overwrite "changed" on the items of a customized form

we want to overwrite the method : setItems for intercepting the changed event of assigned form items

by coding it in this way :

setItems : function (itemList, firstInit) {
this.Super("setItems",arguments);
if (itemList !== null && itemList !== undefined) {
for (var i = 0; i < itemList.length; i++) {
var field = itemList[i];
field.changed = function (form, item, value) {
alert("field has changed ...");
field.Super("changed",arguments);
}
}
}
},

the function "changed" of the field is not called ... what are we missing ?