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接上文Qt5官方demo解析集28——Extending
QML - Signal Support Example

我们常常会在QML代码中使用Animation和bindings。以使得我们的程序具有更好的动态性能。那么，类似NumberAnimation这样的QML类似实际上是提供了一个算法来为属性提供动态变化的数值。或者说是提供了一个值的集合。这里Qt将其称作“属性值来源”(Property Value Source)，并为这些QML类型提供了一个通用的接口。即QQmlPropertyValueSource。通过继承这个类，我们能够实现自己定义的Property
Value Source。

在前面的项目中加入一个类happybirthdaysong，用来自己定义地控制BirthdayParty中announcement属性的变化：

这个demo向我们展示了这个自己定义的过程。happybirthdaysong.h:

#ifndef HAPPYBIRTHDAYSONG_H

#define HAPPYBIRTHDAYSONG_H

#include <QQmlPropertyValueSource>

#include <QQmlProperty>

#include <qqml.h>

#include <QStringList>

// ![0]

class HappyBirthdaySong : public QObject, public QQmlPropertyValueSource // 因为QQmlPropertyValueSource是一个接口类

{                                                                        // 我们还须要继承QObject

    Q_OBJECT

    Q_INTERFACES(QQmlPropertyValueSource)            // 声明接口

// ![0]

    Q_PROPERTY(QString name READ name WRITE setName)  // name属性用来设置生日歌的对象

// ![1]

public:

    HappyBirthdaySong(QObject *parent = 0);

    virtual void setTarget(const QQmlProperty &);  // 用来指明作用的属性对象

// ![1]                                            // <PropertyValueSource> on <property>时被调用

    QString name() const;                          // 自己定义属性的读写函数

    void setName(const QString &);

private slots:

    void advance();                            // 更新函数，每秒输出一句歌词

private:

    int m_line;

    QStringList m_lyrics;

    QQmlProperty m_target;

    QString m_name;

// ![2]

};

// ![2]

#endif // HAPPYBIRTHDAYSONG_H

happybirthdaysong.cpp:

#include "happybirthdaysong.h"

#include <QTimer>

HappyBirthdaySong::HappyBirthdaySong(QObject *parent)

: QObject(parent), m_line(-1)                          // 初始化m_line为-1

{                                                      // 使advance()第一次被调用时输出第一句歌词

    setName(QString());

    QTimer *timer = new QTimer(this);

    QObject::connect(timer, SIGNAL(timeout()), this, SLOT(advance()));

    timer->start(1000);

}

void HappyBirthdaySong::setTarget(const QQmlProperty &p)

{

    m_target = p;                         // 该类型作用于某个属性时，Qt会使用这里的函数

}

QString HappyBirthdaySong::name() const

{

    return m_name;

}

void HappyBirthdaySong::setName(const QString &name)  // 初始化歌词。并带上“姓名”參数

{

    m_name = name;

    m_lyrics.clear();

    m_lyrics << "Happy birthday to you,";

    m_lyrics << "Happy birthday to you,";

    m_lyrics << "Happy birthday dear " + m_name + ",";

    m_lyrics << "Happy birthday to you!";

    m_lyrics << "";

}

void HappyBirthdaySong::advance()            // 循环显示的好方式

{

    m_line = (m_line + 1) % m_lyrics.count();

    m_target.write(m_lyrics.at(m_line));

}

Person类没有变化，而BirthdayParty类则单纯地加入了一个属性announcement来使上面的Source能作用其上，它与其它属性没有不同，类型为QString。用来赋予不同的歌词。

Q_PROPERTY(QString announcement READ announcement WRITE setAnnouncement)

main.cpp也没有修改：

#include <QCoreApplication>

#include <QQmlEngine>

#include <QQmlComponent>

#include <QDebug>

#include "birthdayparty.h"

#include "happybirthdaysong.h"

#include "person.h"

int main(int argc, char ** argv)

{

    QCoreApplication app(argc, argv);

    qmlRegisterType<BirthdayPartyAttached>();

    qmlRegisterType<BirthdayParty>("People", 1,0, "BirthdayParty");

    qmlRegisterType<HappyBirthdaySong>("People", 1,0, "HappyBirthdaySong");

    qmlRegisterType<ShoeDescription>();

    qmlRegisterType<Person>();

    qmlRegisterType<Boy>("People", 1,0, "Boy");

    qmlRegisterType<Girl>("People", 1,0, "Girl");

    QQmlEngine engine;

    QQmlComponent component(&engine, QUrl("qrc:example.qml"));

    BirthdayParty *party = qobject_cast<BirthdayParty *>(component.create());

    if (party && party->host()) {

        qWarning() << party->host()->name() << "is having a birthday!";

        if (qobject_cast<Boy *>(party->host()))

            qWarning() << "He is inviting:";

        else

            qWarning() << "She is inviting:";

        for (int ii = 0; ii < party->guestCount(); ++ii) {

            Person *guest = party->guest(ii);

            QDate rsvpDate;

            QObject *attached =

                qmlAttachedPropertiesObject<BirthdayParty>(guest, false);

            if (attached)

                rsvpDate = attached->property("rsvp").toDate();

            if (rsvpDate.isNull())

                qWarning() << "   " << guest->name() << "RSVP date: Hasn't RSVP'd";

            else

                qWarning() << "   " << guest->name() << "RSVP date:" << qPrintable(rsvpDate.toString());

        }

        party->startParty();

    } else {

        qWarning() << component.errors();

    }

    return app.exec();

}

最后。这个Source须要通过<PropertyValueSource> on <property>这种语句来调用，

因此在QML文件里加入了HappyBirthdaySongonannouncement{name:"BobJones"}，使得这个程序得以循环地为Bob
Jones唱生日快乐歌：

example.qml:

import People 1.0

import QtQuick 2.0  // For QColor

// ![0]

BirthdayParty {

    HappyBirthdaySong on announcement { name: "Bob Jones" }

// ![0]

    onPartyStarted: console.log("This party started rockin' at " + time);

    host: Boy {

        name: "Bob Jones"

        shoe { size: 12; color: "white"; brand: "Nike"; price: 90.0 }

    }

    Boy {

        name: "Leo Hodges"

        BirthdayParty.rsvp: "2009-07-06"

        shoe { size: 10; color: "black"; brand: "Reebok"; price: 59.95 }

    }

    Boy {

        name: "Jack Smith"

        shoe { size: 8; color: "blue"; brand: "Puma"; price: 19.95 }

    }

    Girl {

        name: "Anne Brown"

        BirthdayParty.rsvp: "2009-07-01"

        shoe.size: 7

        shoe.color: "red"

        shoe.brand: "Marc Jacobs"

        shoe.price: 699.99

    }

// ![1]

}

// ![1]

效果例如以下：