Qt：将 C++ 类暴露给 QML 时出错

Question

我在运行应用程序时不断收到相同的错误：

qrc:/main.qml:13: ReferenceError: _myClass is not defined

错误在哪里？

此外，如果我想将信号 myIntChanged 连接到插槽，我应该将连接放在哪里？在 main.cpp 中还是在构造函数 MyClass 中？

myclass.h

#ifndef MYCLASS_H
#define MYCLASS_H

#include <QObject>
#include <QString>
#include <QDebug>
#include <QTimer>

class MyClass : public QObject
{
    Q_OBJECT

public:
    MyClass(QObject *parent = 0);

    Q_PROPERTY(int myInt READ getMyInt WRITE setMyInt NOTIFY myIntChanged)
    int myInt;
    inline int getMyInt() { return myInt; }
    inline void setMyInt(int _value) { myInt = _value; }

private:
    void initVariables();

    QTimer *timer;

private slots:
    void editVariables();

signals:
    void myIntChanged();
};

#endif // MYCLASS_H

myclass.cpp

#include "myclass.h"

MyClass::MyClass(QObject *parent) : QObject (parent)
{
    initVariables();

    timer = new QTimer(this);
    connect(timer, SIGNAL(timeout()), this, SLOT(editVariables()));
    timer -> start(3000);
}

void MyClass::initVariables()
{
    myInt = 0;
}

void MyClass::editVariables()
{
    myInt = myInt + 1;

    qDebug() << "myclass.cpp: timeout: variables edited.";
}

ma​​in.cpp

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>

#include "myclass.h"

int main(int argc, char *argv[])
{
#if defined(Q_OS_WIN)
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
#endif

    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    if (engine.rootObjects().isEmpty())
        return -1;

    MyClass *myClass = new MyClass();
    engine.rootContext() -> setContextProperty("_myClass", myClass);

    return app.exec();
}

ma​​in.qml

import QtQuick 2.9
import QtQuick.Window 2.2

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Exposed C++ Class")

    Text {
        anchors.top: parent.top; anchors.topMargin: 30
        anchors.left: parent.left; anchors.leftMargin: 30
        text: _myClass.myInt
    }
}

Answer 1

问题是因为load()语句正在加载.qml文件，当读取它并验证变量是否存在时，他们意识到这个对象不存在，因为它是通过setContextProperty()传递给他们的，一种可能的解决方案是在加载 .qml 之前将对象传递给它：

int main(int argc, char *argv[])
{
#if defined(Q_OS_WIN)
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
#endif

    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    MyClass *myClass = new MyClass();
    engine.rootContext()->setContextProperty("_myClass", myClass);
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    if (engine.rootObjects().isEmpty())
        return -1;
    return app.exec();
}

没有更新是因为没有通过你声明的信号得到通知，正确的做法是每次属性变化时发出一个信号，你必须修改一些方法：

inline void setMyInt(int _value) {
    if(myInt == _value)
        return;
    myInt = _value;
    emit myIntChanged();
}

---

void MyClass::editVariables()
{
    setMyInt(getMyInt()+1);
    qDebug() << "myclass.cpp: timeout: variables edited.";
}