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".