将自定义 C++ 对象传递给 Qml 错误（无属性）

Question

我正在尝试编写代码，通过信号将一些数据从 C++ 引擎传递到 Qml 脚本，但看起来我做错了一些事情，因为当我在 Qml 中接收信号时，我的对象没有任何方法或特性！看看那个代码：

Signaller - 调用信号的类： signaller.h：

class Signaller : public QObject
{
    Q_OBJECT
public:
    explicit Signaller(QObject *parent = 0);
    Q_INVOKABLE void invokeSignal();
signals:
    void mysignal(TestClass test);
public slots:

};

signaller.cpp：

Signaller::Signaller(QObject *parent) :
    QObject(parent)
{
}

void Signaller::invokeSignal()
{
    TestClass s;
    emit mysignal(s);
}

TestClass - 将传递给 Qml 引擎的类，并且在 Qml 脚本中必须有测试方法：

测试.h：

class TestClass : public QObject
{
    Q_OBJECT
public:
    explicit TestClass(QObject *parent = 0);
             TestClass(const TestClass& obj);
             ~TestClass();
     Q_INVOKABLE void test();

signals:

public slots:

};

Q_DECLARE_METATYPE(TestClass)

Test.cpp：

TestClass::TestClass(QObject *parent) :
    QObject(parent)
{
    qDebug()<<"TestClass::TestClass()";
}

TestClass::TestClass(const TestClass &obj) :
    QObject(obj.parent())
{
    qDebug()<<"TestClass::TestClass(TestClass &obj)";
}


TestClass::~TestClass()
{
    qDebug()<<"TestClass::~TestClass()";
}

void TestClass::test()
{
    qDebug()<<"TestClass::test";
}

这两个类也注册在主函数中：

int main(int argc, char *argv[])
{
    // SailfishApp::main() will display "qml/template.qml", if you need more
    // control over initialization, you can use:
    //
    //   - SailfishApp::application(int, char *[]) to get the QGuiApplication *
    //   - SailfishApp::createView() to get a new QQuickView * instance
    //   - SailfishApp::pathTo(QString) to get a QUrl to a resource file
    //
    // To display the view, call "show()" (will show fullscreen on device).

    qmlRegisterType<TestClass>("Test", 1, 0, "Test");
    qmlRegisterType<Signaller>("Signaller", 1, 0, "Signaller");

    return SailfishApp::main(argc, argv);
}

这是我的带有测试代码的 Qml 文件：

import Signaller 1.0

Page {

    Signaller {
        id: sig
        Component.onCompleted: sig.invokeSignal()
        onMysignal: {
            console.log("signal",typeof test);
            console.log(typeof test.test);
        }
    }
}

还有日志：

[D] TestClass::TestClass:6 - TestClass::TestClass() 
[D] TestClass::TestClass:12 - TestClass::TestClass(TestClass &obj) 
[D] TestClass::TestClass:12 - TestClass::TestClass(TestClass &obj) 
[D] onMysignal:41 - signal object
[D] onMysignal:42 - undefined
[D] TestClass::~TestClass:18 - TestClass::~TestClass() 
[D] TestClass::~TestClass:18 - TestClass::~TestClass() 

从日志中可以看出，TestClass.test 字段在传递给 Qml 后为空。 我做错了什么？

Answer 1

您正在通过信号/插槽系统按值传递 QObject 派生对象 - 您应该永远这样做 (docs)。在堆上创建它并发送它的指针。

在这种情况下，您可能希望通过 QML 控制它的生命周期，您可以通过调用 QQmlEngine::setObjectOwnership( s, QQmlEngine::JavaScriptOwnership) 来设置它。

Answer 2

QObject 不应在信号和槽中按值使用。此外，当QObject 本身隐藏其复制构造函数时，为 QObject 子类实现复制构造函数是一个坏主意。

因此，更改您的信号以将指针传递给 QObject 就可以了。有一个short and good reference on how to communicate between C++ and Qml

ps：您不需要注册声明Q_OBJECT 宏的类。

Answer 3

这是我为自己制作的一个用于测试 C++ 的示例 QML 交互：

//File: animal.h
#ifndef ANIMAL_H
#define ANIMAL_H

#include <QObject>

class Animal : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString animal_name READ get_animal_name WRITE set_animal_name)
public:
    explicit Animal(QObject *parent = 0);
    QString get_animal_name();
    void set_animal_name(QString p_name);
private:
    QString     animal_name;

};

#endif // ANIMAL_H

//File: animal.cpp
#include "animal.h"

Animal::Animal(QObject *parent) : QObject(parent)
{
}
void Animal::set_animal_name(QString p_name) {
    animal_name=p_name;
}
QString Animal::get_animal_name() {
    return animal_name;
}

//File: zoo.h
#ifndef ZOO_H
#define ZOO_H

#include <QObject>

class Animal;
class Zoo : public QObject
{
    Q_OBJECT
public:
    explicit Zoo(QObject *parent = 0);
    Q_INVOKABLE Animal* get_animal_by_index(int index);
    Q_INVOKABLE void add_animal(Animal *a);
    Q_INVOKABLE void dump_animal_info(Animal *a);
    Q_INVOKABLE void emit_signal();
signals:
    void some_signal(Animal *animal_object);
public slots:

private:
       QList<Animal*>       animals;
};

#endif // ZOO_H


//File: zoo.cpp
#include <QDebug>
#include "zoo.h"
#include "animal.h"

Zoo::Zoo(QObject *parent) : QObject(parent)
{
    Animal *a;

    a=new Animal();
    a->set_animal_name("Black Bear");
    add_animal(a);

    a=new Animal();
    a->set_animal_name("Gray Wolf");
    add_animal(a);
}
Animal* Zoo::get_animal_by_index(int index) {
    return animals.at(index);
}
void Zoo::add_animal(Animal *a) {
    animals.append(a);
}
void Zoo::dump_animal_info(Animal *a) {
    qWarning() << "animal_name=" << a->get_animal_name();
}
void Zoo::emit_signal() {
    Animal *a;
    a=animals.at(0);
    emit some_signal(a);
}

//File: main.cpp
#include <QGuiApplication>
#include <QQmlApplicationEngine>

#include "animal.h"
#include "zoo.h"

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

    qmlRegisterType<Zoo>("zoo",1,0,"Zoo");
    qmlRegisterType<Animal>("zoo.animal",1,0,"Animal");

    QQmlApplicationEngine engine;
    engine.load(QUrl(QLatin1String("qrc:/main.qml")));

    return app.exec();
}

//File: main.qml
import QtQuick 2.7
import QtQuick.Controls 2.0
import QtQuick.Layouts 1.0

import zoo 1.0
import zoo.animal 1.0

ApplicationWindow {
    visible: true
    width: 640; height: 480; title: qsTr("Zoo")

    Zoo {
        id: zoopark
    }
    Column {
        Button {
            text: "Test C++ <--> QML data exchange by Method"
            onClicked: {
                var animal=zoopark.get_animal_by_index(1);
                zoopark.dump_animal_info(animal);
            }
        }
        Button {
            text: "Text C++ <--> QML data exchage by Signal"
            onClicked: {
                zoopark.emit_signal();
            }
        }
    }
    Connections {
        target: zoopark
        onSome_signal: {
            console.log('signal received');
            console.log('from qml: animal name=' + animal_object.animal_name)
            console.log('dumping animal info from c++:')
            zoopark.dump_animal_info(animal_object)
        }
    }

    function process_signal() {
        console.log('from qml animal name=' + animal_object.animal_name)
        zoopark.dump_animal_info(animal_object)
    }
}