Qt5官方demo解析集29——Extending QML - Property Value Source Example

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本系列所有文章可以在这里查看http://blog.csdn.net/cloud_castle/article/category/2123873

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QObjectQ_OBJECTQ_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

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”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:";elseqWarning() << "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";elseqWarning() << "   " << guest->name() << "RSVP date:" << qPrintable(rsvpDate.toString());}party->startParty();} else {qWarning() << component.errors();}return app.exec();
}

最后，这个Source需要通过<PropertyValueSource> on <property>这样的语句来调用，

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: 7shoe.color: "red"shoe.brand: "Marc Jacobs"shoe.price: 699.99 }// ![1]
}
// ![1]

效果如下：