCount = d->winEventNotifierList.count();              Q_ASSERT(nCount < MAXIMUM_WAIT_OBJECTS - 1);              for (int i=0; i<(int)nCount; i++)                  pHandles[i] = d->winEventNotifierList.at(i)->handle();              emit aboutToBlock();              waitRet = MsgWaitForMultipleObjectsEx(nCount, pHandles, INFINITE, QS_ALLINPUT, MWMO_ALERTABLE);              emit awake();              if (waitRet >= WAIT_OBJECT_0 && waitRet < WAIT_OBJECT_0 + nCount) {                  d->activateEventNotifier(d->winEventNotifierList.at(waitRet - WAIT_OBJECT_0));                  retVal = true;              }          }      } while (canWait);      return retVal;  }
     

小结：关于详解 QT 源码之 Qt 事件机制原理的内容介绍完了，基本属于代码实现的内容，最后希望本文对你有帮助！