Sunday, January 22, 2012

Animating element on curve in QML

I got comment on my post "Animating object along with curve in Qt using QPainterPath" that how similar thing can be achieved in QML. 

I tried to answer that comment in this post.

Originally I intend to achieve it using only QML but did not find any easy way. So I created a custom QDeclarativeItem that uses QPainterPath and implement required functionality.

Well code it quite simple, but it required little hack so that I can use some QML element like PathQuad or PathLine into my c++ code. 

So let's now move to code, I will try to explain as much as I can.

So let's begin with QML, what I want my QML code to look like. My QML has one rect item which act as circle and my custom QML element which act as curve.

I want my curve element to be drawn on screen and I also want circle to follow the curve. In code path is list which contain different kind of path element.

I was not able to intrepreat QML Path element into C++ class , so I end up creating my own path property. My curve also has pointAtPercentage function which provide point at certain progress on curve, which I use with timer to make dot follow that curve.
import QtQuick 1.0
import MyCurve 1.0

Rectangle {
    width: 500; height: 500

    Rectangle {
        width:10;height:10; radius: 5

    MyCurve {
                PathQuad { x: 400; y: 400; controlX: 100; controlY: 600 },
                PathLine{x: 120; y: 120},
                PathQuad { x: 120; y: 25; controlX: 260; controlY: 75 },
                PathQuad { x: 120; y: 100; controlX: -20; controlY: 75 }

        property variant progress:0;
        interval: 100; running: true; repeat: true
        onTriggered: {
            progress += 0.005;
            if( progress > 1 ) {
                progress  = 0;
            var point = curve.pointAtPercent(progress);
            dot.x = point.x
            dot.y = point.y
Now lets look at MyCurve, custom QDeclaraiveItem class, which holds the QPainterPath. MyCurve is quite simple class if we don't include code to interpret PathQuad or PathLine QML element. We could have populated QPainterPath directly in c++ class. But I choose not to do so, to make it more reusable and also that I can try my hands on QDeclarativeListProperty and can show how we can read property of QML element.

#ifndef MYCURVE_H
#define MYCURVE_H

#include <QDeclarativeItem>
#include <QPainterPath>
#include <QGraphicspathItem>
#include <QDeclarativeListProperty>

class MyCurve : public QDeclarativeItem
    Q_PROPERTY(QDeclarativeListProperty<QObject> path READ path)
    explicit MyCurve(QDeclarativeItem *parent = 0);

public slots:
    QPointF pointAtPercent(double Percent);

    QDeclarativeListProperty<QObject> path();

    static void appendItem(QDeclarativeListProperty<QObject> *list,
         QObject *pathItem);

    void appendPathQuad( QObject* pathQual);
    void appendPathLine( QObject* pathLine);

    QGraphicsPathItem* _path;
    QPainterPath _painterPath;
#endif // MYCURVE_H
Only Interesting about header is appendItem static method, which I will explain after showing its implementation. So here is implementation of header.
#include "mycurve.h"
#include <QDebug>
#include <QDeclarativeItem>
#include <QPen>

MyCurve::MyCurve(QDeclarativeItem *parent) :
    _path = new QGraphicsPathItem(this);

QPointF MyCurve::pointAtPercent(double Percent) {
    return _painterPath.pointAtPercent(Percent);

QDeclarativeListProperty<QObject> MyCurve::path()
    return QDeclarativeListProperty<QObject>(this,0,&MyCurve::appendItem);

void MyCurve::appendItem(QDeclarativeListProperty<QObject> *list, 
    QObject *pathItem)
    MyCurve *myCurve = qobject_cast<MyCurve *>(list->object);
    if (myCurve) {
         if( QString(pathItem->metaObject()->className())
                == QString("QDeclarativePathQuad") ) {
         } else if (QString(pathItem->metaObject()->className()) 
               == QString("QDeclarativePathLine")){

void MyCurve::appendPathQuad( QObject* pathQuad) {
    double x = pathQuad->property("x").toDouble();
    double y = pathQuad->property("y").toDouble();
    double controlX = pathQuad->property("controlX").toDouble();
    double controlY = pathQuad->property("controlY").toDouble();


void MyCurve::appendPathLine( QObject* pathLine) {
    double x = pathLine->property("x").toDouble();
    double y = pathLine->property("y").toDouble();
There are quite a few interesting things here.

Usage of QDeclarativeListProperty(I am using it for first time), QDeclarativeListProperty is used to create list of property in QML. To operate on QDeclarativeListProperty, we need to provide static method which take pointer of list object and list item. appendItem is similar static method which add path element to our QPainterPath. To know more how other operation can be implemented on QDeclarativeListProperty, please visit this or this link.

In above code I am using metaObject to identify which QML Path element is being added by appendItem ( which is kind of dirty hack ) and after identifying element I am using property method to get its value and finally using those value to call appropriate QPainterPath method.

And finally here is how it looks,