QAbstractTableModel and emit dataChanged for a single row - c++

Question

I derived a model from QAbstractTableModel and now I want to notify, that the data of a whole row has been changed. If for example the data of a row with index 5 is changed (4 columns), than using the following code works as expected.
emit dataChanged(index(5,0), index(5, 0));
emit dataChanged(index(5,1), index(5, 1));
emit dataChanged(index(5,2), index(5, 2));
emit dataChanged(index(5,3), index(5, 3));
But if I try to achieve the same with only one emit, ALL columns of ALL rows in the view are updated.
emit dataChanged(index(5, 0), index(5, 3));
What I am doing wrong here?
Minimal example (C++11, QTCreator 4.7.1, Windows 10 (1803), 64 Bit)
demo.h
#pragma once
#include <QAbstractTableModel>
#include <QTime>
#include <QTimer>
class Demo : public QAbstractTableModel
{
Q_OBJECT
QTimer * t;
public:
Demo()
{
t = new QTimer(this);
t->setInterval(1000);
connect(t, SIGNAL(timeout()) , this, SLOT(timerHit()));
t->start();
}
QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override
{
int c = index.column();
if (role == Qt::DisplayRole)
{
QString strTime = QTime::currentTime().toString();
if (c == 0) return "A" + strTime;
if (c == 1) return "B" + strTime;
if (c == 2) return "C" + strTime;
if (c == 3) return "D" + strTime;
}
return QVariant();
}
int rowCount(const QModelIndex &) const override { return 10; }
int columnCount(const QModelIndex &) const override { return 4; }
private slots:
void timerHit()
{
//Works
emit dataChanged(index(5,0), index(5, 0));
emit dataChanged(index(5,1), index(5, 1));
emit dataChanged(index(5,2), index(5, 2));
emit dataChanged(index(5,3), index(5, 3));
//emit dataChanged(index(5,0), index(5, 3)); // <-- Doesn't work
}
};
main.cpp
#include "demo.h"
#include <QApplication>
#include <QTreeView>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QTreeView dataView;
Demo dataModel{};
dataView.setModel( &dataModel );
dataView.show();
return a.exec();
}

Answer 1

I think the problem lies with certain assumptions you're making with regard the behaviour of QTreeView when the QAbstractItemModel::dataChanged signal is emitted.
Specifically, you assume that the view will only invoke QAbstractItemModel::data on those indexes that are specified in the signal. That's not necessarily the case.
Looking at the source for QAbstractItemView::dataChanged (Qt 5.11.2) you'll see...
void QAbstractItemView::dataChanged(const QModelIndex &topLeft, const QModelIndex &bottomRight, const QVector<int> &roles)
{
Q_UNUSED(roles);
// Single item changed
Q_D(QAbstractItemView);
if (topLeft == bottomRight && topLeft.isValid()) {
const QEditorInfo &editorInfo = d->editorForIndex(topLeft);
//we don't update the edit data if it is static
if (!editorInfo.isStatic && editorInfo.widget) {
QAbstractItemDelegate *delegate = d->delegateForIndex(topLeft);
if (delegate) {
delegate->setEditorData(editorInfo.widget.data(), topLeft);
}
}
if (isVisible() && !d->delayedPendingLayout) {
// otherwise the items will be update later anyway
update(topLeft);
}
} else {
d->updateEditorData(topLeft, bottomRight);
if (isVisible() && !d->delayedPendingLayout)
d->viewport->update();
}
#ifndef QT_NO_ACCESSIBILITY
if (QAccessible::isActive()) {
QAccessibleTableModelChangeEvent accessibleEvent(this, QAccessibleTableModelChangeEvent::DataChanged);
accessibleEvent.setFirstRow(topLeft.row());
accessibleEvent.setFirstColumn(topLeft.column());
accessibleEvent.setLastRow(bottomRight.row());
accessibleEvent.setLastColumn(bottomRight.column());
QAccessible::updateAccessibility(&accessibleEvent);
}
#endif
d->updateGeometry();
}
The important point is that this code behaves differently depending on whether or not the signal specifies a single QModelIndex -- e.g. topLeft is the same as bottomRight. If they are the same then the view tries to ensure that only that model index is updated. However, if multiple model indexes are specified then it will invoke...
d->viewport->update();
which will, presumably, result in the data for all visible model indexes being queried.
Since your implementation of Demo::data always returns new data based on the current time you will see the entire visible part of the view update giving the impression that the dataChanged signal was emitted for all rows and columns.
So the fix is really to make your data model more ``stateful'' -- it needs to keep track of values rather than simply generating them on demand.

Answer 2

Not sure whether this is what you're looking for but I'll put it up anyways.
Even using emit dataChanged(...), you would still see that clicks/selection on rows will cause them to self-update (doing this from a Mac, so might be different).
Instead of using the QAbstractItemModel::dataChanged signal, I will be using the QAbstractItemModel::setData() function.
This is my implementation of demo.h
#pragma once
#include <QAbstractTableModel>
#include <QTime>
#include <QTimer>
class Demo : public QAbstractTableModel
{
Q_OBJECT
public:
Demo()
{
int cCount = columnCount(index(0, 0));
int rCount = rowCount(index(0, 0));
// populate model data with *static* values
QString strTime = QTime::currentTime().toString();
QStringList temp;
for (int j = 0; j < cCount; j++)
temp.append(strTime);
for (int i = 0; i < rCount; i++)
demoModelData.append(temp);
// nothing new here
t = new QTimer(this);
t->setInterval(1000);
connect(t, SIGNAL(timeout()) , this, SLOT(timerHit()));
t->start();
}
QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override
{
// tells the *view* what to display
// if this was dynamic (e.g. like your original strTime implementation)
// then the view (QTreeView in main.cpp) will constantly update
if (role == Qt::DisplayRole)
return demoModelData.at(index.row()).at(index.column()); // retrieve data from model
return QVariant();
}
// reimplemented from QAbstractTableModel
virtual bool setData(const QModelIndex &index, const QVariant &value, int role = Qt::DisplayRole) override
{
if (role == Qt::DisplayRole)
{
demoModelData[index.row()][index.column()] = value.toString(); // set the new data
emit dataChanged(index, index); // explicitly emit dataChanged signal, notifies TreeView to update by
// calling this->data(index, Qt::DisplayRole)
}
return true;
}
int rowCount(const QModelIndex &) const override { return 10; }
int columnCount(const QModelIndex &) const override { return 4; }
private slots:
void timerHit()
{
QString strTime = QTime::currentTime().toString();
setData(index(5, 0), QVariant(strTime), Qt::DisplayRole); // only changes index at (row = 5, col = 0)
}
private:
QTimer *t;
QList<QStringList> demoModelData; // stores the table model's data
};
Since the class is a "model", there should be some way of storing/retrieving data for display. Here, I've used a QList<QStringList>, but you can store data in other ways that suit you as well (e.g. a tree, QVector, QMap).

Answer 3

This is a genuine efficiency bug in Qt.
The Qt project is no longer accepting changes to Qt 5, so I made the change and pushed it to my fork on GitHub. You can see a fix for the issue you've encountered here.
If you want to build your own copy of 5.15.2, you may be interested in my other fixes.