QT拖拽事件順序問題

需求：從右邊拖動一個按鈕到左邊，然後生成一個對應的控件。

效果一：

這樣的效果顯然不是我們想要的，於是查了下Qt的幫助文檔，增加一句代碼：

 ui->widget->layout()->setAlignment(Qt::AlignTop);

效果如下：

Qt要想實現拖拽，一般重寫以下函數：

protected:
    void dragEnterEvent(QDragEnterEvent *event) override;
    void dragMoveEvent(QDragMoveEvent *event) override;
    void dropEvent(QDropEvent *event) override;
    void mousePressEvent(QMouseEvent *event) override;

下面是Qt自帶拖拽例子部分代碼：

void DragWidget::dragEnterEvent(QDragEnterEvent *event)
{
    if (event->mimeData()->hasFormat("application/x-dnditemdata")) {
        if (event->source() == this) {
            event->setDropAction(Qt::MoveAction);
            event->accept();
        } else {
            event->acceptProposedAction();
        }
    } else {
        event->ignore();
    }
}

void DragWidget::dragMoveEvent(QDragMoveEvent *event)
{
    if (event->mimeData()->hasFormat("application/x-dnditemdata")) {
        if (event->source() == this) {
            event->setDropAction(Qt::MoveAction);
            event->accept();
        } else {
            event->acceptProposedAction();
        }
    } else {
        event->ignore();
    }
}

void DragWidget::dropEvent(QDropEvent *event)
{
    if (event->mimeData()->hasFormat("application/x-dnditemdata")) {
        QByteArray itemData = event->mimeData()->data("application/x-dnditemdata");
        QDataStream dataStream(&itemData, QIODevice::ReadOnly);

        QPixmap pixmap;
        QPoint offset;
        dataStream >> pixmap >> offset;

        QLabel *newIcon = new QLabel(this);
        newIcon->setPixmap(pixmap);
        newIcon->move(event->pos() - offset);
        newIcon->show();
        newIcon->setAttribute(Qt::WA_DeleteOnClose);

        if (event->source() == this) {
            event->setDropAction(Qt::MoveAction);
            event->accept();
        } else {
            event->acceptProposedAction();
        }
    } else {
        event->ignore();
    }
}

//! [1]
void DragWidget::mousePressEvent(QMouseEvent *event)
{

    qDebug()<<"mousePressEvent"<<event->pos();
    QLabel *child = static_cast<QLabel*>(childAt(event->pos()));
    if (!child)
        return;

    QPixmap pixmap = *child->pixmap();

    QByteArray itemData;
    QDataStream dataStream(&itemData, QIODevice::WriteOnly);
    dataStream << pixmap << QPoint(event->pos() - child->pos());
//! [1]

//! [2]
    QMimeData *mimeData = new QMimeData;
    mimeData->setData("application/x-dnditemdata", itemData);
//! [2]

//! [3]
    QDrag *drag = new QDrag(this);
    drag->setMimeData(mimeData);
    drag->setPixmap(pixmap);
    drag->setHotSpot(event->pos() - child->pos());
//! [3]

    QPixmap tempPixmap = pixmap;
    QPainter painter;
    painter.begin(&tempPixmap);
    painter.fillRect(pixmap.rect(), QColor(127, 127, 127, 127));
    painter.end();

    child->setPixmap(tempPixmap);

    if (drag->exec(Qt::CopyAction | Qt::MoveAction, Qt::CopyAction) == Qt::MoveAction) {
        child->close();
    } else {
        child->show();
        child->setPixmap(pixmap);
    }
}

實現效果如下：