复制粘贴——QT实现原理

QT 剪贴板相关类

QClipboard

对外通用的剪贴板类，一般通过QGuiApplication::clipboard() 来获取对应的剪贴板实例。

// qtbase/src/gui/kernel/qclipboard.h
class Q_GUI_EXPORT QClipboard : public QObject
{
    Q_OBJECT
private:
    explicit QClipboard(QObject *parent);
    ~QClipboard();

public:
    enum Mode { Clipboard, Selection, FindBuffer, LastMode = FindBuffer };

    void clear(Mode mode = Clipboard);

    bool supportsSelection() const;
    bool supportsFindBuffer() const;

    bool ownsSelection() const;
    bool ownsClipboard() const;
    bool ownsFindBuffer() const;

    QString text(Mode mode = Clipboard) const;
    QString text(QString& subtype, Mode mode = Clipboard) const;
    void setText(const QString &, Mode mode = Clipboard);

    const QMimeData *mimeData(Mode mode = Clipboard ) const;
    void setMimeData(QMimeData *data, Mode mode = Clipboard);

    QImage image(Mode mode = Clipboard) const;
    QPixmap pixmap(Mode mode = Clipboard) const;
    void setImage(const QImage &, Mode mode  = Clipboard);
    void setPixmap(const QPixmap &, Mode mode  = Clipboard);

Q_SIGNALS:
    void changed(QClipboard::Mode mode);
    void selectionChanged();
    void findBufferChanged();
    void dataChanged();

protected:
    friend class QApplication;
    friend class QApplicationPrivate;
    friend class QGuiApplication;
    friend class QBaseApplication;
    friend class QDragManager;
    friend class QPlatformClipboard;

private:
    Q_DISABLE_COPY(QClipboard)

    bool supportsMode(Mode mode) const;
    bool ownsMode(Mode mode) const;
    void emitChanged(Mode mode);
};

QPlatformClipboard

系统剪切板平台接口类，各种桌面平台（Windows，X11，Wayland等）通过这个类提供统一的剪贴板操作接口。

// qtbase/src/gui/kernel/qplatformclipboard.h
class Q_GUI_EXPORT QPlatformClipboard
{
public:
    virtual ~QPlatformClipboard();

    virtual QMimeData *mimeData(QClipboard::Mode mode = QClipboard::Clipboard);
    virtual void setMimeData(QMimeData *data, QClipboard::Mode mode = QClipboard::Clipboard);
    virtual bool supportsMode(QClipboard::Mode mode) const;
    virtual bool ownsMode(QClipboard::Mode mode) const;
    void emitChanged(QClipboard::Mode mode);
};

QXcbClipboard

X11平台实现的剪贴板接口类，继承自QPlatformClipboard，它主要实现了基类的大部分接口，除了emitChanged 这个接口。

// qtbase/src/plugins/platforms/xcb/qxcbclipboard.h
class QXcbClipboard : public QXcbObject, public QPlatformClipboard
{
public:
    QXcbClipboard(QXcbConnection *connection);
    ~QXcbClipboard();

    QMimeData *mimeData(QClipboard::Mode mode) override;
    void setMimeData(QMimeData *data, QClipboard::Mode mode) override;

    bool supportsMode(QClipboard::Mode mode) const override;
    bool ownsMode(QClipboard::Mode mode) const override;
...
};

QWindowsClipboard

Windows平台下的剪贴板接口类，继承自QPlatformClipboard 。

// qtbase/src/plugins/platforms/windows/qwindowsclipboard.h
class QWindowsClipboard : public QPlatformClipboard
{
public:
    QWindowsClipboard();
    ~QWindowsClipboard();
    void registerViewer(); // Call in initialization, when context is up.
    void cleanup();

    QMimeData *mimeData(QClipboard::Mode mode = QClipboard::Clipboard) override;
    void setMimeData(QMimeData *data, QClipboard::Mode mode = QClipboard::Clipboard) override;
    bool supportsMode(QClipboard::Mode mode) const override;
    bool ownsMode(QClipboard::Mode mode) const override;
...
}

可以看出，同一目录下还有其他各种平台的实现接口：

QWaylandClipboard

Wayland平台实现的剪贴板接口.

// qtwayland/src/client/qwaylandclipboard_p.h
class Q_WAYLAND_CLIENT_EXPORT QWaylandClipboard : public QPlatformClipboard
{
public:
    QWaylandClipboard(QWaylandDisplay *display);

    ~QWaylandClipboard() override;

    QMimeData *mimeData(QClipboard::Mode mode = QClipboard::Clipboard) override;
    void setMimeData(QMimeData *data, QClipboard::Mode mode = QClipboard::Clipboard) override;
    bool supportsMode(QClipboard::Mode mode) const override;
    bool ownsMode(QClipboard::Mode mode) const override;

private:
    QWaylandDisplay *mDisplay = nullptr;
    QMimeData m_emptyData;
};

QT 剪贴板相关接口

通过查看QClipboard 类的定义，我们比较关心的接口有：

    const QMimeData *mimeData(Mode mode = Clipboard ) const;
    void setMimeData(QMimeData *data, Mode mode = Clipboard);
Q_SIGNALS:
    void changed(QClipboard::Mode mode);
    void selectionChanged();
    void findBufferChanged();
    void dataChanged();

获取剪贴板最基础的应该是mimeData 这个接口：

const QMimeData* QClipboard::mimeData(Mode mode) const
{
    // 获取一个QPlatformClipboard对象，根据不同平台返回的应该是不同的子类，比如x11下就返回的是QXcbClipboard
    QPlatformClipboard *clipboard = QGuiApplicationPrivate::platformIntegration()->clipboard();
    if (!clipboard->supportsMode(mode)) return 0;
    return clipboard->mimeData(mode);
}

可以看出，最终是通过X11接口拿到的。

另外，我们关系剪贴板变化的信号在什么情况下发出来，从实现可以看出，基本是在emitChanged 里发出来的。

/*!
    \internal
    Emits the appropriate changed signal for \a mode.
*/
void QClipboard::emitChanged(Mode mode)
{
    switch (mode) {
        case Clipboard:
            emit dataChanged();
        break;
        case Selection:
            emit selectionChanged();
        break;
        case FindBuffer:
            emit findBufferChanged();
        break;
        default:
        break;
    }
    emit changed(mode);
}

还有一个地方会通过emitChanged发出变化的信号：

void QPlatformClipboard::emitChanged(QClipboard::Mode mode)
{
    if (!QGuiApplicationPrivate::is_app_closing) // QTBUG-39317, prevent emission when closing down.
        QGuiApplication::clipboard()->emitChanged(mode);
}

可以再往下看下谁会调用emitChanged ，可以发现是QPlatformClipboard 的子类QXcbClipboard ：

// qtbase/src/plugins/platforms/xcb/qxcbclipboard.cpp
void QXcbClipboard::setMimeData(QMimeData *data, QClipboard::Mode mode)
{
    if (mode > QClipboard::Selection)
        return;

    QXcbClipboardMime *xClipboard = 0;
    // verify if there is data to be cleared on global X Clipboard.
    if (!data) {
        xClipboard = qobject_cast<QXcbClipboardMime *>(mimeData(mode));
        if (xClipboard) {
            if (xClipboard->isEmpty())
                return;
        }
    }

    if (!xClipboard && (m_clientClipboard[mode] == data))
        return;

    xcb_atom_t modeAtom = atomForMode(mode);
    xcb_window_t newOwner = XCB_NONE;

    if (m_clientClipboard[mode]) {
        if (m_clientClipboard[QClipboard::Clipboard] != m_clientClipboard[QClipboard::Selection])
            delete m_clientClipboard[mode];
        m_clientClipboard[mode] = 0;
        m_timestamp[mode] = XCB_CURRENT_TIME;
    }

    if (connection()->time() == XCB_CURRENT_TIME)
        connection()->setTime(connection()->getTimestamp());

    if (data) {
        newOwner = owner();

        m_clientClipboard[mode] = data;
        m_timestamp[mode] = connection()->time();
    }

    xcb_set_selection_owner(xcb_connection(), newOwner, modeAtom, connection()->time());

    if (getSelectionOwner(modeAtom) != newOwner) {
        qWarning("QXcbClipboard::setMimeData: Cannot set X11 selection owner");
    }

    emitChanged(mode);
}

void QXcbClipboard::handleXFixesSelectionRequest(xcb_xfixes_selection_notify_event_t *event)
{
    QClipboard::Mode mode = modeForAtom(event->selection);
    if (mode > QClipboard::Selection)
        return;

    // Note1: Here we care only about the xfixes events that come from other processes.
    // Note2: If the QClipboard::clear() is issued, event->owner is XCB_NONE,
    // so we check selection_timestamp to not handle our own QClipboard::clear().
    if (event->owner != owner() && event->selection_timestamp > m_timestamp[mode]) {
        if (!m_xClipboard[mode]) {
            m_xClipboard[mode].reset(new QXcbClipboardMime(mode, this));
        } else {
            m_xClipboard[mode]->reset();
        }
        emitChanged(mode);
    } else if (event->subtype == XCB_XFIXES_SELECTION_EVENT_SELECTION_CLIENT_CLOSE ||
               event->subtype == XCB_XFIXES_SELECTION_EVENT_SELECTION_WINDOW_DESTROY)
        emitChanged(mode);
}

至此，我们可以知道QClipboard是如何发出剪贴板内容变化的信号了：

1. QClipboard设置剪贴板内容（setMimeData），QXcbClipboard设置完剪贴板内容，emitChanged通知内容变化
2. QXcbClipboard收到X11剪贴板变化的事件，主动emitChanged通知QClipboard剪贴板变化

总结

qt的剪贴板底层是由各个平台的剪贴板接口驱动的，如果是X11平台，那么整个剪贴板就是X11接口驱动的。关于如何使用X11原生剪贴板接口，参考：https://stackoverflow.com/questions/27378318/c-get-string-from-clipboard-on-linux