onBeforeStateChange

fires before the moment when the state of a two-state button item is changed

boolean onBeforeStateChange(string|number id,boolean state);

id	string\|number	id of an item
state	boolean	new state of an item

boolean

true to allow state changing, false to forbid it

myToolbar.attachEvent("onBeforeStateChange", function(id, state){
    // your code here
});