fpsitem.cpp

#include "fpsitem.h"

#include <QElapsedTimer>

FPSItem::FPSItem(QQuickItem *parent)
    : QQuickItem(parent)
    , m_averageMsPerFrame(0)
{
    setFlag(ItemHasContents);
}

int FPSItem::fps() const
{
    return 1000 / m_averageMsPerFrame;
}

class AutoStartingElapsedTimer : public QElapsedTimer
{
public:
    AutoStartingElapsedTimer() : QElapsedTimer() { start(); }
};

QSGNode* FPSItem::updatePaintNode(QSGNode *oldNode, UpdatePaintNodeData *data)
{
    Q_UNUSED(data);

    static AutoStartingElapsedTimer timer;
    static qreal smoothingFactor = 0.1;

    m_averageMsPerFrame = (m_averageMsPerFrame * (1.0 - smoothingFactor))
            + (timer.restart() * smoothingFactor);

    emit fpsChanged();

    update(); // Ensure that we'll be called again for the next frame

    return oldNode;
}

fpsitem.h

#ifndef FPSITEM_H
#define FPSITEM_H

#include <QQuickItem>

class FPSItem : public QQuickItem
{
    Q_OBJECT
    Q_PROPERTY(int fps READ fps NOTIFY fpsChanged)
public:
    explicit FPSItem(QQuickItem *parent = 0);
    int fps() const;

signals:
    void fpsChanged();

protected:
    virtual QSGNode *updatePaintNode(QSGNode *, UpdatePaintNodeData *);

private:
    qreal m_averageMsPerFrame;
};

QML_DECLARE_TYPE(FPSItem)

#endif // FPSITEM_H