将 qml 项目值传递给 cpp 后如何与它们进行交互?

您在QML 中创建的后端对象

BackEnd {
    id: backend
}

它与C++中创建的那个不同：

BackEnd m_can;

有几种可能的解决方案：

1. 通过 setContextProperty() 导出后端对象

与其他建议的解决方案相比，它的优势在于现在可以从任何QML(如console.log())访问后端对象。

#include <QQmlContext>
// ...
int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);
BackEnd m_can;
m_can.oneShotConnectCan(); //creating CAN device
m_can.run(); //sending the CAN message
QQmlApplicationEngine engine;
engine.rootContext()->setContextProperty("backend", &m_can);
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
return app.exec();
}

*.qml

Window {
id: window
objectName: "window"
visible: true
visibility: Window.FullScreen
onWindowStateChanged: {
console.log( "onWindowStateChanged (Window), state: " + windowState );
}
Dial {
id: dial
x: 181
y: 38
stepSize: 1
to: 255
onValueChanged: backend.elemVal = value
}
}

与其他答案相比，一个优势是不使用可能产生问题的旧连接样式，因为它的验证是在运行时而不是在编译时完成的。此外，代码依赖于 QML 结构。

我还假设你想在 elemVal发生变化时发送一个帧，所以理想的情况是在 elemValChanged 和 sendCanFrame() 之间的信号之间建立连接：

BackEnd::BackEnd(QObject *parent) : QObject(parent)
{
connect(this, &Backend::elemValChanged, this, &Backend::sendCanFrame);
}

2. 创建 QML 类型

有时需要在创建对象后启动某些资源，在这些情况下，您可以在 QML 中使用 Component.onDone 或使用 QQmlParserStatus，在这种情况下我将使用第二种方法。

*.h

class BackEnd : public QObject, public QQmlParserStatus
{
Q_OBJECT
Q_INTERFACES(QQmlParserStatus)
Q_PROPERTY(int elemVal READ getElemVal WRITE setElemVal NOTIFY elemValChanged)
public:
explicit BackEnd(QObject *parent = nullptr);
void classBegin();
void componentComplete();
//elemVal
int getElemVal();
void setElemVal(const int &elemVal);
signals:
void elemValChanged();
public slots:
void sendCanFrame();
private:
//can
void run();
void oneShotConnectCan();
QCanBusDevice *m_canDevice = nullptr;
int m_elemVal;
};

*。.cpp

BackEnd::BackEnd(QObject *parent) : QObject(parent)
{
connect(this, &Backend::elemValChanged, this, &Backend::sendCanFrame);
}
void BackEnd::classBegin(){}
void BackEnd::componentComplete()
{
oneShotConnectCan(); //creating CAN device
run()
}
//elemVal get set
int BackEnd::getElemVal()
{
return m_elemVal;
}
void BackEnd::setElemVal(const int &elemVal)
{
if(elemVal == m_elemVal)
return;
m_elemVal = elemVal;
emit elemValChanged();
qDebug() << "elemVal is: " << m_elemVal;
}
//end of elemVal get set
// ... 
// CAN Bus initialization
// ...
void BackEnd::sendCanFrame()
{
quint32 frameid = 131;
QByteArray payload;
payload[0] = 0x04;
payload[1] = 0x03;
payload[2] = m_elemVal;
QCanBusFrame testFrame(frameid, payload);
testFrame.setFrameType(QCanBusFrame::DataFrame);
m_canDevice->writeFrame(testFrame);
if (m_canDevice->writeFrame(testFrame)) {
qDebug() << "test frame: " << testFrame.toString();
}
else {
qFatal("Write failed");
}
}

主.cpp

static void registerTypes()
{
qmlRegisterType<BackEnd>("io.qt.examples.backend", 1, 0, "BackEnd");
}
Q_COREAPP_STARTUP_FUNCTION(registerTypes)
int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
return app.exec();
}

*.qml

Window {
id: window
objectName: "window"
visible: true
visibility: Window.FullScreen
onWindowStateChanged: {
console.log( "onWindowStateChanged (Window), state: " + windowState );
}
BackEnd{
id: backend
elemVal : dial.value
}
Dial {
id: dial
x: 181
y: 38
stepSize: 1
to: 255
Component.onCompleted: value = backend.elemVal
}
}

• 创建一个可调用的方法，一旦 QML 窗口实例化，就可以调用该方法
• 调用用于在 elemval 更改时发送消息的方法：

后端.h：

#ifndef BACKEND_H
#define BACKEND_H
#include <QObject>
#include <QCanBusDevice>
class BackEnd : public QObject
{
Q_OBJECT
public:
explicit BackEnd(QObject *parent = nullptr);
// elemVal
Q_PROPERTY(int elemVal READ getElemVal WRITE setElemVal NOTIFY elemValChanged)
int getElemVal();
void setElemVal(const int &elemVal);
int m_elemVal;
// The method for your first message
Q_INVOKABLE void sendFirstMessage();
//can
void run();
void oneShotConnectCan();
QCanBusDevice *m_canDevice = nullptr;
signals:
void elemValChanged();
public slots:
void sendCanFrame();
};

后端.cpp：

// Your actual backend.cpp here.
void Backend::sendFirstMessage() {
oneShotConnectCan(); //creating CAN device
run(); //sending the CAN message
}

main.qml：

Window {
id: window
objectName: "window"
visible: true
visibility: Window.FullScreen
onWindowStateChanged: {
console.log( "onWindowStateChanged (Window), state: " + windowState );
}
BackEnd{
id: backend
elemVal: dial.value
onElemValChanged: backend.sendCanFrame()    // You should code this on the C++ side.
}
Dial {
id: dial
x: 181
y: 38
stepSize: 1
to: 255
}
// Wiil be executed when window instanciation has just ended.
Component.onCompleted: backend.sendFirstMessage();
}

有关Component.onCompleted的更多信息，请查看以下内容：https://doc.qt.io/qt-5/qml-qtqml-component.html#completed-signal。

避免从C++端操作 QML。尽可能多地分离后端和前端(这就是为什么我对 eyllanesc 和 Romha Korev 的答案都投了反对票的原因)。