QTimer的一些注意事项和探索

注意事项：

1、QTimer's accuracy depends on the underlying operating system and hardware.Most platforms support an accuracy of 20 milliseconds; some provide more.

2、If Qt is unable to deliver the requested number of timer events, it will silently discard some.

问题：当开启一个定时器，设置时间间隔为50ms，它对应的QTimerEvnet处理函数要执行一段耗时的操作，如何让界面快速响应用户的鼠标事件呢

代码：

#include "QtGuiApplication2.h"
#include <QDebug>
#include <QTimerEvent>
#include <QDateTime>
#include <QTime>
#include <QThread>

QtGuiApplication2::QtGuiApplication2(QWidget *parent)
    : QDialog(parent)
{
    ui.setupUi(this);
    startTimer();
    connect(ui.pushButton, &QPushButton::clicked, this, &QtGuiApplication2::showCurrentTime);
}

void QtGuiApplication2::timerEvent(QTimerEvent *event)
{
    //QThread::msleep(2000);这个和下面的效果是一样的
    QTime qTime1 = QTime::currentTime();
    while (qTime1.msecsTo(QTime::currentTime())<)
        qDebug() << "do nothing just for test";
}

void QtGuiApplication2::showCurrentTime()
{
    ui.label->setText(QDateTime::currentDateTime().toString("hh:mm:ss.zzz"));
}

Ui:

当运行程序的时候，点击按钮发现textLabel的响应时间差不多是2秒，通过text显示的时间可以计算出来

解决方法：

void QtGuiApplication2::timerEvent(QTimerEvent *event)
{
    //QThread::msleep(2000);
     QTime qTime1 = QTime::currentTime();
    while (qTime1.msecsTo(QTime::currentTime()) < ) {
        QCoreApplication::processEvents(QEventLoop::AllEvents, );
        qDebug() << "do nothing just for test";
    }

}

这个解决了操作耗时的情况，但是当你选择QThread::msleep(2000);那怎么都会卡了

---

疑问：

假设当前时刻为0;程序开始运行；100ms时刻，我按下按钮；此时事件队列里面应该有QTimerEvent;QTimerEvent;QMousePress这三个事件要处理，按理说Label的时间相应应该是4s左右啊，可能的解释就是If Qt is unable to deliver the requested number of timer events, it will silently discard some.因此它丢弃了QMousePress之前相同的QTimerEvent

此实验验证了这个观念：

#include <QtWidgets/QDialog>
#include <QTime>
#include "ui_QtGuiApplication2.h"

class QtGuiApplication2 : public QDialog
{
    Q_OBJECT

public:
    QtGuiApplication2(QWidget *parent = Q_NULLPTR);
protected:
    void timerEvent(QTimerEvent *event) override;
    void showCurrentTime();
private:
    Ui::QtGuiApplication2Class ui;
    int id;
    QTime lastTime;
};

QtGuiApplication2::QtGuiApplication2(QWidget *parent)
    : QDialog(parent)
{
    ui.setupUi(this);
    startTimer();
    id = startTimer();
    lastTime = QTime::currentTime();
}

void QtGuiApplication2::timerEvent(QTimerEvent *event)
{
    qDebug() << "current timer id:  " << event->timerId();
    if (event->timerId() == id) {
        ui.label->setText(QString::number(lastTime.msecsTo(QTime::currentTime())));
        lastTime= QTime::currentTime();
    }
}

通过界面的Label显示基本都是1014左右；多出来的14毫秒就是执行50msQTimerEvent的输出语句和 lastTime= QTime::currentTime()的总耗时；

因此可以推测出：

当有个定时器在事件队列里有很多个QTimerEvent;那么当有另外一个事件A到了，在执行完当前的QTimerEvent之后它会丢弃A之前其他QTimerEvent；直接执行事件A

所以14毫秒就是执行50msQTimerEvent的输出语句和 lastTime= QTime::currentTime()的总耗时