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Creating a custom QML element with Qt

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This snippet shows how to create a custom QML element.

Article Metadata
Tested with
Devices(s): Nokia 900
Platform(s): S60 5th Edition
Keywords: QML, QDeclarativeItem
Created: kratsan (17 Jun 2010)
Last edited: hamishwillee (17 Oct 2012)



Sometimes the elements provided by the Declarative module are not enough and a new element must be implemented.

In the following example we create a Line element which simply draws a line. The coordinates of the beginning and end of the line must be given. It is also possible to adjust the color, pen width, and smoothing of the line.


  • Qt 4.7 or higher is installed on your platform.


QT       += core gui declarative
TARGET = qmlapp
SOURCES += main.cpp
HEADERS += line.h
OTHER_FILES += ui.qml

The new element is derived from QDeclarativeItem which gives us the functionality of a QML Item, such as x, y, width and height. We declare the new properties with the macro Q_PROPERTY and state which methods to use when properties are read and updated. The NOTIFY feature is used to notify bound properties to update their value when the property here changes. The paint method handles the painting of our custom element. Finally, at the end of the header, we declare the class Line as a QML type.


#ifndef LINE_H
#define LINE_H
#include <QDeclarativeItem>
#include <QPainter>
class Line : public QDeclarativeItem
Q_PROPERTY(int x1 READ x1 WRITE setX1 NOTIFY x1Changed);
Q_PROPERTY(int y1 READ y1 WRITE setY1 NOTIFY y1Changed);
Q_PROPERTY(int x2 READ x2 WRITE setX2 NOTIFY x2Changed);
Q_PROPERTY(int y2 READ y2 WRITE setY2 NOTIFY y2Changed);
Q_PROPERTY(QColor color READ color WRITE setColor NOTIFY colorChanged);
Q_PROPERTY(int penWidth READ penWidth WRITE setPenWidth NOTIFY penWidthChanged);
Line(QDeclarativeItem *parent = 0) :
QDeclarativeItem(parent), m_x1(0), m_y1(0), m_x2(0), m_y2(0),
m_color(Qt::black), m_penWidth(1)
// Important, otherwise the paint method is never called
setFlag(QGraphicsItem::ItemHasNoContents, false);
void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
QPen pen(m_color, m_penWidth);
if(smooth() == true) {
painter->setRenderHint(QPainter::Antialiasing, true);
int x = qMin(m_x1, m_x2) - m_penWidth/2;
int y = qMin(m_y1, m_y2) - m_penWidth/2;
painter->drawLine(m_x1 - x, m_y1 - y, m_x2 - x, m_y2 - y);
// Get methods
int x1() const { return m_x1; }
int y1() const { return m_y1; }
int x2() const { return m_x2; }
int y2() const { return m_y2; }
QColor color() const { return m_color; }
int penWidth() const { return m_penWidth; }
// Set methods
void setX1(int x1) {
if(m_x1 == x1) return;
m_x1 = x1;
emit x1Changed();
void setY1(int y1) {
if(m_y1 == y1) return;
m_y1 = y1;
emit y1Changed();
void setX2(int x2) {
if(m_x2 == x2) return;
m_x2 = x2;
emit x2Changed();
void setY2(int y2) {
if(m_y2 == y2) return;
m_y2 = y2;
emit x2Changed();
void setColor(const QColor &color) {
if(m_color == color) return;
m_color = color;
emit colorChanged();
void setPenWidth(int newWidth) {
if(m_penWidth == newWidth) return;
m_penWidth = newWidth;
emit penWidthChanged();
void x1Changed();
void y1Changed();
void x2Changed();
void y2Changed();
void colorChanged();
void penWidthChanged();
void updateSize() {
setX(qMin(m_x1, m_x2) - m_penWidth/2);
setY(qMin(m_y1, m_y2) - m_penWidth/2);
setWidth(qAbs(m_x2 - m_x1) + m_penWidth);
setHeight(qAbs(m_y2 - m_y1) + m_penWidth);
int m_x1;
int m_y1;
int m_x2;
int m_y2;
QColor m_color;
int m_penWidth;
#endif // LINE_H

In main.cpp, we register the C++ type Line in the QML system with the name Line to the library CustomComponents with version number 1.0.


#include "line.h"
#include <QApplication>
#include <QDeclarativeView>
int main(int argc, char *argv[])
QApplication a(argc, argv);
qmlRegisterType<Line>("CustomComponents", 1, 0, "Line");
QDeclarativeView view;
#if defined(Q_WS_S60) || defined(Q_WS_MAEMO)
view.setGeometry(100,100, 800, 480);;
return a.exec();

In the QML document, we import the CustomComponents library which contains the Line element.


import CustomComponents 1.0
import Qt 4.7
Rectangle {
property bool evenClick : false
anchors.fill: parent; color: "lightsteelblue"
Line {
id: diagonalLine
Behavior on x1 { NumberAnimation { duration: 1000 } }
Behavior on y1 { NumberAnimation { duration: 1000 } }
Behavior on x2 { NumberAnimation { duration: 1000 } }
Behavior on y2 { NumberAnimation { duration: 1000 } }
x1: parent.x + 20; y1: parent.height / 2
x2: parent.width - 20; y2: parent.height / 2
color: "tomato"; penWidth: 3; smooth: true
MouseArea {
anchors.fill: parent
onClicked: {
if(evenClick) { diagonalLine.x1 = mouseX; diagonalLine.y1 = mouseY }
else { diagonalLine.x2 = mouseX; diagonalLine.y2 = mouseY }
evenClick = !evenClick
Text {
id: textX1Y1
anchors.left: parent.left;
text: "x1: " + diagonalLine.x1 + " y1: " + diagonalLine.y1
Text {
anchors.left: parent.left; textX1Y1.bottom; anchors.topMargin: 10
text: "x2: " + diagonalLine.x2 + " y2: " + diagonalLine.y2


The custom QML element Line was created with Qt by deriving the class QDeclarativeItem, adding a few new properties and overwriting the paint method. The Qt class was registered as QML type and then used in a Qt Quick application.

This page was last modified on 17 October 2012, at 06:09.
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