When is an anonymous function evaluated?

Question

Hi everyone.
I used two kinds of anonymous function in my AS 3.0 Flash
file, in order to implement the roll-over animation of menu
buttons. menu_1, menu_2, ... are the buttons already on the stage,
and menu_list is an array of them. The code is about adding event
listeners on the menu buttons to the corresponding movie clips
("shades" in the code).
It is supposed that when MOUSE_OVER happens on menu_1, then
shade_list[0] plays itself from a frame named "colorIn". However,
when complied and run, all of the event listeners are linked to the
final element of shade_list, namely shade_list[5]. No matter what
button your mouse is going over, a shade covers the sixth menu
button.
If I put the for statement away and hard-code the whole
thing, like menu_list[0].addEventListener( ... );
menu_list[1].addEventListener( ... );
menu_list[2].addEventListener( ... ); ..., then it works fine as I
intended. But I'd really appreciate if I can do this with for
statement, since this time it should be an extensive menu
design.

Answer 1

The problem was because of the concept of "scope." In the
code, the local variable named menuNumber is used in an anonymous
function about shade_list. However, shade_list[menuNumber] is not
evaluated until it is called. When it is actually called,
menuNumber already has the final value (namely the length of
menu_list) because the for statement is over. This is why every
event listener is linked to the final element of shade_list. So I
have to force menuNumber to be evaluated when addEventListener
happens. I can do this by adding an extra scope.
Thus
function (MouseEvent) {
shade_list[menuNumber].gotoAndPlay("colorIn") }
should be rewritten like
function (menuNumber) { return function (MouseEvent) {
shade_list[menuNumber].gotoAndPlay("colorIn") } }
(menuNumber)