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

我们经常会在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这种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，我们可以实现自定义的Property Value Source。

在前面的项目中添加一个类happybirthdaysong，用来自定义地控制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 // 由于QQmlPropertyValueSource是一个接口类
{                                                                        // 我们还需要继承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

#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”参数
{
    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类则单纯地添加了一个属性announcement来使上面的Source能作用其上，它与其他属性没有不同，类型为QString，用来赋予不同的歌词。

Q_PROPERTY(QString announcement READ announcement WRITE setAnnouncement)

main.cpp也没有改动：

#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();
}

因此在QML文件中添加了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]