How to synchronize scrolling of the two QListWidgets? - c++

Question

There are two QListWidget with the same number of the items. How can synchronize their scrolling?
I mean when I scroll one of them, the other one should get same scrolling.

Answer 1

Suppose you have two QListWidget elements listWidget_1 and listWidget_2 in your UI, then you can use valueChanged/setValue signal/slot pair to connect vertical sliders of both listwidgets, indeed, I didn't find any issue of signals re-bouncing in this "two way" connection because eventually both values will be same and I think no more signals would be emitted, thus you can set such sufficient connections:
connect(this->ui->listWidget_1->verticalScrollBar(), &QScrollBar::valueChanged,
this->ui->listWidget_2->verticalScrollBar(), &QScrollBar::setValue);
connect(this->ui->listWidget_2->verticalScrollBar(), &QScrollBar::valueChanged,
this->ui->listWidget_1->verticalScrollBar(), &QScrollBar::setValue);
// test lists:
QList<QString> lw11, lw22;
for (int x=0; x <200; x++){
lw11.append("ListWidget1_" + QVariant(x).toString());
lw22.append("The Other lw is at: " + QVariant(x).toString());
}
this->ui->listWidget_1->addItems(lw11);
this->ui->listWidget_2->addItems(lw22);
If signal rebounce should be blocked anyway, then the model can be adjusted by adding a single slot to handle scrolling for both widgets and connect both to that slot:
connect(this->ui->listWidget_1->verticalScrollBar(),&QScrollBar::valueChanged
, this, &MainWindow::handleScroll);
connect(this->ui->listWidget_2->verticalScrollBar(),&QScrollBar::valueChanged
, this, &MainWindow::handleScroll);
and the slot logic can be :
void MainWindow::handleScroll(int value)
{
// Logic for detecting sender() can be used ... but I don't see it's important
// fast way to check which listWidget emitted the signal ...
if (this->ui->listWidget_1->verticalScrollBar()->value() == value){
qDebug() << "lw1 is in charge ...............";
disconnect(this->ui->listWidget_2->verticalScrollBar(), &QScrollBar::valueChanged,this, &MainWindow::handleScroll); // prevent signal rebounce from the other lw
this->ui->listWidget_2->verticalScrollBar()->setValue(value);
connect(this->ui->listWidget_2->verticalScrollBar(), &QScrollBar::valueChanged,this, &MainWindow::handleScroll);
}else{
qDebug() << "lw2 is in charge ...............";
disconnect(this->ui->listWidget_1->verticalScrollBar(), &QScrollBar::valueChanged,this, &MainWindow::handleScroll);
this->ui->listWidget_1->verticalScrollBar()->setValue(value);
connect(this->ui->listWidget_1->verticalScrollBar(), &QScrollBar::valueChanged,this, &MainWindow::handleScroll);
}
}

Answer 2

You must use the valueChanged() signal from the verticalScrollBar() of QListWidget, since the connection is bidirectional, it will cause unnecessary tasks to be executed for it using blockSignals():
In the next section I show an example:
#include <QApplication>
#include <QListWidget>
#include <QHBoxLayout>
#include <QScrollBar>
class Widget: public QWidget{
Q_OBJECT
QListWidget w1;
QListWidget w2;
public:
Widget(QWidget *parent=Q_NULLPTR):QWidget(parent){
auto layout = new QHBoxLayout{this};
layout->addWidget(&w1);
layout->addWidget(&w2);
connect(w1.verticalScrollBar(), &QScrollBar::valueChanged, [this](int value){
w2.verticalScrollBar()->blockSignals(true);
w2.verticalScrollBar()->setValue(value);
w2.verticalScrollBar()->blockSignals(false);
});
connect(w2.verticalScrollBar(), &QScrollBar::valueChanged, [this](int value){
w1.verticalScrollBar()->blockSignals(true);
w1.verticalScrollBar()->setValue(value);
w1.verticalScrollBar()->blockSignals(false);
});
for(int i=0; i<100; i++){
w1.addItem(QString("item %1 of 1").arg(i));
w2.addItem(QString("item %1 of 2").arg(i));
}
}
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Widget w;
w.show();
return a.exec();
}
#include "main.moc"