Efficient Qt GUI Refresh: Alternatives to QWindow::requestUpdate()

Functionality

• It simply signals the windowing system that an update might be necessary.
• QWindow::requestUpdate() doesn't directly trigger an immediate redraw of the window.

Better Alternatives

Qt offers more efficient ways to update the GUI:

• update() method
  In custom widgets or for more granular control, you can call the update() method on the specific widget that needs refreshing. This schedules a repaint event for the widget.
• Widget Updates
  Most Qt widgets, like QLabel or QPushButton, handle updates automatically when their properties change (e.g., setting a new text or image).

When to use requestUpdate() (Less Common)

There are rare situations where requestUpdate() might be useful:

• Custom Rendering
  If you're implementing very low-level rendering using a QBackingStore or QOpenGLContext, you might need to call requestUpdate() after manually painting to the window.

---

Example 1: Updating a QLabel with new text

#include <QApplication>
#include <QLabel>

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

  // Create a label widget
  QLabel label;
  label.setText("This is the initial text");
  label.show();

  // Function to update the label text
  void updateLabelText(const QString& newText) {
    label.setText(newText);
  }

  // Update the label text later
  QTimer timer;
  QObject::connect(&timer, &QTimer::timeout, [&updateLabelText]() {
    updateLabelText("The text has been updated!");
  });
  timer.start(2000); // Update every 2 seconds

  return app.exec();
}

In this example, the QLabel updates automatically when its setText method is called. The timer triggers the update function after a delay.

Example 2: Repainting a custom widget

#include <QApplication>
#include <QWidget>
#include <QPainter>

class MyWidget : public QWidget {
  Q_OBJECT

public:
  MyWidget(QWidget* parent = nullptr) : QWidget(parent) {}

protected:
  void paintEvent(QPaintEvent* event) override {
    QPainter painter(this);
    // Your custom drawing code here
    painter.fillRect(0, 0, width(), height(), Qt::red);
  }

  // Function to trigger a repaint
  void updateWidget() {
    update(); // Calls the base class update() which schedules a repaint event
  }
};

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

  MyWidget widget;
  widget.show();

  // Update the widget content later
  QTimer timer;
  QObject::connect(&timer, &QTimer::timeout, [&widget]() {
    widget.updateWidget(); // Triggers repaint event for custom drawing
  });
  timer.start(1000); // Update every 1 second

  return app.exec();
}

This example shows a custom MyWidget with custom painting in its paintEvent. The updateWidget function calls update() to schedule a repaint for the widget when needed.

---

1. Automatic Updates
   

• Most Qt widgets (like QLabel, QPushButton, QLineEdit) handle updates automatically when their properties change. For example, setting a new text for a QLabel or a new value for a QSlider will trigger an update without any additional code.

1. update() method
   

• For custom widgets or more granular control, use the update() method on the specific widget that needs refreshing. This schedules a repaint event for that particular widget, ensuring an update only for the necessary area.

Method	Description	Use Case
Automatic Updates	Widget updates its content automatically when its properties change.	Most common scenario for standard Qt widgets.
update() method	Schedules a repaint event for a specific widget.	Custom widgets or for more precise control.
requestUpdate()	Signals the windowing system that an update might be necessary (less common).	Very specific situations like custom low-level rendering.

• Avoid requestUpdate() unless you're dealing with very low-level rendering using a QBackingStore or QOpenGLContext.
• In most cases, rely on the automatic updates of Qt widgets or the update() method on specific widgets. These are more efficient and recommended for standard GUI updates.