QTreeWidget childAt(int x, int y) returns NULL

Question

I am trying to get the QTreeWidgetItem (node) at the position where the mouse pointer is. The QTreeWidget class has a method called childAt(int x, int y) which does not seem to be documented here: http://qt-project.org/doc/qt-5.1/qtwidgets/qtreewidget.html and I have no idea why. May be there is a reason behind it. The method always returns NULL for me.

I have extended the QTreeWidget class so I can capture the mouseMoveEvent.

class CustomTreeWidget : public QTreeWidget 
{
  Q_OBJECT

  public:
      explicit CustomTreeWidget(QWidget *parent = 0);

  signals:
      void OnMouseMove(int x, int y);

  public slots:

  private:
      void mouseMoveEvent(QMouseEvent *event);

 };

Then in my main cpp file:

void CustomTreeWidget::mouseMoveEvent(QMouseEvent *event)
{
    QTreeWidget::mouseMoveEvent(event);

    POINT p;
    if (GetCursorPos(&p))
    {
       qDebug(QString("GetCursorPos() OK: X=" + QString::number(p.x) + " Y=" + QString::number(p.y)).toLocal8Bit().data());

       QTreeWidgetItem *item = dynamic_cast<QTreeWidgetItem *> (this->childAt(p.x, p.y));

       if (item == NULL) return;

       qDebug(item->text(0).toLocal8Bit().data());
    }

    emit OnMouseMove(p.x, p.y);
}

Then in my MainWindow file:

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)

{
   ui->setupUi(this);

   XTreeWidget *treeWidget = new XTreeWidget(this);

   QTreeWidgetItem *rootItem = new QTreeWidgetItem(treeWidget, QTreeWidgetItem::UserType);
   rootItem->setText(0, "Item 1");
   treeWidget->addTopLevelItem(rootItem);

   rootItem = new QTreeWidgetItem(treeWidget, QTreeWidgetItem::UserType);
   rootItem->setText(0, "Item 2");
   treeWidget->addTopLevelItem(rootItem);

   this->setCentralWidget(treeWidget);
}

I run the code and the following statement never runs:

qDebug(item->text(0).toLocal8Bit().data());

So in the mouseMoveEvent in the CustomTreeWidget class, the following statement returns true and the method returns:

if (item == NULL) return;

What I am I doing wrong? dynamic_cast fails.

I also tried ScreenToClient((HWND)this->winId(), &p)) and passing px and py to childAt() as well as event.pos.x() and event.pos.y(). I am really confused.

I checked my px and py in the log and they always valid.

I even show a tooltip at the x and y and they are valid, yet, childAt() fails.

Answer 1

The problem is that you use "the position of the mouse cursor, in screen coordinates.", but it requires coordinates in QTreeWidget's coordinates system. Thus I would suggest to drop using WinAPI and use QMouseEvent::pos() function instead. Your mouseMoveEvent will look like:

void CustomTreeWidget::mouseMoveEvent(QMouseEvent *event)
{
    QTreeWidget::mouseMoveEvent(event);    
    QTreeWidgetItem *item = itemAt(event->pos();    
    if (item != NULL)
        qDebug(item->text(0).toLocal8Bit().data());
}

Please note the usage of QTableWidget::itemAt() function instead of childAt() .