Qt WidgetsのQMdiArea::timerEvent()：タイマーを活用したサブウィンドウ操作のベストプラクティス

Understanding QMdiArea::timerEvent()

The QMdiArea::timerEvent() method is a virtual protected member function that gets called whenever a QTimer associated with the QMdiArea widget emits a timeout signal. This method provides a mechanism for handling timed events and implementing custom behavior based on the timer's expiration.

Implementing QMdiArea::timerEvent()

To implement the QMdiArea::timerEvent() method, you'll need to override the virtual protected function in your QMdiArea subclass. The method takes a single argument, a QTimerEvent object, which provides information about the timer that triggered the event.

Within the QMdiArea::timerEvent() implementation, you can perform the desired actions based on the timer's expiration. For instance, you could update the state of the QMdiArea widget, manipulate its subwindows, or trigger other events.

Example Usage

Consider a scenario where you want to automatically cascade the subwindows within an QMdiArea widget every 5 seconds. To achieve this, you can follow these steps:

1. Create a QTimer object and set its interval to 5000 milliseconds (5 seconds).

2. Connect the QTimer object's timeout signal to the QMdiArea::cascadeSubWindows() slot.

3. Start the QTimer object.

4. Override the QMdiArea::timerEvent() method in your QMdiArea subclass.

5. Inside the QMdiArea::timerEvent() method, check if the timer that triggered the event is the one you created for cascading subwindows.

6. If it is, call the QMdiArea::cascadeSubWindows() method to cascade the subwindows.

• Use QTimer::stop() to stop timers when they are no longer needed to avoid unnecessary resource consumption.

• If you have multiple timers associated with the QMdiArea widget, you'll need to distinguish between them based on the QTimerEvent object's timer() method.

• When overriding QMdiArea::timerEvent(), ensure you call the base class implementation using super::timerEvent(event). This ensures that any default behavior associated with the event is handled correctly.

---

#include <QApplication>
#include <QMdiArea>
#include <QTimer>

class MyMdiArea : public QMdiArea {
public:
    MyMdiArea(QWidget *parent = nullptr) : QMdiArea(parent) {
        // Create and start the timer
        timer = new QTimer(this);
        timer->setInterval(5000);
        connect(timer, &QTimer::timeout, this, &MyMdiArea::cascadeSubWindows);
        timer->start();
    }

protected:
    void timerEvent(QTimerEvent *event) override {
        // Check if the timer is the one for cascading subwindows
        if (event->timer() == timer) {
            cascadeSubWindows();
        } else {
            // Handle other timers if present
            // ...
        }

        // Call the base class implementation
        super::timerEvent(event);
    }

private:
    QTimer *timer;
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    MyMdiArea mdiArea;
    mdiArea.setWindowTitle("QMdiArea Timer Example");
    mdiArea.show();

    // Add some subwindows to the QMdiArea
    for (int i = 0; i < 3; ++i) {
        QMdiSubWindow *subWindow = new QMdiSubWindow(&mdiArea);
        subWindow->setWindowTitle(QString("Subwindow %1").arg(i + 1));
        subWindow->show();
    }

    return app.exec();
}

---

1. Using QTimer::singleShot()
   

   The QTimer::singleShot() method can be employed to execute a specific action once after a specified interval. To cascade subwindows every 5 seconds, you could:

   QTimer::singleShot(5000, this, &MyMdiArea::cascadeSubWindows);
   

   This approach is suitable for one-time actions or when you want to trigger the event only at specific intervals.

2. Utilizing QEventLoop::processEvents()
   

   The QEventLoop::processEvents() method can be used to process pending events, including timer events, in a non-blocking manner. You could create a custom timer class that periodically calls processEvents() to check for timer events and cascade subwindows if necessary.

   class MyTimer : public QObject {
   public:
       MyTimer(int interval) : QObject() {
           timer = new QTimer(this);
           timer->setInterval(interval);
           connect(timer, &QTimer::timeout, this, &MyTimer::handleTimeout);
           timer->start();
       }
   
   private:
       void handleTimeout() {
           QEventLoop loop;
           loop.processEvents();
   
           // Check for timer event and cascade subwindows if necessary
           // ...
       }
   
       QTimer *timer;
   };
   

   This approach is suitable for scenarios where you need to handle other events alongside the timer event and have more control over the event processing loop.

3. Employing Signals and Slots
   

   You can create a custom signal that emits whenever you want to cascade subwindows. Connect this signal to a slot that performs the cascading operation. You can trigger the signal using a timer, a user action, or any other event.

   class MyMdiArea : public QMdiArea {
   public:
       MyMdiArea(QWidget *parent = nullptr) : QMdiArea(parent) {
           cascadeSignal = new QSignal(this);
           connect(cascadeSignal, &QSignal::emitted, this, &MyMdiArea::cascadeSubWindows);
       }
   
   signals:
       void cascadeSubWindows();
   
   private:
       QSignal *cascadeSignal;
   };
   

   This approach provides flexibility in decoupling the timer from the cascading action and allows for triggering the cascading from various sources.