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QXcbClipboardQPlatformClipboard *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。

/*!\internalEmits 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