Qt 5.11 | Qt Widgets | Qt Widgets Examples | Digital Clock Example

Qt 5.11.3 Reference Documentation

# Digital Clock Example

The Digital Clock example shows how to use QLCDNumber to display a number with LCD-like digits.

Screenshot of the Digital Clock example

This example also demonstrates how QTimer can be used to update a widget at regular intervals.

## DigitalClock Class Definition

The `DigitalClock` class provides a clock widget showing the time with hours and minutes separated by a blinking colon. We subclass QLCDNumber and implement a private slot called `showTime()` to update the clock display:

      class DigitalClock : public QLCDNumber
      {
          Q_OBJECT

      public:
          DigitalClock(QWidget *parent = 0);

      private slots:
          void showTime();
      };

## DigitalClock Class Implementation

      DigitalClock::DigitalClock(QWidget *parent)
          : QLCDNumber(parent)
      {
          setSegmentStyle(Filled);

          QTimer *timer = new QTimer(this);
          connect(timer, &QTimer::timeout, this, &DigitalClock::showTime);
          timer->start(1000);

          showTime();

          setWindowTitle(tr("Digital Clock"));
          resize(150, 60);
      }

In the constructor, we first change the look of the LCD numbers. The QLCDNumber::Filled style produces raised segments filled with the foreground color (typically black). We also set up a one-second timer to keep track of the current time, and we connect its timeout() signal to the private `showTime()` slot so that the display is updated every second. Then, we call the `showTime()` slot; without this call, there would be a one-second delay at startup before the time is shown.

      void DigitalClock::showTime()
      {
          QTime time = QTime::currentTime();
          QString text = time.toString("hh:mm");
          if ((time.second() % 2) == 0)
              text[2] = ' ';
          display(text);
      }

The `showTime()` slot is called whenever the clock display needs to be updated.

The current time is converted into a string with the format "hh:mm". When QTime::second() is a even number, the colon in the string is replaced with a space. This makes the colon appear and vanish every other second.

Finally, we call QLCDNumber::display() to update the widget.

Files:

- widgets/digitalclock/digitalclock.cpp
- widgets/digitalclock/digitalclock.h
- widgets/digitalclock/digitalclock.pro
- widgets/digitalclock/main.cpp