Add the QAbstractItemModel::canDropMimeData method.

Can be used by views to indicate whether a drop is allowed (eg
adequete permissions in a filesystem model).

Change-Id: Iefedb5399e44c8edc5f5df1403c8d5c0da618612
Reviewed-by: Peter Penz <peter.penz19@gmail.com>
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/itemmodels/qabstractitemmodel.cpp

@@ -1752,6 +1752,23 @@ QMimeData *QAbstractItemModel::mimeData(const QModelIndexList &indexes) const
     return data;
 }
 
+/*!
+    Returns whether a model can accept a drop of data.
+
+    This can be used to indicate whether a drop of certain data is allowed, for example
+    by using a 'forbidden' emblem on a mouse cursor during a drag operation.
+
+    This method returns true by default.
+
+    \sa dropMimeData(), {Using drag and drop with item views}
+ */
+bool QAbstractItemModel::canDropMimeData(const QMimeData *data, Qt::DropAction action,
+                                         int row, int column,
+                                         const QModelIndex &parent) const
+{
+    return true;
+}
+
 /*!
     Handles the \a data supplied by a drag and drop operation that ended with
     the given \a action.
@@ -1773,7 +1790,7 @@ QMimeData *QAbstractItemModel::mimeData(const QModelIndexList &indexes) const
     greater than or equal zero, it means that the drop occurred just before the
     specified \a row and \a column in the specified \a parent.
 
-    \sa supportedDropActions(), {Using drag and drop with item views}
+    \sa supportedDropActions(), canDropMimeData(), {Using drag and drop with item views}
 */
 bool QAbstractItemModel::dropMimeData(const QMimeData *data, Qt::DropAction action,
                                       int row, int column, const QModelIndex &parent)

src/corelib/itemmodels/qabstractitemmodel.h

@@ -193,6 +193,8 @@ public:
 
     virtual QStringList mimeTypes() const;
     virtual QMimeData *mimeData(const QModelIndexList &indexes) const;
+    virtual bool canDropMimeData(const QMimeData *data, Qt::DropAction action,
+                                 int row, int column, const QModelIndex &parent) const;
     virtual bool dropMimeData(const QMimeData *data, Qt::DropAction action,
                               int row, int column, const QModelIndex &parent);
     virtual Qt::DropActions supportedDropActions() const;

tests/auto/corelib/itemmodels/qabstractitemmodel/tst_qabstractitemmodel.cpp

@@ -72,6 +72,7 @@ private slots:
     void match();
     void dropMimeData_data();
     void dropMimeData();
+    void canDropMimeData();
     void changePersistentIndex();
     void movePersistentIndex();
 
@@ -150,6 +151,9 @@ public:
                      const QModelIndex &destinationParent, int destinationChild);
     void reset();
 
+    bool canDropMimeData(const QMimeData *data, Qt::DropAction action,
+                                 int row, int column, const QModelIndex &parent) const;
+
     int cCount, rCount;
     mutable bool wrongIndex;
     QVector<QVector<QString> > table;
@@ -315,6 +319,25 @@ void QtTestModel::reset()
     QAbstractItemModel::reset();
 }
 
+bool QtTestModel::canDropMimeData(const QMimeData *data, Qt::DropAction action,
+                                 int row, int column, const QModelIndex &parent) const
+{
+    Q_UNUSED(data);
+    Q_UNUSED(action);
+
+    // For testing purposes, we impose some arbitrary rules on what may be dropped.
+    if (!parent.isValid() && row < 0 && column < 0) {
+        // a drop in emtpy space in the view is allowed.
+        // For example, in a filesystem view, a file may be dropped into empty space
+        // if it represents a writable directory.
+        return true;
+    }
+
+    // We then arbitrarily decide to only allow drops on odd rows.
+    // A filesystem view/model might be able to drop onto (writable) directories.
+    return row % 2 == 0;
+}
+
 /**
  * The source Model *must* be initialized before the _data function, since the _data function uses QModelIndexes to reference the items in the tables.
  * Therefore, we must initialize it globally.
@@ -755,6 +778,15 @@ void tst_QAbstractItemModel::dropMimeData()
     }
 }
 
+void tst_QAbstractItemModel::canDropMimeData()
+{
+    QtTestModel model(3, 3);
+
+    QVERIFY(model.canDropMimeData(0, Qt::CopyAction, -1, -1, QModelIndex()));
+    QVERIFY(model.canDropMimeData(0, Qt::CopyAction, 0, 0, QModelIndex()));
+    QVERIFY(!model.canDropMimeData(0, Qt::CopyAction, 1, 0, QModelIndex()));
+}
+
 void tst_QAbstractItemModel::changePersistentIndex()
 {
     QtTestModel model(3, 3);