the method is called when a drag operation is initiated
| source |
| string | a text which will be used in a dnd placeholder, returning false or an empty string will block dnd |
The method is not purposed for direct call! It is an extension point.
You can override it with your own logic, but check drag-n-drop related events first, as they provide a much more convenient way for drag-n-drop handling.
Pointer has two states: "mouse" and "touch".