QListView drag and drop

[buster]

I thought I had fixed this, but discovered that if I drag an item (string) from listA and drop it over an existing item in listB then the existing item is overwritten. From the docs it seems that this behaviour can be prevented by calling setDragDropOverwriteMode(false), but that doesn't change anything. What is the correct way to prevent overwriting an existing item?

[d_stranz]

According to the QAbstractItemView docs:

dragDropOverwriteMode : bool

This property holds the view's drag and drop behavior

If its value is true, the selected data will overwrite the existing item data when dropped, while moving the data will clear the item. If its value is false, the selected data will be inserted as a new item when the data is dropped. When the data is moved, the item is removed as well.

The default value is false, as in the QListView and QTreeView subclasses. In the QTableView subclass, on the other hand, the property has been set to true.

Note: This is not intended to prevent overwriting of items. The model's implementation of flags() should do that by not returning Qt::ItemIsDropEnabled.

The last sentence (Note) is the important one. If you have already derived from QStringListModel to implement supportedDropActions(), then you will also need to implement the flags() method in your model class to mask out the ItemIsDropEnabled flag. ( return QStringListModel::flags( index ) & ~Qt::ItemIsDropEnabled; ) [Edit: added "index" argument to superclass call]

This doesn't make a lot of sense to me. It sounds like dragDropOverwriteMode applies to the entire model when something is dropped, not individual items, but if the default is true for QTableView, who would want behavior when dropping something erased the entire table?

[buster]

According to the QAbstractItemView docs:



The last sentence (Note) is the important one. If you have already derived from QStringListModel to implement supportedDropActions(), then you will also need to implement the flags() method in your model class to mask out the ItemIsDropEnabled flag. ( return QStringListModel::flags() & ~Qt::ItemIsDropEnabled; )

This doesn't make a lot of sense to me. It sounds like dragDropOverwriteMode applies to the entire model when something is dropped, not individual items, but if the default is true for QTableView, who would want behavior when dropping something erased the entire table?

Do you mean?

virtual Qt::ItemFlags flags(const QModelIndex &index) const override
{
    return QStringListModel::flags() | ~Qt::ItemIsDropEnabled;
}

virtual Qt::ItemFlags flags(const QModelIndex &index) const override
{
    return QStringListModel::flags() | ~Qt::ItemIsDropEnabled;
}

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Probably not, as it doesn't compile.

[d_stranz]

No, you should be returning the logical AND of whatever flags there already are (QStringListModel:: flags()) with the bitwise NOT of Qt:: ItemIsDropEnabled. My original post had a logical OR, and I realized the mistake and fixed it a bit later.

return QStringListModel::flags( index ) & ~Qt::ItemIsDropEnabled;

return QStringListModel::flags( index ) & ~Qt::ItemIsDropEnabled;

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Your code doesn't compile because you left out the "index" argument to the superclass call. In this case, I also left out the argument, but seriously, you have to look at what the compiler is telling you and figure out what is wrong.

[buster]

No, you should be returning the logical AND of whatever flags there already are (QStringListModel:: flags()) with the bitwise NOT of Qt:: ItemIsDropEnabled.

return QStringListModel::flags( index ) & ~Qt::ItemIsDropEnabled;

return QStringListModel::flags( index ) & ~Qt::ItemIsDropEnabled;

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Yes, you are right. However, this prevents all drop actions in the view. The solution to the problem, which can be found here, is to AND in Qt::ItemIsDropEnabled only if (index.isValid()). So now the problem is fixed.