the method is called when a drag operation is initiated
|source||HTMLElement||a DOM element on which a dnd action has started|
|ev||Event||a native HTML event|
|pointer||string||defines what pointer started dnd|
|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".