Parameters:

• id of item;
• selected state of item (true when item is selected by action)

  view.attachEvent("onBeforeSelect", function (id, state){
       //any custom logic here
       return true;
  });

Event is blockable, if non-true value returned from custom method, default reaction will be blocked

In case of multi-select, event occurs only for items which was clicked, and not occurs for items which was selected by shift-click or similar actions.