Qt Create a QML Slider Sensitive to Touch Events - c++

Question

I am creating a game for touch screens that requires 2-4 players to each have access to a pair of slider controls. The problem is that the QML Slider control responds to touch as a mouse event and seizes the focus. Then only one player can access a single control at a time. I need multiple sliders to respond to touch events simultaneously. My question is how to do that?
With the help of a variety of stack overflow posts, I have been able to create my own answer that so far seems to work. I detail the answer below in the answer section to save other newbies like me the trouble.

Answer 1

There is a pure qml solution to this problem. The TouchSlider C++ object in my first answer (elsewhere in this thread) was unnecessary. Here I have modified the code to the TouchSlider qml code to eliminate references to touchslider (the TouchSlider C++ object).
TouchPoint.qml:
import QtQuick 2.0
import QtQuick 2.5
import QtQuick.Controls 1.4
import QtQuick.Layouts 1.2
Item {
property string sliderTitle
property real sliderMin
property real sliderMax
property real sliderVal
ColumnLayout{
id: column1
Label {
text: qsTr(sliderTitle)
font.pointSize: 10
}
Slider {
id: touchSlider1
minimumValue: sliderMin
maximumValue: sliderMax
orientation: Qt.Vertical
value: sliderVal
onValueChanged: function(){
sliderVal = Math.round(this.value);
labelSliderValue.text = qsTr(JSON.stringify(sliderVal));
}
}
Label {
id: labelSliderValue
text: qsTr(JSON.stringify(sliderVal))
font.pointSize: 10
}
function sliderSetValueFromTouch(position){
// Assume qs a vertical slider
var minvalue = touchSlider1.minimumValue;
var maxvalue = touchSlider1.maximumValue;
// Since this is a vertical slider, by assumption, get the height
var height = touchSlider1.height;
// Compute the new value based on position coordinate
var newvalue = (height-position)/height * (maxvalue-minvalue);
if (newvalue<minvalue) newvalue = minvalue;
if (newvalue>maxvalue) newvalue = maxvalue;
//qDebug() << newvalue;
// Set the value of the slider
touchSlider1.value = newvalue;
}
MultiPointTouchArea{
anchors.fill: touchSlider1
touchPoints: [
TouchPoint {
id: point1
onPressedChanged: function(){
if(pressed){
//console.log("pressed");
//console.log(touchslider.testStringReturn());
//touchslider.sliderSetValueFromTouch(touchSlider1,point1.y);
column1.sliderSetValueFromTouch(point1.y);
}
}
}
]
onTouchUpdated: function(){
//touchslider.sliderSetValueFromTouch(touchSlider1,point1.y);
column1.sliderSetValueFromTouch(point1.y);
}
}
}
}
The touchslider.h and touchslider.cpp files add no value.

Answer 2

I could not find a pure QML way to solve the problem but I wanted to minimize the use of C++. Using C++, I create an object TouchSlider and add it to my qml scene. The TouchSlider object has a simple function to update the value of a vertical slider according to a position argument. Then in the QML code, I add a MultiPointTouchArea on top of a regular slider and respond to the touch events by calling C++ function.
Here are all my files for a project called SliderPair.
SliderPair.pro:
QT += qml quick widgets
QT += quickcontrols2
QT += core
CONFIG += c++11
SOURCES += main.cpp \
touchslider.cpp
RESOURCES += \
qml.qrc
# Additional import path used to resolve QML modules in Qt Creator's code model
QML_IMPORT_PATH += qml
# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target
HEADERS += \
touchslider.h
DISTFILES +=
main.cpp:
#include <QApplication>
#include <QQmlApplicationEngine>
// add following includes for exposing new class TouchSlider to QML
#include <QQmlEngine>
#include <QQmlContext>
#include "touchslider.h"
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
//Create an object of type TouchSlider
//When a scoped pointer goes out of scope the object is deleted from memory. Good housekeeping:
QScopedPointer<TouchSlider> touchslider (new TouchSlider);
QQmlApplicationEngine engine;
engine.addImportPath(QStringLiteral("qml"));
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
//QML can now refer to the TouchSlider object using the handle "touchslider":
engine.rootContext()->setContextProperty("touchslider",touchslider.data());
return app.exec();
}
touchslider.h:
#ifndef TOUCHSLIDER_H
#define TOUCHSLIDER_H
#include <QObject>
#include <QDebug>
#include <QtQuickControls2>
class TouchSlider : public QObject
{
Q_OBJECT
public:
explicit TouchSlider(QObject *parent = 0);
//call Q_INVOKABLE macro to set up functions for QML
Q_INVOKABLE void testDebug(); //hello world from C++
Q_INVOKABLE QString testStringReturn(); //hello world to javascript
Q_INVOKABLE void sliderSetValueFromTouch(QQuickItem *qs,int position );//use touch event to set slider value
signals:
public slots:
};
#endif // TOUCHSLIDER_H
touchslider.cpp:
#include "touchslider.h"
TouchSlider::TouchSlider(QObject *parent) : QObject(parent)
{
}
void TouchSlider::testDebug()
{
qDebug() << "Hello from C++";
}
QString TouchSlider::testStringReturn()
{
QString message = "Hi from C++";
return message;
}
void TouchSlider::sliderSetValueFromTouch(QQuickItem *qs, int position)
{
// Assume qs a vertical slider
// Get its properties (its slider properties are accessible even though it is declared as QQuickItem)
// minimumValue and maximumValue are of type QVariant so we need to cast them as double
double minvalue = qs->property("minimumValue").value<double>();
double maxvalue = qs->property("maximumValue").value<double>();
// Since this is a vertical slider, by assumption, get the height
double height = qs->property("height").value<double>();
// Compute the new value based on position coordinate
double newvalue = (height-position)/height * (maxvalue-minvalue);
if (newvalue<minvalue) newvalue = minvalue;
if (newvalue>maxvalue) newvalue = maxvalue;
//qDebug() << newvalue;
// Set the value of the slider
qs->setProperty("value",newvalue);
}
TouchSlider.qml:
import QtQuick 2.0
import QtQuick 2.5
import QtQuick.Controls 1.4
import QtQuick.Layouts 1.2
Item {
property string sliderTitle
property real sliderMin
property real sliderMax
property real sliderVal
ColumnLayout{
Label {
text: qsTr(sliderTitle)
font.pointSize: 10
}
Slider {
id: touchSlider1
minimumValue: sliderMin
maximumValue: sliderMax
orientation: Qt.Vertical
value: sliderVal
onValueChanged: function(){
sliderVal = Math.round(this.value);
labelSliderValue.text = qsTr(JSON.stringify(sliderVal));
}
}
Label {
id: labelSliderValue
text: qsTr(JSON.stringify(sliderVal))
font.pointSize: 10
}
MultiPointTouchArea{
anchors.fill: touchSlider1
touchPoints: [
TouchPoint {
id: point1
onPressedChanged: function(){
if(pressed){
//console.log("pressed");
//console.log(touchslider.testStringReturn());
touchslider.sliderSetValueFromTouch(touchSlider1,point1.y);
}
}
}
]
onTouchUpdated: function(){
touchslider.sliderSetValueFromTouch(touchSlider1,point1.y);
}
}
}
}
PlayerControls.qml:
import QtQuick 2.0
import QtQuick.Controls 1.4
import QtQuick.Layouts 1.2
Item {
// These properties act as constants, useable outside this QML file
property string playerName
property real priceMin
property real priceMax
property real qualityMin
property real qualityMax
property real priceValue
property real qualityValue
property int sliderWidth
ColumnLayout{
id: columnLayout1
width: 640
height: 480
Layout.minimumWidth: 640
Layout.fillWidth: true
anchors.fill: parent
spacing: 10.2
Label {
Layout.alignment: Qt.AlignHCenter | Qt.AlignVCenter
id: labelPlayer1
text: qsTr(playerName)
font.pointSize: 10
}
RowLayout{
ColumnLayout{
Label {
text: qsTr("")
font.pointSize: 10
width: 50
}
}
TouchSlider {
width: sliderWidth
sliderTitle: "Price"
sliderMin: priceMin
sliderMax: priceMax
sliderVal: priceValue
}
TouchSlider {
width: sliderWidth
sliderTitle: "Quality"
sliderMin: qualityMin
sliderMax: qualityMax
sliderVal: qualityValue
}
}
}
}
main.qml:
import QtQuick 2.7
import QtQuick.Controls 2.0
import QtQuick.Layouts 1.0
ApplicationWindow {
visible: true
width: 640
height: 480
title: qsTr("SliderPair Test")
Item {
PlayerControls{
playerName: "Player 1"
priceMin: 0
priceMax: 200
priceValue: 100
qualityMin: 0
qualityMax: 50
qualityValue: 25
sliderWidth: 200
}
}
}
The result should look like this:
On a touch screen like my Surface Pro, I can control each slider simultaneously with two fingers. Since Windows supports up to 10 simultaneous touches that should mean I can have 2-4 players without a problem. We shall see.