How to synchronize scrolling of the two QListWidgets?

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);
    }
}