qml与c++交互，不涉及传参

注意事项：

新建的c++类必须是QObject的，我这边的Qt在我新建完一个类的时候，是不会自动导入头文件的，所以需要自己导入头文件。

进入正题

当我右键 -> 添加新文件 -> C++ class

qt新建QObject.png

需要交互，则c++必须是个槽函数

#include <QObject> //这里需要手动导入
class UA4Qml2 : public QObject // 这里必须是QObject的子类
{
    Q_OBJECT
public:
    UA4Qml2(QObject *parent = 0);

signals:

public slots:
    void loadUrl(); // 这里可以由qml进行调用
};

然后需要在入口函数加入以下代码

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    QQmlApplicationEngine *engine = new QQmlApplicationEngine();
    engine->rootContext()->setContextProperty("$SigDispatcher", new UA4Qml2); //这里请注意
    engine->load(QUrl(QStringLiteral("qrc:/main.qml")));

    return app.exec();
}

然后是qml，正常调用

import QtQuick 2.5
import QtQuick.Controls 1.4

ApplicationWindow {
    visible: true
    width: 640
    height: 480

    Button{
        id:loadUrl
        text: "测试加载url"
        onClicked: {
            // 这里就是调用的地方
            $SigDispatcher.loadUrl();
        }
    }
}

然后就可以正常响应了

话外音

然后我们来看一下，如果不加QObject 会报什么问题

class  UA4QML{
public :
    UA4QML();
};

然后在

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
#include "ua4qml2.h"
#include "ua4qml.h"

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    QQmlApplicationEngine *engine = new QQmlApplicationEngine();
    //这里注意是调用的UA4Qml，而非上面的UA4Qml2
    //这里调用的不是QObject的子类
    engine->rootContext()->setContextProperty("$SigDispatcher", new UA4QML);
    engine->load(QUrl(QStringLiteral("qrc:/main.qml")));

    return app.exec();
}

查看报的错误

../testCurlPrj/main.cpp: In function ‘int main(int, char**)’:
../testCurlPrj/main.cpp:12:75: error: ‘QVariant::QVariant(void*)’ is private within this context
     engine->rootContext()->setContextProperty("$SigDispatcher", new UA4QML);
                                                                           ^
In file included from /usr/include/x86_64-linux-gnu/qt5/QtCore/qlocale.h:43:0,
                 from /usr/include/x86_64-linux-gnu/qt5/QtGui/qguiapplication.h:46,
                 from /usr/include/x86_64-linux-gnu/qt5/QtGui/QGuiApplication:1,
                 from ../testCurlPrj/main.cpp:1:
/usr/include/x86_64-linux-gnu/qt5/QtCore/qvariant.h:471:12: note: declared private here
     inline QVariant(void *) Q_DECL_EQ_DELETE;
            ^~~~~~~~
../testCurlPrj/main.cpp:12:75: error: use of deleted function ‘QVariant::QVariant(void*)’
     engine->rootContext()->setContextProperty("$SigDispatcher", new UA4QML);
                                                                           ^
In file included from /usr/include/x86_64-linux-gnu/qt5/QtCore/qlocale.h:43:0,
                 from /usr/include/x86_64-linux-gnu/qt5/QtGui/qguiapplication.h:46,
                 from /usr/include/x86_64-linux-gnu/qt5/QtGui/QGuiApplication:1,
                 from ../testCurlPrj/main.cpp:1:
/usr/include/x86_64-linux-gnu/qt5/QtCore/qvariant.h:471:12: note: declared here
     inline QVariant(void *) Q_DECL_EQ_DELETE;
            ^~~~~~~~

其实,我表示我看的时候，特别的头大。报错说：‘QVariant::QVariant(void*)’ is private within this context 看的一顿晕眩。恶心。

如果我们不放到槽函数里面会怎样

#ifndef UA4QML2_H
#define UA4QML2_H

#include <QObject>
class UA4Qml2 : public QObject
{
    Q_OBJECT
public:
    UA4Qml2(QObject *parent = 0);

    void loadUrl(); //这里不是槽函数
signals:

public slots:
};

#endif // UA4QML2_H

查看报的错误：

qrc:/main.qml:14: TypeError: Property 'loadUrl' of object UA4Qml2(0x564d6d5a3c40) is not a function

看着这个错误，还是有点意思的，不是一个方法。

我们不加头文件 #include <QObject>

#ifndef UA4QML2_H
#define UA4QML2_H

//#include <QObject> //这里被我注释掉了
class UA4Qml2 : public QObject
{
    Q_OBJECT
public:
    UA4Qml2(QObject *parent = 0);

    void loadUrl();
signals:

public slots:
};

#endif // UA4QML2_H

看下报的错误

In file included from ../testCurlPrj/ua4qml2.cpp:1:0:
../testCurlPrj/ua4qml2.h:6:1: error: expected class-name before ‘{’ token
 {
 ^
../testCurlPrj/ua4qml2.h:7:5: error: ‘Q_OBJECT’ does not name a type
     Q_OBJECT
     ^~~~~~~~
../testCurlPrj/ua4qml2.h:12:1: error: ‘signals’ does not name a type
 signals:
 ^~~~~~~
../testCurlPrj/ua4qml2.cpp:4:17: error: expected constructor, destructor, or type conversion before ‘(’ token
 UA4Qml2::UA4Qml2(QObject *parent) : QObject(parent)
                 ^
Makefile:480: recipe for target 'ua4qml2.o' failed

额，看到这样的错误，也是一顿抽搐，不知所云。话说可能我是java转过来的，所以看上去就不是特别的友好，说好的c++ 是最好的语言呢？