qt中用treewidget创建横向表格，实现两个treewidget能相互拖动item，并且只能拖动到到根节点的子节点同级节点

实现这个功能需要以下几个步骤：

1. 创建两个QTreeWidget，一个作为源，一个作为目标。

2. 设置源QTreeWidget为可拖动，目标QTreeWidget为可接受拖放。

sourceWidget->setDragEnabled(true);
targetWidget->setAcceptDrops(true);

3. 在源QTreeWidget的mousePressEvent事件中，记录被拖动的item和它的父节点。

if (event->button() == Qt::LeftButton) {
    QTreeWidgetItem *item = sourceWidget->itemAt(event->pos());
    if (item && item->parent()) {
        m_draggedItem = item;
        m_draggedParent = item->parent();
    }
}

4. 在源QTreeWidget的mouseMoveEvent事件中，如果鼠标移动的距离大于一定阈值，就开始拖拽操作。

if (m_draggedItem && (event->buttons() & Qt::LeftButton)) {
    int distance = (event->pos() - m_pressPos).manhattanLength();
    if (distance >= QApplication::startDragDistance()) {
        QDrag *drag = new QDrag(this);
        QMimeData *mimeData = new QMimeData;
        mimeData->setData("application/x-qabstractitemmodeldatalist",
                          sourceWidget->model()->itemData(m_draggedItem).value<QByteArray>());
        drag->setMimeData(mimeData);
        drag->exec(Qt::CopyAction | Qt::MoveAction, Qt::CopyAction);
    }
}

5. 在目标QTreeWidget的dragEnterEvent和dragMoveEvent事件中，检查被拖动的item是否可以拖放到指定位置。如果不能，就设置禁止拖放的光标。

if (event->mimeData()->hasFormat("application/x-qabstractitemmodeldatalist")) {
    QTreeWidgetItem *targetItem = this->itemAt(event->pos());
    QTreeWidgetItem *parentItem = targetItem ? targetItem->parent() : this->invisibleRootItem();
    if (parentItem == m_draggedParent) {
        event->accept();
    } else {
        event->ignore();
        this->setCursor(Qt::ForbiddenCursor);
    }
} else {
    event->ignore();
}

6. 在目标QTreeWidget的dropEvent事件中，将被拖动的item插入到指定位置。

if (event->mimeData()->hasFormat("application/x-qabstractitemmodeldatalist")) {
    QByteArray data = event->mimeData()->data("application/x-qabstractitemmodeldatalist");
    QDataStream stream(&data, QIODevice::ReadOnly);
    QList<QVariant> itemData;
    QPoint offset;
    stream >> itemData >> offset;
    QTreeWidgetItem *targetItem = this->itemAt(event->pos());
    QTreeWidgetItem *parentItem = targetItem ? targetItem->parent() : this->invisibleRootItem();
    if (parentItem == m_draggedParent) {
        QTreeWidgetItem *newItem = new QTreeWidgetItem(parentItem, itemData);
        parentItem->insertChild(targetItem ? this->indexOfTopLevelItem(targetItem) : parentItem->childCount(), newItem);
        event->accept();
    } else {
        event->ignore();
    }
}

完整的代码示例：

#include <QtWidgets>

class TreeWidget : public QWidget {
public:
    TreeWidget(QWidget *parent = nullptr) : QWidget(parent) {
        QVBoxLayout *layout = new QVBoxLayout(this);
        QHBoxLayout *hlayout = new QHBoxLayout;
        layout->addLayout(hlayout);

        m_sourceWidget = new QTreeWidget(this);
        m_sourceWidget->setColumnCount(1);
        m_sourceWidget->setHeaderHidden(true);
        m_sourceWidget->setDragEnabled(true);
        hlayout->addWidget(m_sourceWidget);

        m_targetWidget = new QTreeWidget(this);
        m_targetWidget->setColumnCount(1);
        m_targetWidget->setHeaderHidden(true);
        m_targetWidget->setAcceptDrops(true);
        hlayout->addWidget(m_targetWidget);

        QTreeWidgetItem *rootItem = new QTreeWidgetItem(m_sourceWidget, {"Root"});
        QTreeWidgetItem *childItem = new QTreeWidgetItem(rootItem, {"Child"});
        rootItem->addChild(childItem);

        connect(m_sourceWidget, &QTreeWidget::itemPressed, this, &TreeWidget::onSourceItemPressed);
        connect(m_targetWidget, &QTreeWidget::dragEnterEvent, this, &TreeWidget::onTargetDragEnter);
        connect(m_targetWidget, &QTreeWidget::dragMoveEvent, this, &TreeWidget::onTargetDragMove);
        connect(m_targetWidget, &QTreeWidget::dropEvent, this, &TreeWidget::onTargetDrop);
    }

    void onSourceItemPressed(QTreeWidgetItem *item, int column) {
        Q_UNUSED(column)
        m_draggedItem = nullptr;
        m_draggedParent = nullptr;
        m_pressPos = QCursor::pos();
        if (item && item->parent()) {
            m_draggedItem = item;
            m_draggedParent = item->parent();
        }
    }

    void onTargetDragEnter(QDragEnterEvent *event) {
        if (event->mimeData()->hasFormat("application/x-qabstractitemmodeldatalist")) {
            QTreeWidgetItem *targetItem = m_targetWidget->itemAt(event->pos());
            QTreeWidgetItem *parentItem = targetItem ? targetItem->parent() : m_targetWidget->invisibleRootItem();
            if (parentItem == m_draggedParent) {
                event->accept();
            } else {
                event->ignore();
                m_targetWidget->setCursor(Qt::ForbiddenCursor);
            }
        } else {
            event->ignore();
        }
    }

    void onTargetDragMove(QDragMoveEvent *event) {
        if (event->mimeData()->hasFormat("application/x-qabstractitemmodeldatalist")) {
            QTreeWidgetItem *targetItem = m_targetWidget->itemAt(event->pos());
            QTreeWidgetItem *parentItem = targetItem ? targetItem->parent() : m_targetWidget->invisibleRootItem();
            if (parentItem == m_draggedParent) {
                event->accept();
            } else {
                event->ignore();
                m_targetWidget->setCursor(Qt::ForbiddenCursor);
            }
        } else {
            event->ignore();
        }
    }

    void onTargetDrop(QDropEvent *event) {
        if (event->mimeData()->hasFormat("application/x-qabstractitemmodeldatalist")) {
            QByteArray data = event->mimeData()->data("application/x-qabstractitemmodeldatalist");
            QDataStream stream(&data, QIODevice::ReadOnly);
            QList<QVariant> itemData;
            QPoint offset;
            stream >> itemData >> offset;
            QTreeWidgetItem *targetItem = m_targetWidget->itemAt(event->pos());
            QTreeWidgetItem *parentItem = targetItem ? targetItem->parent() : m_targetWidget->invisibleRootItem();
            if (parentItem == m_draggedParent) {
                QTreeWidgetItem *newItem = new QTreeWidgetItem(parentItem, itemData);
                parentItem->insertChild(targetItem ? m_targetWidget->indexOfTopLevelItem(targetItem) : parentItem->childCount(), newItem);
                event->accept();
            } else {
                event->ignore();
            }
        } else {
            event->ignore();
        }
    }

private:
    QTreeWidget *m_sourceWidget;
    QTreeWidget *m_targetWidget;
    QTreeWidgetItem *m_draggedItem;
    QTreeWidgetItem *m_draggedParent;
    QPoint m_pressPos;
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    TreeWidget widget;
    widget.show();

    return app.exec();
}