29——Extending QML – Property Value Source Example
最后更新于：2022-04-01 07:22:10
本系列所有文章可以在这里查看[http://blog.csdn.net/cloud_castle/article/category/2123873](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](http://blog.csdn.net/cloud_castle/article/details/37512407)
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
~~~
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:";
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需要通过 on 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: 7
shoe.color: "red"
shoe.brand: "Marc Jacobs"
shoe.price: 699.99
}
// ![1]
}
// ![1]
~~~
效果如下：
