How do I synchronize model views that have been hidden? - qt

Question

I have three views of the same model: two QListViews and one QTableView. Generally only one is visible but the user can switch views, hiding the current view and making one of the others visible. Qt does not update hidden views so I need to scroll a view that has been set visible to match a previous view. The issue is that when a QListView is set visible it takes time to paint and setup the scrollbars etc. How can I reliably determine when the view is ready to accept a scrollTo request?
I have tried calculating the maximum value of the scrollbars and then checking the horizontalScrollBar()->maximum(), but there is still a delay after the widget scrollbars have reached maximum size. I have also monitored all the view widget events, which include multiple paint events until the widget is finished. I would prefer to poll the QListView on a timer to determine when it is ready, but I do not know what to call.
void MW::gridDisplay()
{
// hide thumbView (QListView)
thumbView->setVisible(false);
// show gridView (QListView)
gridView->setVisible(true);
gridView->setFocus();
// scrollTo not working if gridView was hidden unless delay by about 300ms
// until gridView is ready
gridView->scrollTo(gridView->currentIndex(), gridView->ScrollHint::PositionAtCenter);
}

Answer 1

I figured out what was causing the delay in rendering the QListView, requiring a delay before calling QListView::scrollTo(). I had included a QListView::setLayoutMode(QListView::Batched) which really slows things down. Here is some code that illustrates the problem:
#include <QtWidgets>
#include <QDebug>
int main(int argc, char* argv[])
{
QApplication a(argc, argv);
auto f = new QFrame;
auto blank = new QListView;
auto view = new QListView;
view->setViewMode(QListView::IconMode);
view->setResizeMode(QListView::Adjust);
// this prevents the scrollTo working in time
view->setLayoutMode(QListView::Batched);
auto stack = new QStackedLayout;
f->setLayout(stack);
f->layout()->addWidget(blank);
f->layout()->addWidget(view);
QFile file("D:/Pictures/Avatars/frog.jpg"); // substitute your own image
QImage image;
QImageReader thumbReader;
thumbReader.setFileName("D:/Pictures/Avatars/frog.jpg");
thumbReader.setScaledSize(QSize(40,40));
image = thumbReader.read();
auto model = new QStandardItemModel;
view->setModel(model);
for (int row = 0; row < 50000; ++row) {
auto item = new QStandardItem();
item->setData(QString::number(row), Qt::DisplayRole);
item->setIcon(QPixmap::fromImage(image));
model->appendRow(item);
}
stack->setCurrentIndex(1);
// scrollTo does not work if view->setLayoutMode(QListView::Batched) without a delay
view->scrollTo(model->index(40000,0), QAbstractItemView::PositionAtCenter);
f->show();
return a.exec();
}
Many thanks for the help and advice to create a minimal reproducible example. Lesson learned.

Answer 2

I really tried to reproduce your error, but I just cannot see the same behavior as you did. The app has two views, which can be hidden and modified by either drag and drop or by adding items at the end.
Still, the update works as expected. Maybe there is still something different with your app. (I'm using Qt 5.13.0)
#include <QApplication>
#include <QDebug>
#include <QPushButton>
#include <QCheckBox>
#include <QGroupBox>
#include <QHBoxLayout>
#include <QFrame>
#include <QTreeView>
#include <QStandardItemModel>
int main(int argc, char* argv[])
{
QApplication a(argc, argv);
auto f = new QFrame;
auto viewLeft = new QTreeView;
auto viewRight = new QTreeView;
auto model = new QStandardItemModel;
viewLeft->setDragDropMode(QAbstractItemView::DragDropMode::InternalMove);
viewRight->setDragDropMode(QAbstractItemView::DragDropMode::InternalMove);
viewLeft->setModel(model);
viewRight->setModel(model);
auto item1 = new QStandardItem("Item1");
model->appendRow(item1);
auto item2 = new QStandardItem("Item2");
model->appendRow(item2);
auto item3 = new QStandardItem("Item3");
item2->appendRow(item3);
f->setLayout(new QHBoxLayout);
f->layout()->addWidget(viewLeft);
f->layout()->addWidget(viewRight);
auto grpBox = new QGroupBox;
grpBox->setLayout(new QHBoxLayout);
auto chkLeft = new QCheckBox("Left");
auto chkRight = new QCheckBox("Right");
grpBox->layout()->addWidget(chkLeft);
grpBox->layout()->addWidget(chkRight);
f->layout()->addWidget(grpBox);
auto btnAdd = new QPushButton("Add");
QObject::connect(btnAdd, &QPushButton::clicked, [&]() {
model->appendRow(new QStandardItem("New Item"));
});
f->layout()->addWidget(btnAdd);
QObject::connect(chkLeft, &QCheckBox::toggled, [&](auto checked) { viewLeft->setVisible(checked); });
QObject::connect(chkRight, &QCheckBox::toggled, [&](auto checked) { viewRight->setVisible(checked); });
f->show();
return a.exec();
}