在 QML 中显示帧率

在 QML/C++ 应用程序中是否有一种“简单”的方法来显示 FPS（frame rate 帧率） ？

为了达到这个目的，我们必须创建自己的获取 FPS 的 QQuickItem（或 QQuickPaintedItem） 类，并在 main.cpp 中完成注册，以便在 QML 代码中使用它。

头文件 FPSText.h 的代码如下：

#ifndef FPSTEXT_H_7FE54CFA_E610_4C32_A4D7_A7AF0E56FF04
#define FPSTEXT_H_7FE54CFA_E610_4C32_A4D7_A7AF0E56FF04

#include <QQuickPaintedItem>

class FPSText : public QQuickPaintedItem
{
    Q_OBJECT
    Q_PROPERTY(int fps READ fps NOTIFY fpsChanged)
public:
    FPSText(QQuickItem *parent = nullptr);
    ~FPSText();

    void paint(QPainter *painter);

    int fps() const {
        return _currentFPS;
    }

signals:
    void fpsChanged(int);

private:
    void recalculateFPS();
    int _currentFPS;
    int _cacheCount;
    QVector<qint64> _times;
};

#endif // FPSTEXT_H_7FE54CFA_E610_4C32_A4D7_A7AF0E56FF04

实现文件 FPSText.cpp 的代码如下：

#include "FPSText.h"
#include <QDateTime>
#include <QPainter>

FPSText::FPSText(QQuickItem *parent)
    : QQuickPaintedItem(parent)
    , _currentFPS(0)
    , _cacheCount(0)
{
    _times.clear();
    setFlag(QQuickItem::ItemHasContents);
}

FPSText::~FPSText()
{
}

void FPSText::paint(QPainter *painter)
{
    recalculateFPS();

    painter->save();

    QBrush brush(Qt::yellow);

    painter->setBrush(brush);
    painter->setPen(Qt::NoPen);
    painter->setRenderHint(QPainter::Antialiasing);
    painter->drawRoundedRect(0, 0, boundingRect().width(), boundingRect().height(), 0, 0);

    painter->restore();
    update();
}

void FPSText::recalculateFPS()
{
    qint64 currentTime = QDateTime::currentDateTime().toMSecsSinceEpoch();
    _times.push_back(currentTime);

    while (currentTime - _times.at(0) > 1000) {
        _times.pop_front();
    }

    int currentCount = _times.length();
    _currentFPS = (currentCount + _cacheCount) / 2;

    if (currentCount != _cacheCount) {
        emit fpsChanged(_currentFPS);
    }
    _cacheCount = currentCount;
}

在 main.cpp 中进行注册的代码如下：

#include <QGuiApplication>
#include <QQmlApplicationEngine>

#include <QQmlProperty>
#include <QQmlComponent>

#include "FPSText.h"

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);

    qmlRegisterType<FPSText>("FPSText", 1, 0, "FPSText"); // 注册

    QQmlApplicationEngine engine;
    const QUrl url(QStringLiteral("qrc:/main.qml"));
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                     &app, [url](QObject *obj, const QUrl &objUrl) {
        if (!obj && url == objUrl)
            QCoreApplication::exit(-1);
    }, Qt::QueuedConnection);
    engine.load(url);

    return app.exec();
}

测试使用的 QML 的代码如下：

import QtQuick 2.12
import QtQuick.Window 2.12
import FPSText 1.0

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello FPSText")

    FPSText {
        width: 200
        height: 200
        anchors.centerIn: parent

        Text {
            anchors.centerIn: parent
            text: parent.fps.toFixed(2)
        }
    }
}

参考链接： Show FPS in QML