事件循环

  事件是一个类对象具有特定的类型，事件多数情况下是被分发到一个队列中(事件队列)，当队列中有事件时就不停的将队列中的事件发送给QObject对象，当队列为空时就阻塞地等待事件，这个过程就是事件循环。
  事件循环具有以下特点：

• 当事件循环处理完一个事件后，就从“事件循环队列”中取出下一个事件处理之。
• 事件循环的本质就是以队列的方式再次分配线程时间片。
• 事件循环是可以嵌套的，一层套一层，子层的事件循环执行exec()的时候，父层事件循环就处于中断状态；当子层事件循环跳出exec()后，父层事件循环才能继续循环下去。如果某个子事件循环仍然有效，但其父循环被强制跳出，此时父循环不会立即执行跳出，而是等待子事件循环跳出后，父循环才会跳出。

  事件处理流程如下：
  

  对于主程序而言，其事件循环是由QApplication::exec()开启的，开启之后，我们可以通过sendEvent直接进行事件的处理，也可以通过postEvent将事件加入循环队列进行处理。
  在主程序开启事件循环过程中，其子层也可以开启，比如一个QDialog的exec()，那么这个窗口将会处理其循环队列中的事件，而上一层exec循环已经中断了。当QDialog的循环终止时，上一层循环才会重新开启。

postevent和sendevent

  从图中我们可以看出：postevent实际上是一种异步通信，而sendevent则是直接调用，是一种同步通信。

事件过滤

  事件的传送和处理流程的第一站是事件过滤器eventFilter()，某个对象A可以通过给另一个对象B安装事件处理器，实现对对象B事件的监听或者拦截功能。我们可以给A取名监听器，B取名接收器。一个对象可以监听多个对象，一个对象也可以被多个事件监听。事件过滤器返回true则表示事件已经处理完毕，否则传递给下一个监听器或者接收器本身。

Widget::Widget(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Widget)
{
    ui->setupUi(this);
 
    ui->lineEdit_user->setText(QString("lee"));
    focusNextChild();
    ui->lineEdit_password->setText(QString("*******"));
 
    //监听控件
    ui->lineEdit_user->installEventFilter(this);
    ui->lineEdit_password->installEventFilter(this);
    ui->button_accept->installEventFilter(this);
    ui->button_cancel->installEventFilter(this);
}
 
bool Widget::eventFilter(QObject *watched, QEvent *event)
{
    //定义事件处理动作
    if (watched == ui->lineEdit_user || watched == ui->lineEdit_password
        || watched == ui->button_accept || watched == ui->button_cancel)
    {
        if (event->type() == QEvent::KeyPress)
        {
            QKeyEvent *e = static_cast<QKeyEvent *>(event);
            if(e->key() == Qt::Key_Space)
            {
                focusNextChild();
                return true;
            }
        }
    }
    return QWidget::eventFilter(watched, event);
}
 
Widget::~Widget()
{
    delete ui;
}
 
void Widget::on_button_cancel_clicked()
{
    qApp->quit();
}

  在上述代码中，A对象即Widget，B对象即lineEdit，Widget给lineedit安装了事件过滤器，监听其KeySpace事件，当按下space时，跳转到下一个子控件。

源码剖析

int QCoreApplication::exec()

int QCoreApplication::exec()
{
    if (!QCoreApplicationPrivate::checkInstance("exec"))
        return -1;
    //判断是否处于当前线程，如果不是，则退出，这里的当前线程就是指主线程
    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;
    }
    if (!threadData->eventLoops.isEmpty()) { //如果事件循环已启动，则不能重复启动
        qWarning("QCoreApplication::exec: The event loop is already running");
        return -1;
    }

    threadData->quitNow = false;
    QEventLoop eventLoop;
    self->d_func()->in_exec = true;
    self->d_func()->aboutToQuitEmitted = false;
    int returnCode = eventLoop.exec();  //创建eventLoop
    threadData->quitNow = false;
    if (self) {
        self->d_func()->in_exec = false;  //程序退出
        if (!self->d_func()->aboutToQuitEmitted)
            emit self->aboutToQuit(QPrivateSignal());
        self->d_func()->aboutToQuitEmitted = true;  
        sendPostedEvents(0, QEvent::DeferredDelete);
    }

    return returnCode;
}

int QEventLoop::exec(ProcessEventsFlags flags)

int QEventLoop::exec(ProcessEventsFlags flags)
{
    Q_D(QEventLoop);
    //we need to protect from race condition with QThread::exit
    QMutexLocker locker(&static_cast<QThreadPrivate *>(QObjectPrivate::get(d->threadData->thread))->mutex);
    if (d->threadData->quitNow)
        return -1;

    if (d->inExec) {
        qWarning("QEventLoop::exec: instance %p has already called exec()", this);
        return -1;
    }

    struct LoopReference {
        QEventLoopPrivate *d;
        QMutexLocker &locker;

        bool exceptionCaught;
        LoopReference(QEventLoopPrivate *d, QMutexLocker &locker) : d(d), locker(locker), exceptionCaught(true)
        {
            d->inExec = true;
            d->exit.storeRelease(false);
            ++d->threadData->loopLevel;
            d->threadData->eventLoops.push(d->q_func()); //把当前事件入栈，当前对象是QEventLoop
            locker.unlock();
        }

        ~LoopReference()
        {
            if (exceptionCaught) {
                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");
            }
            locker.relock();
            QEventLoop *eventLoop = d->threadData->eventLoops.pop();  //退出事件时，将事件对象从线程对象中取出
            Q_ASSERT_X(eventLoop == d->q_func(), "QEventLoop::exec()", "internal error");
            Q_UNUSED(eventLoop); // --release warning
            d->inExec = false;
            --d->threadData->loopLevel;
        }
    };
    LoopReference ref(d, locker);

    // remove posted quit events when entering a new event loop
    //这里的app是主线程，如果当前线程是主线程，那么在进入新的事件循环后就将quit事件删除，否则主线程会推出
    QCoreApplication *app = QCoreApplication::instance();
    if (app && app->thread() == thread())
        QCoreApplication::removePostedEvents(app, QEvent::Quit);

    while (!d->exit.loadAcquire())  //这里才是真正事件循环的地方
        processEvents(flags | WaitForMoreEvents | EventLoopExec);

    ref.exceptionCaught = false;
    return d->returnCode.load();
}

bool QEventLoop::processEvents(ProcessEventsFlags flags)

bool QEventLoop::processEvents(ProcessEventsFlags flags)
{
    Q_D(QEventLoop);
    if (!d->threadData->eventDispatcher.load())
        return false;
    return d->threadData->eventDispatcher.load()->processEvents(flags); //调用不同平台下的事件分发器进行事件分发
}

bool QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags flags)

//windows下的事件分发器
bool QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags flags)
{
    Q_D(QEventDispatcherWin32);

    if (!d->internalHwnd) {
        createInternalHwnd();
        wakeUp(); // trigger a call to sendPostedEvents()
    }

    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;
                msg = d->queuedUserInputEvents.takeFirst();
            } else if(!(flags & QEventLoop::ExcludeSocketNotifiers) && !d->queuedSocketEvents.isEmpty()) {
                // process queued socket events
                haveMessage = true;
                msg = d->queuedSocketEvents.takeFirst();
            } else {
                haveMessage = PeekMessage(&msg, 0, 0, 0, PM_REMOVE);
                if (haveMessage) {
                    if ((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)) {
                        // queue user input events for later processing
                        d->queuedUserInputEvents.append(msg);
                        continue;
                    }
                    if ((flags & QEventLoop::ExcludeSocketNotifiers)
                        && (msg.message == WM_QT_SOCKETNOTIFIER && msg.hwnd == d->internalHwnd)) {
                        // queue socket events for later processing
                        d->queuedSocketEvents.append(msg);
                        continue;
                    }
                }
            }
            if (!haveMessage) {
                // no message - check for signalled objects
                for (int i=0; i<(int)nCount; i++)
                    pHandles[i] = d->winEventNotifierList.at(i)->handle();
                waitRet = MsgWaitForMultipleObjectsEx(nCount, pHandles, 0, QS_ALLINPUT, MWMO_ALERTABLE);
                if ((haveMessage = (waitRet == WAIT_OBJECT_0 + nCount))) {
                    // a new message has arrived, process it
                    continue;
                }
            }
            if (haveMessage) {  //使用windows下的事件机制进行消息分发处理
                // WinCE doesn't support hooks at all, so we have to call this by hand :(
                if (!d->getMessageHook)
                    (void) qt_GetMessageHook(0, PM_REMOVE, (LPARAM) &msg);

                if (d->internalHwnd == msg.hwnd && msg.message == WM_QT_SENDPOSTEDEVENTS) {
                    if (seenWM_QT_SENDPOSTEDEVENTS) {
                        // when calling processEvents() "manually", we only want to send posted
                        // events once
                        needWM_QT_SENDPOSTEDEVENTS = true;
                        continue;
                    }
                    seenWM_QT_SENDPOSTEDEVENTS = true;
                } else 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;
                }

                if (!filterNativeEvent(QByteArrayLiteral("windows_generic_MSG"), &msg, 0)) {
                    TranslateMessage(&msg);
                    DispatchMessage(&msg);
                }
            } else if (waitRet - WAIT_OBJECT_0 < nCount) {
                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
        canWait = (!retVal
                   && !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 | MWMO_INPUTAVAILABLE);
            emit awake();
            if (waitRet - WAIT_OBJECT_0 < nCount) {
                d->activateEventNotifier(d->winEventNotifierList.at(waitRet - WAIT_OBJECT_0));
                retVal = true;
            }
        }
    } while (canWait);

    if (!seenWM_QT_SENDPOSTEDEVENTS && (flags & QEventLoop::EventLoopExec) == 0) {
        // when called "manually", always send posted events
        sendPostedEvents(); //分发事件
    }

    if (needWM_QT_SENDPOSTEDEVENTS)
        PostMessage(d->internalHwnd, WM_QT_SENDPOSTEDEVENTS, 0, 0);

    return retVal;
}

void QEventDispatcherWin32::sendPostedEvents()

void QEventDispatcherWin32::sendPostedEvents()
{
    Q_D(QEventDispatcherWin32);
    QCoreApplicationPrivate::sendPostedEvents(0, 0, d->threadData);
}

void QCoreApplicationPrivate::sendPostedEvents(QObject *receiver, int event_type,
                                               QThreadData *data)
{
    if (event_type == -1) {
        // we were called by an obsolete event dispatcher.
        event_type = 0;
    }

    if (receiver && receiver->d_func()->threadData != data) {
        qWarning("QCoreApplication::sendPostedEvents: Cannot send "
                 "posted events for objects in another thread");
        return;
    }

    ++data->postEventList.recursion;

    QMutexLocker locker(&data->postEventList.mutex);

    // by default, we assume that the event dispatcher can go to sleep after
    // processing all events. if any new events are posted while we send
    // events, canWait will be set to false.
    data->canWait = (data->postEventList.size() == 0);

    if (data->postEventList.size() == 0 || (receiver && !receiver->d_func()->postedEvents)) {
        --data->postEventList.recursion;
        return;
    }

    data->canWait = true;

    // okay. here is the tricky loop. be careful about optimizing
    // this, it looks the way it does for good reasons.
    int startOffset = data->postEventList.startOffset;
    int &i = (!event_type && !receiver) ? data->postEventList.startOffset : startOffset;
    data->postEventList.insertionOffset = data->postEventList.size();

    // Exception-safe cleaning up without the need for a try/catch block
    struct CleanUp {
        QObject *receiver;
        int event_type;
        QThreadData *data;
        bool exceptionCaught;

        inline CleanUp(QObject *receiver, int event_type, QThreadData *data) :
            receiver(receiver), event_type(event_type), data(data), exceptionCaught(true)
        {}
        inline ~CleanUp()
        {
            if (exceptionCaught) {
                // since we were interrupted, we need another pass to make sure we clean everything up
                data->canWait = false;
            }

            --data->postEventList.recursion;
            if (!data->postEventList.recursion && !data->canWait && data->hasEventDispatcher())
                data->eventDispatcher.load()->wakeUp();

            // clear the global list, i.e. remove everything that was
            // delivered.
            if (!event_type && !receiver && data->postEventList.startOffset >= 0) {
                const QPostEventList::iterator it = data->postEventList.begin();
                data->postEventList.erase(it, it + data->postEventList.startOffset);
                data->postEventList.insertionOffset -= data->postEventList.startOffset;
                Q_ASSERT(data->postEventList.insertionOffset >= 0);
                data->postEventList.startOffset = 0;
            }
        }
    };
    CleanUp cleanup(receiver, event_type, data);

    while (i < data->postEventList.size()) {  //事件队列实际是个链表，遍历该链表并依次执行
        // avoid live-lock
        if (i >= data->postEventList.insertionOffset)
            break;

        const QPostEvent &pe = data->postEventList.at(i);
        ++i;

        if (!pe.event)
            continue;
        if ((receiver && receiver != pe.receiver) || (event_type && event_type != pe.event->type())) {
            data->canWait = false;
            continue;
        }

        if (pe.event->type() == QEvent::DeferredDelete) {
            // DeferredDelete events are sent either
            // 1) when the event loop that posted the event has returned; or
            // 2) if explicitly requested (with QEvent::DeferredDelete) for
            //    events posted by the current event loop; or
            // 3) if the event was posted before the outermost event loop.

            int eventLevel = static_cast<QDeferredDeleteEvent *>(pe.event)->loopLevel();
            int loopLevel = data->loopLevel + data->scopeLevel;
            const bool allowDeferredDelete =
                (eventLevel > loopLevel
                 || (!eventLevel && loopLevel > 0)
                 || (event_type == QEvent::DeferredDelete
                     && eventLevel == loopLevel));
            if (!allowDeferredDelete) {
                // cannot send deferred delete
                if (!event_type && !receiver) {
                    // we must copy it first; we want to re-post the event
                    // with the event pointer intact, but we can't delay
                    // nulling the event ptr until after re-posting, as
                    // addEvent may invalidate pe.
                    QPostEvent pe_copy = pe;

                    // null out the event so if sendPostedEvents recurses, it
                    // will ignore this one, as it's been re-posted.
                    const_cast<QPostEvent &>(pe).event = 0;

                    // re-post the copied event so it isn't lost
                    data->postEventList.addEvent(pe_copy);
                }
                continue;
            }
        }

        // first, we diddle the event so that we can deliver
        // it, and that no one will try to touch it later.
        pe.event->posted = false;
        QEvent *e = pe.event;
        QObject * r = pe.receiver;  //获取事件的接收者

        --r->d_func()->postedEvents;
        Q_ASSERT(r->d_func()->postedEvents >= 0);

        // next, update the data structure so that we're ready
        // for the next event.
        const_cast<QPostEvent &>(pe).event = 0;

        struct MutexUnlocker
        {
            QMutexLocker &m;
            MutexUnlocker(QMutexLocker &m) : m(m) { m.unlock(); }
            ~MutexUnlocker() { m.relock(); }
        };
        MutexUnlocker unlocker(locker);

        QScopedPointer<QEvent> event_deleter(e); // will delete the event (with the mutex unlocked)

        // after all that work, it's time to deliver the event.
        QCoreApplication::sendEvent(r, e);  //开始执行事件

        // careful when adding anything below this point - the
        // sendEvent() call might invalidate any invariants this
        // function depends on.
    }

    cleanup.exceptionCaught = false;
}

inline bool QCoreApplication::sendEvent(QObject *receiver, QEvent *event)

inline bool QCoreApplication::sendEvent(QObject *receiver, QEvent *event)
{  if (event) event->spont = false; return notifyInternal2(receiver, event); }

bool QCoreApplication::notifyInternal2(QObject *receiver, QEvent *event)
{
    bool selfRequired = QCoreApplicationPrivate::threadRequiresCoreApplication();
    if (!self && selfRequired)
        return false;

    // Make it possible for Qt Script to hook into events even
    // though QApplication is subclassed...
    bool result = false;
    void *cbdata[] = { receiver, event, &result };
    if (QInternal::activateCallbacks(QInternal::EventNotifyCallback, cbdata)) {
        return result;
    }

    // Qt enforces the rule that events can only be sent to objects in
    // the current thread, so receiver->d_func()->threadData is
    // equivalent to QThreadData::current(), just without the function
    // call overhead.
    QObjectPrivate *d = receiver->d_func();
    QThreadData *threadData = d->threadData;
    QScopedScopeLevelCounter scopeLevelCounter(threadData);
    if (!selfRequired)
        return doNotify(receiver, event);
    return self->notify(receiver, event);  //通知事件执行
}

bool QCoreApplication::notify(QObject *receiver, QEvent *event)
{
    // no events are delivered after ~QCoreApplication() has started
    if (QCoreApplicationPrivate::is_app_closing)
        return true;
    return doNotify(receiver, event);
}

static bool doNotify(QObject *receiver, QEvent *event)
{
    if (receiver == 0) {                        // serious error
        qWarning("QCoreApplication::notify: Unexpected null receiver");
        return true;
    }

#ifndef QT_NO_DEBUG
    QCoreApplicationPrivate::checkReceiverThread(receiver);
#endif

    return receiver->isWidgetType() ? false : QCoreApplicationPrivate::notify_helper(receiver, event);
}

bool QCoreApplicationPrivate::notify_helper(QObject *receiver, QEvent * event)
{
    // send to all application event filters (only does anything in the main thread)
    if (QCoreApplication::self
            && receiver->d_func()->threadData->thread == mainThread()
            && QCoreApplication::self->d_func()->sendThroughApplicationEventFilters(receiver, event))
        return true;
    // send to all receiver event filters
    if (sendThroughObjectEventFilters(receiver, event)) //事件过滤的地方
        return true;
    // deliver the event
    return receiver->event(event);  //这里就进入event了，我们可以通过重写其来改变响应
}

bool QObject::event(QEvent *e)

bool QObject::event(QEvent *e)
{
    switch (e->type()) {
    case QEvent::Timer:
        timerEvent((QTimerEvent*)e);
        break;

    case QEvent::ChildAdded:
    case QEvent::ChildPolished:
    case QEvent::ChildRemoved:
        childEvent((QChildEvent*)e);
        break;

    case QEvent::DeferredDelete:
        qDeleteInEventHandler(this);
        break;

    case QEvent::MetaCall:
        {
            QMetaCallEvent *mce = static_cast<QMetaCallEvent*>(e);

            QConnectionSenderSwitcher sw(this, const_cast<QObject*>(mce->sender()), mce->signalId());

            mce->placeMetaCall(this);
            break;
        }

    case QEvent::ThreadChange: {
        Q_D(QObject);
        QThreadData *threadData = d->threadData;
        QAbstractEventDispatcher *eventDispatcher = threadData->eventDispatcher.load();
        if (eventDispatcher) {
            QList<QAbstractEventDispatcher::TimerInfo> timers = eventDispatcher->registeredTimers(this);
            if (!timers.isEmpty()) {
                // do not to release our timer ids back to the pool (since the timer ids are moving to a new thread).
                eventDispatcher->unregisterTimers(this);
                QMetaObject::invokeMethod(this, "_q_reregisterTimers", Qt::QueuedConnection,
                                          Q_ARG(void*, (new QList<QAbstractEventDispatcher::TimerInfo>(timers))));
            }
        }
        break;
    }

    default:
        if (e->type() >= QEvent::User) { //用户自定义事件，将执行自定义的customEvent
            customEvent(e);
            break;
        }
        return false;
    }
    return true;
}

  这段代码中并没有涉及到鼠标、键盘等事件，但是在bool QWidget::event(QEvent *event)中，有对应的响应函数，并在末尾，执行 QObject::event(event)，从而确保事件层层向上传递。

参考链接1: https://blog.csdn.net/simonyucsdy/article/details/82749539
参考链接2: https://blog.csdn.net/weixin_42981623/article/details/107593343
参考链接3: https://blog.csdn.net/imxiangzi/article/details/48176417