Reordering rows of QTableView with drag and drop

[Rodrigo B.]

I have a QTableView on my own implementation of QAbstractItemModel.

The project is a simple drag and drop implementation with some options for testing on-the-fly, available on GitHub:

I would like to be able to quickly sort/re-order the rows of the table with drag and drop.

I followed the instructions in Qt's documentation on "Using Drag and Drop with View Items", more specifically "Using model/view classes".

With that, I can select an entire row (let's call it D) and re-position it by dragging it into the narrow space between two other rows.

That is sort of what I wanted, but it is not very agile or natural. When I drop D on another row R, I would like D to "push" R down and insert itself in R's original place. This way, I don't have to carefully "aim" in between rows.

However, dropping D on R overwrites R, even if the QTableView's property dragDropOverwriteMode is false.

What I want seems to be a very common way to reorder rows in applications in general, so I believe Qt must have a simple way of providing that. What would that be?

A couple of more details:

• I used both QAbstractItemView::InternalMove (which seems ideally named for what I want) and QAbstractItemView::DragDrop for the QTableView's dragDropMode property, but as far as I can tell they behave exactly the same.

• I have also set the model's supportedDropActions to return Qt::MoveAction only.

[SGaist]

Hi,

AFAIK, what you are looking for is QAbstractItemView::InternalMove.

[kshegunov]

@Rodrigo-B said in Reordering rows of QTableView with drag and drop:

What I want seems to be a very common way to reorder rows in applications in general, so I believe Qt must have a simple way of providing that. What would that be?

Yes, you would believe that, but unfortunately no. You have to hack-away in the event handlers to make this work. I'll see if I can dig you up some code ... (may take a day or two)

[Rodrigo B.]

@SGaist, as mentioned in the post, I tried InternalMove but it didn't do anything different from DragDrop.

Also, with all due respect, it looks to me that InternalMove doesn't do what you think it does. I've been trying to get drag and drop to work for a long time now, and searching through these archives I see you have said a few times that the solution was to use InternalMove, but it never was the solution. As far as I can tell, InternalMove doesn't differ much from DragDrop. Maybe it blocks drops from external views but when it's about drops from the same view everything seems to work exactly the same.

[Rodrigo B.]

@kshegunov I was afraid that was going to be the answer. Yes, if you could send me some examples that would be great, as I have been trying to get this to work for a long time now.

[kshegunov]

I think this should get you started:

MyTableView::MyTableView(QWidget *parent)
    :  QTableView(parent), m_dropRow(0)
{
   setSelectionMode(QAbstractItemView::SingleSelection);
   setSelectionBehavior(QAbstractItemView::SelectRows);
   setDragEnabled(true);
   setAcceptDrops(true);
   setDragDropMode(QAbstractItemView::DragDrop);
   setDefaultDropAction(Qt::MoveAction);
   setDragDropOverwriteMode(false);
   setDropIndicatorShown(true);
}

int MyTableView::selectedRow() const
{
   QItemSelectionModel *selection = selectionModel();
   return selection->hasSelection() ? selection->selectedRows().front().row() : -1;
}

void MyTableView::reset()
{
   QTableView::reset();

    QObject::connect(model(), &QAbstractTableModel::rowsInserted, this, [this](const QModelIndex &parent, int first, int last) {
        m_dropRow = first;
    });
}

void MyTableView::dropEvent(QDropEvent *e)
{
   if (e->source() != this || e->dropAction() != Qt::MoveAction)
      return;
   int dragRow = selectedRow();

   QTableView::dropEvent(e);  // m_dropRow is set by inserted row

   if (m_dropRow > dragRow)
      --m_dropRow;

   QMetaObject::invokeMethod(this, std::bind(&MyTableView::selectRow, this, m_dropRow), Qt::QueuedConnection);  // Postpones selection
}

[Rodrigo B.]

@kshegunov

Thank you!

I tried using the code but got stuck on the QMetaObject::invokeMethod call, which gives me the following error message:

error: no matching function for call to 'invokeMethod'

Any idea of what may be causing it?

Here's the full compilation unit and .pro file in case they are useful:

#include "mainwindow.h"
#include "ui_mainwindow.h"

#include <QItemSelectionModel>
#include <QMetaObject>
#include <QTableView>
#include <QDropEvent>


class MyTableView: QTableView {
    
    Q_OBJECT
    
    int m_dropRow;
    
    MyTableView(QWidget *parent)
        :  QTableView(parent), m_dropRow(0)
    {
       setSelectionMode(QAbstractItemView::SingleSelection);
       setSelectionBehavior(QAbstractItemView::SelectRows);
       setDragEnabled(true);
       setAcceptDrops(true);
       setDragDropMode(QAbstractItemView::DragDrop);
       setDefaultDropAction(Qt::MoveAction);
       setDragDropOverwriteMode(false);
       setDropIndicatorShown(true);
    }
    
    int selectedRow() const
    {
       QItemSelectionModel *selection = selectionModel();
       return selection->hasSelection() ? selection->selectedRows().front().row() : -1;
    }
    
    void reset()
    {
       QTableView::reset();
    
        QObject::connect(model(), &QAbstractTableModel::rowsInserted, this, [this](const QModelIndex &parent, int first, int last) {
            m_dropRow = first;
        });
    }
    
    void dropEvent(QDropEvent *e)
    {
       if (e->source() != this || e->dropAction() != Qt::MoveAction)
          return;
       int dragRow = selectedRow();
    
       QTableView::dropEvent(e);  // m_dropRow is set by inserted row
    
       if (m_dropRow > dragRow)
          --m_dropRow;
       
       QMetaObject::invokeMethod(this, 
                                 std::bind(&MyTableView::selectRow, this, m_dropRow), 
                                 Qt::QueuedConnection);  // Postpones selection
    }
    
    
};

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}

MainWindow::~MainWindow()
{
    delete ui;
}

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

CONFIG += c++11

# You can make your code fail to compile if it uses deprecated APIs.
# In order to do so, uncomment the following line.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0

SOURCES += \
    main.cpp \
    mainwindow.cpp

HEADERS += \
    mainwindow.h

FORMS += \
    mainwindow.ui

# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target

[kshegunov]

Wait, what Qt version is this?

[Rodrigo B.]

@kshegunov I believe 5.15.2 because my Qt Creator project's Run Settings show an environment variable QTDIR set to /Users/xxxxxx/Qt/5.15.2/clang_64.

I also have an Anaconda Qt package 5.9 installed because I was using PyQt, but given that I am running the project directly from within the Qt Creator project with the 5.15.2 setting, I believe that's the version being used.

PS: just checked that the header files being included are definitely the ones in the 5.15 .2 install.

[kshegunov]

There's something fishy, because these are excerpts from code that is built and runs against Qt 5.12.x. Could you please check the kit you have in your creator, the compiler version and also if substituting with this makes it compile:

Q_INVOKABLE int selectedRow() const
// ...

and

QMetaObject::invokeMethod(this, "selectRow", Q_ARG(int, m_dropRow), Qt::QueuedConnection);

[Rodrigo B.]

@kshegunov Thank you again.

I made the changes you suggest in the code but unfortunately I still get the same error message.

I've placed the project files for download if that is helpful.

I've installed Qt Creator 4.15.2 just two months ago. I'm using the Clang 5.15.2 Clang 64-bit kit.

As for compiler:

$ /usr/bin/clang --version
Apple clang version 12.0.0 (clang-1200.0.32.29)
Target: x86_64-apple-darwin19.6.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

Hopefully that sheds light on it. Thank you.

[kshegunov]

I don't use MacOS, but I'll build it on linux later this evening just to make sure.

[Rodrigo B.]

@kshegunov friendly reminder about this, if you can. Thanks!

[kshegunov]

I'm sorry! I completely forgot.
I just downloaded it. The std::bind version works out of the box when you fix the inheritance error (look down for details).

Otherwise there's an argument order error for the runtime resolution method:

QMetaObject::invokeMethod(this, "selectRow", Qt::QueuedConnection, Q_ARG(int, m_dropRow));

The inheritance error, though, affects both invocations:

class MyTableView: QTableView

inherits privately, it should be:

class MyTableView: public QTableView

[Rodrigo B.]

@kshegunov said in Reordering rows of QTableView with drag and drop:

QMetaObject::invokeMethod(this, "selectRow", Qt::QueuedConnection, Q_ARG(int, m_dropRow));

Thank you, that fixed my compilation errors! I will experiment with it tomorrow to try to solve my original problem and let you know.

[Rodrigo B.]

Hi @kshegunov ,

Now I had time to experiment with your suggestion but I am not seeing how it may solve the problem.

I've updated with GitHub repository with it if anyone wishes to run it.

There are several things I do not understand:

1. The main one is that I don't see the stated desired effect: I wanted the dropping a row into another to insert itself rather than overwrite, but it still overwrites.
2. Another thing that puzzles me is that, when I drag a row in between two rows, it used to insert itself there (as is the standard behavior), but now it also overwrites. That is to say, it seems to have gone in the opposite direction of what was desired.
3. It seems to me your code has the purpose of selecting the row after drag and drop, but I don't see that behavior. There is no selection after the operation.

Would you please shed some light on these questions? Thanks again.

[kshegunov]

@Rodrigo-B said in Reordering rows of QTableView with drag and drop:

Now I had time to experiment with your suggestion but I am not seeing how it may solve the problem.

That's because I'd forgotten a method. I'm sorry, it happens sometimes when you snip pieces from existing code without thinking too much. I've created a pull request for you, so you could check it out.

[Rodrigo B.]

@kshegunov Thank you, I really appreciate that you went over the code and provided a pull request! However, it still doesn't seem to work. When I drop the first row (Lion) on third row (Mouse), I would expect the Lion row to be inserted right below Mouse, and Gazelle to move up and be the first row. Instead, Lion overwrites Gazelle in the second row and remains in the first row. As far as I can tell, everything is being overwritten rather than moved.

In any case, I am starting to see the idea here... modifying variables in dropMimeData so data goes where we want. So if you don't have the time to look into this I will probably be able to mess around and find a solution. Thanks!

[kshegunov]

@Rodrigo-B said in Reordering rows of QTableView with drag and drop:

Instead, Lion overwrites Gazelle in the second row and remains in the first row. As far as I can tell, everything is being overwritten rather than moved.

That's odd, because it works on linux as described.
Video: https://drive.google.com/file/d/14vvAHgGdyRoJkKqgm_tbT8uRwKt_3JhL/view?usp=sharing

[Rodrigo B.]

@kshegunov Wow, that's mindblogging! Here's my video showing that my local code is sync'ed with the repository containing your change, and behaving completely differently:

https://drive.google.com/file/d/11m9d5xOGGhJN-WM-OhRjHAhGK1_t7vMR/view?usp=sharing

Not quite sure how to proceed now other than submitting as a bug...