Source vote and image changes

Some applications might require that the source be able to change its vote, its drag images, or its drag offsets based on the target, the operation, or both. For example, the source might want to allow certain operations only if the item is being dragged over the source. In this case, the source would need an opportunity to change its vote whenever the target changed.

To support this requirement, the dragChangeCallback on the source adapter is activated whenever the target or the operation changes. The callData includes the items being dragged, the target widget, target item, target model, and the current operation. It also includes the drag images and offsets and the most recent source vote and source model. The application can change these values to implement behaviors like those outlined in the example above.