如何在C++代码中连接QML代码中的信号

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在QML应用设计中，C++在很多的时候作为一个语言来为应用做一些需要计算或拓展QML功能的选项．在今天的例程中，我们来介绍如何连接QML代码中的信号．具体更多的阅读，可以参阅文章＂Interacting with QML Objects from C++＂．

为了说明问题的方便，我们还是来贴上我们的代码：

myclass.h

#ifndef MYCLASS_H#define MYCLASS_H#include <QObject>#include <QQuickItem>#include <QDebug>class MyClass : public QObject{    Q_OBJECTpublic:    MyClass(QObject *parent = 0);public slots:    // This is a slot for QString only    void cppSlot(const QString &msg) {        qDebug() << "Called the C++ slot with message:" << msg;    }    // This is the slot for a generic data type    void cppSlotForGenericData(const QVariant &v) {        qDebug() << "Called the C++ slot with value:" << v;        QQuickItem *item = qobject_cast<QQuickItem*>(v.value<QObject*>());        qDebug() << "Item dimensions:" << item->width() << item->height();    }};#endif // MYCLASS_H

在这里，我们定义了两个slots．第一个slot是用来接受一个QString来作为参数的．第二个slot是用一个通用的QVariant来作为参数，也即它可以用任何一个类型的数据来传人，包括第一种情况．具体关于QML的基本数据类型，可以参阅文章＂QML Basic Types＂．

main.cpp

#include <QGuiApplication>#include <QQmlApplicationEngine>#include <QQuickView>#include <QQuickItem>#include "myclass.h"int main(int argc, char *argv[]){    QGuiApplication app(argc, argv);    QQuickView view;    view.setSource(QUrl(QStringLiteral("qrc:///Main.qml")));    view.setResizeMode(QQuickView::SizeRootObjectToView);    QObject *item = view.rootObject();    MyClass myClass;    // Connect a QString data type slot    QObject::connect(item, SIGNAL(qmlSignal(QString)),                     &myClass, SLOT(cppSlot(QString)));    // Connect a generic slot    QObject::connect(item, SIGNAL(qmlSignalGeneric(QVariant)),                         &myClass, SLOT(cppSlotForGenericData(QVariant)));    view.show();    return app.exec();}

在上面的代码中，我们把view的rootObject，也即我们下面即将讲到的MainView，连到我们定义的两个signal:

    signal qmlSignal(string msg)    signal qmlSignalGeneric(var anObject)

我们的Main.qml设计也非常简单：

Main.qml

import QtQuick 2.0import Ubuntu.Components 1.1/*!    \brief MainView with a Label and Button elements.*/MainView {    id: main    // objectName for functional testing purposes (autopilot-qt5)    objectName: "mainView"    // Note! applicationName needs to match the "name" field of the click manifest    applicationName: "connectiontoqmlsignal.liu-xiao-guo"    /*     This property enables the application to change orientation     when the device is rotated. The default is false.    */    //automaticOrientation: true    // Removes the old toolbar and enables new features of the new header.    useDeprecatedToolbar: false    signal qmlSignal(string msg)    signal qmlSignalGeneric(var anObject)    width: units.gu(60)    height: units.gu(85)    Page {        id: page        title: i18n.tr("connectiontoqmlsignal")        Column {            anchors.fill: parent            spacing: units.gu(2)            Button {                width: parent.width                text: i18n.tr("Send signal to C++!")                onClicked: {                    main.qmlSignal("Hello from QML!")                }            }            Button {                width: parent.width                text: "Send a generic data to C++"                onClicked: {                    main.qmlSignalGeneric(main)                }            }        }    }}

运行我们的应用：

应用的输出：

QML debugging is enabled. Only use this in a safe environment.Loading module: 'libubuntu_application_api_touch_mirclient.so.3.0.0'Called the C++ slot with message: "Hello from QML!"Called the C++ slot with value: QVariant(QObject*, Main_QMLTYPE_46(0xdebec0, name = "mainView") )Item dimensions: 540 919

整个项目的源码在：https://github.com/liu-xiao-guo/connectiontoqmlsignal

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