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>

2 files changed, 8 insertions, 1 deletions

diff --git a/src/plugins/platforms/windows/qwindowsdrag.cpp b/src/plugins/platforms/windows/qwindowsdrag.cpp
index 955f5792a9..719553fa7c 100644
--- a/src/plugins/platforms/windows/qwindowsdrag.cpp
+++ b/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);
diff --git a/src/plugins/platforms/windows/qwindowsdrag.h b/src/plugins/platforms/windows/qwindowsdrag.h
index 9a5e0b17f2..b031faa884 100644
--- a/src/plugins/platforms/windows/qwindowsdrag.h
+++ b/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;