Qt官方示例：Fridge Magnets Example（冰箱贴）

冰箱贴类：

#include "draglabel.h"
#include 

DragLabel::DragLabel(const QString &text, QWidget *parent)
    : QLabel(parent)
{
    QFontMetrics metric(font());
    QSize size = metric.size(Qt::TextSingleLine, text);

    QImage image(size.width() + 12, size.height() + 12, QImage::Format_ARGB32_Premultiplied);
    image.fill(qRgba(0, 0, 0, 0));

    QLinearGradient gradient(0, 0, 0, image.height()-1);
    gradient.setColorAt(0.0, Qt::white);
    gradient.setColorAt(0.2, QColor(200, 200, 255));
    gradient.setColorAt(0.8, QColor(200, 200, 255));
    gradient.setColorAt(1.0, QColor(127, 127, 200));

    QPainter painter(&image);
    painter.setRenderHint(QPainter::Antialiasing);
    painter.setBrush(gradient);
    painter.drawRoundedRect(QRectF(0.5, 0.5, image.width()-1, image.height()-1),25, 25, Qt::RelativeSize);
    painter.setFont(font());
    painter.setBrush(Qt::black);
    painter.drawText(QRect(QPoint(6, 6), size), Qt::AlignCenter, text);

    setPixmap(QPixmap::fromImage(image));
    m_labelText = text;
}

QString DragLabel::labelText() const
{
    return m_labelText;
}

一个自定义的 QLabel，根据创建时传入的文本设确定一个图片的尺寸，使用渐变的画刷在图片绘制一个圆角矩形和文本，然后设置标签显示此图片。

然后是一个白色的窗口：

DragWidget::DragWidget(QWidget *parent)
    : QWidget(parent)
{
    QFile dictionaryFile(QStringLiteral(":/dictionary/words.txt"));
    dictionaryFile.open(QFile::ReadOnly);
    QTextStream inputStream(&dictionaryFile);

    int x = 5;
    int y = 5;

    while (!inputStream.atEnd())
    {
        QString word;
        inputStream >> word;
        if (!word.isEmpty())
        {
            DragLabel *wordLabel = new DragLabel(word, this);
            wordLabel->move(x, y);
            wordLabel->show();
            wordLabel->setAttribute(Qt::WA_DeleteOnClose);
            x += wordLabel->width() + 2;
            if (x >= 245)
            {
                x = 5;
                y += wordLabel->height() + 2;
            }
        }
    }

    QPalette newPalette = palette();
    newPalette.setColor(QPalette::Window, Qt::white);
    setPalette(newPalette);

    setMinimumSize(400, qMax(200, y));
    setWindowTitle(tr("Fridge Magnets"));

    setAcceptDrops(true);
}

从一个文本文件中读取内容：

每次读取一行，读取一行就创建一个冰箱贴并移动到相应的位置。最后设置窗口支持拖放。

鼠标按下的操作：

void DragWidget::mousePressEvent(QMouseEvent *event)
{
    DragLabel *child = static_cast(childAt(event->position().toPoint()));
    if (!child)
        return;

    QPoint hotSpot = event->position().toPoint() - child->pos();//鼠标按在child上的位置

    QByteArray itemData;
    QDataStream dataStream(&itemData, QIODevice::WriteOnly);
    dataStream << child->labelText() << QPoint(hotSpot);

    QMimeData *mimeData = new QMimeData;
    mimeData->setData(fridgetMagnetsMimeType(), itemData);
    mimeData->setText(child->labelText());

    QDrag *drag = new QDrag(this);
    drag->setMimeData(mimeData);
    drag->setPixmap(child->pixmap());
    drag->setHotSpot(hotSpot);

    child->hide();

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

这里定义了一种自定义MIME类型：

static inline QString fridgetMagnetsMimeType() { return QStringLiteral("application/x-fridgemagnet"); }

内联的非成员函数，这种写法值得学习。

其中包含了了冰箱贴的文本和鼠标按下的位置。

开始拖放操作后，如果是拖放移到那么冰箱贴就隐藏了，拖动过程中显示的只是 QDrag::pixmap()。

拖动进入事件和拖动移到事件：

void DragWidget::dragEnterEvent(QDragEnterEvent *event)
{
    if (event->mimeData()->hasFormat(fridgetMagnetsMimeType()))
    {
        if (children().contains(event->source()))
        {
            event->setDropAction(Qt::MoveAction);
            event->accept();
        }
        else
        {
            event->acceptProposedAction();
        }
    }
    else if (event->mimeData()->hasText())
    {
        event->acceptProposedAction();
    }
    else
    {
        event->ignore();
    }
}

void DragWidget::dragMoveEvent(QDragMoveEvent *event)
{
    if (event->mimeData()->hasFormat(fridgetMagnetsMimeType()))
    {
        if (children().contains(event->source()))
        {
            event->setDropAction(Qt::MoveAction);
            event->accept();
        }
        else
        {
            event->acceptProposedAction();
        }
    }
    else if (event->mimeData()->hasText())
    {
        event->acceptProposedAction();
    }
    else
    {
        event->ignore();
    }
}

这两个没啥好说的，主要是拖动放下事件：

void DragWidget::dropEvent(QDropEvent *event)
{
    if (event->mimeData()->hasFormat(fridgetMagnetsMimeType()))
    {
        const QMimeData *mime = event->mimeData();
        QByteArray itemData = mime->data(fridgetMagnetsMimeType());
        QDataStream dataStream(&itemData, QIODevice::ReadOnly);

        QString text;
        QPoint offset;
        dataStream >> text >> offset;

        DragLabel *newLabel = new DragLabel(text, this);
        newLabel->move(event->position().toPoint() - offset);
        newLabel->show();
        newLabel->setAttribute(Qt::WA_DeleteOnClose);

        if (event->source() == this)
        {
            event->setDropAction(Qt::MoveAction);
            event->accept();
        }
        else
        {
            event->acceptProposedAction();
        }
    }
    else if (event->mimeData()->hasText())
    {
        QStringList pieces = event->mimeData()->text().split(QRegularExpression(QStringLiteral("\\s+")), Qt::SkipEmptyParts);
        QPoint position = event->position().toPoint();

        for (const QString &piece : pieces)
        {
            DragLabel *newLabel = new DragLabel(piece, this);
            newLabel->move(position);
            newLabel->show();
            newLabel->setAttribute(Qt::WA_DeleteOnClose);

            position += QPoint(newLabel->width(), 0);
        }

        event->acceptProposedAction();
    }
    else
    {
        event->ignore();
    }
}

第一个分支在拖动放下的位置新建冰箱贴，从 QMimeData 取出数据设置其文本和位置。

第二个分支指示拖动能用空白符分割的文本到当前窗口，则使用其文本创建冰箱贴。

涉及到的类：

• QFontMetrics 
• QLabel 
• QFile 
• QTextStream 
• QMimeData 
• QDataStream
• QDrag