Implement canceling of Qt-initiated drags.

- Add new virtual QPlatformDrag::cancelDrag()
  [avoiding a conflict with existing QBasicDrag::cancel()]
- Implement on Windows by returning DRAGDROP_S_CANCEL
  from IOleDropSource::QueryContinueDrag() as suggested on report.
- Implement in QBasicDrag by calling QBasicDrag::cancel()
  and quitting the event loop.
- Add new API static void QDrag::cancel() for it.

Task-number: QTBUG-47004
Change-Id: I4b4bb52e5fc226c8e04688ac1b0f9550daaf918e
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>
Reviewed-by: David Faure <david.faure@kdab.com>

src/gui/kernel/qdrag.cpp

@@ -33,6 +33,8 @@
 
 #include <qdrag.h>
 #include "private/qguiapplication_p.h"
+#include "qpa/qplatformintegration.h"
+#include "qpa/qplatformdrag.h"
 #include <qpixmap.h>
 #include <qpoint.h>
 #include "qdnd_p.h"
@@ -223,6 +225,8 @@ QObject *QDrag::target() const
     loop. Other events are still delivered to the application while
     the operation is performed. On Windows, the Qt event loop is
     blocked during the operation.
+
+    \sa cancel()
 */
 
 Qt::DropAction QDrag::exec(Qt::DropActions supportedActions)
@@ -377,6 +381,21 @@ Qt::DropAction QDrag::defaultAction() const
     Q_D(const QDrag);
     return d->default_action;
 }
+
+/*!
+    Cancels a drag operation initiated by Qt.
+
+    \note This is currently implemented on Windows and X11.
+
+    \since 5.6
+    \sa exec()
+*/
+void QDrag::cancel()
+{
+    if (QPlatformDrag *platformDrag = QGuiApplicationPrivate::platformIntegration()->drag())
+        platformDrag->cancelDrag();
+}
+
 /*!
     \fn void QDrag::actionChanged(Qt::DropAction action)
 

src/gui/kernel/qdrag.h

@@ -77,6 +77,8 @@ public:
     Qt::DropActions supportedActions() const;
     Qt::DropAction defaultAction() const;
 
+    static void cancel();
+
 Q_SIGNALS:
     void actionChanged(Qt::DropAction action);
     void targetChanged(QObject *newTarget);

src/gui/kernel/qplatformdrag.cpp

@@ -154,6 +154,20 @@ Qt::DropAction QPlatformDrag::defaultAction(Qt::DropActions possibleActions,
     return default_action;
 }
 
+/*!
+    \brief Cancels the currently active drag (only for drags of
+    the current application initiated by QPlatformDrag::drag()).
+
+    The default implementation does nothing.
+
+    \since 5.6
+ */
+
+void QPlatformDrag::cancelDrag()
+{
+    Q_UNIMPLEMENTED();
+}
+
 /*!
     \brief Called to notify QDrag about changes of the current action.
  */

src/gui/kernel/qplatformdrag.h

@@ -92,6 +92,7 @@ public:
     virtual QMimeData *platformDropData() = 0;
 
     virtual Qt::DropAction drag(QDrag *m_drag) = 0;
+    virtual void cancelDrag();
     void updateAction(Qt::DropAction action);
 
     virtual Qt::DropAction defaultAction(Qt::DropActions possibleActions, Qt::KeyboardModifiers modifiers) const;

src/gui/kernel/qsimpledrag.cpp

@@ -191,6 +191,14 @@ Qt::DropAction QBasicDrag::drag(QDrag *o)
     return m_executed_drop_action;
 }
 
+void QBasicDrag::cancelDrag()
+{
+    if (m_eventLoop) {
+        cancel();
+        m_eventLoop->quit();
+    }
+}
+
 void QBasicDrag::restoreCursor()
 {
     if (m_restoreCursor) {

src/gui/kernel/qsimpledrag_p.h

@@ -66,6 +66,7 @@ public:
     virtual ~QBasicDrag();
 
     virtual Qt::DropAction drag(QDrag *drag) Q_DECL_OVERRIDE;
+    void cancelDrag() Q_DECL_OVERRIDE;
 
     virtual bool eventFilter(QObject *o, QEvent *e) Q_DECL_OVERRIDE;
 

src/plugins/platforms/windows/qwindowsdrag.cpp

@@ -397,7 +397,7 @@ QWindowsOleDropSource::QueryContinueDrag(BOOL fEscapePressed, DWORD grfKeyState)
 {
     HRESULT hr = S_OK;
     do {
-        if (fEscapePressed) {
+        if (fEscapePressed || QWindowsDrag::isCanceled()) {
             hr = ResultFromScode(DRAGDROP_S_CANCEL);
             break;
         }
@@ -677,6 +677,8 @@ QWindowsOleDropTarget::Drop(LPDATAOBJECT pDataObj, DWORD grfKeyState,
     \ingroup qt-lighthouse-win
 */
 
+bool QWindowsDrag::m_canceled = false;
+
 QWindowsDrag::QWindowsDrag() :
     m_dropDataObject(0), m_cachedDropTargetHelper(0)
 {
@@ -806,6 +808,7 @@ Qt::DropAction QWindowsDrag::drag(QDrag *drag)
     Qt::DropAction dragResult = Qt::IgnoreAction;
 
     DWORD resultEffect;
+    QWindowsDrag::m_canceled = false;
     QWindowsOleDropSource *windowDropSource = new QWindowsOleDropSource(this);
     windowDropSource->createCursors();
     QWindowsOleDataObject *dropDataObject = new QWindowsOleDataObject(dropData);

src/plugins/platforms/windows/qwindowsdrag.h

@@ -87,6 +87,8 @@ public:
     Qt::DropAction drag(QDrag *drag) Q_DECL_OVERRIDE;
 
     static QWindowsDrag *instance();
+    void cancelDrag() Q_DECL_OVERRIDE { QWindowsDrag::m_canceled = true; }
+    static bool isCanceled() { return QWindowsDrag::m_canceled; }
 
     IDataObject *dropDataObject() const             { return m_dropDataObject; }
     void setDropDataObject(IDataObject *dataObject) { m_dropDataObject = dataObject; }
@@ -98,6 +100,8 @@ public:
     QPixmap defaultCursor(Qt::DropAction action) const;
 
 private:
+    static bool m_canceled;
+
     QWindowsDropMimeData m_dropData;
     IDataObject *m_dropDataObject;