Actionscript :: How To Synchronize Methods

The question is how could I stop a method being called twice, where the first call has not "completed" because its handler is waiting for a url to load for example?

I have written a flash client which interfaces with a java server using a binary encrypted protocol (I would love to not have had to re-invent the whole client/server object communcation stack, but I had to encrypt the data in such a way that simple tools like tamper data and charles proxy could not pick them up if using SSL).The API presents itself to flas as an actionscript swf file, and the API itself is a singleton. the api exposes some simple methods, including:

login()
getBalance()
startGame()
endGame()

Each method will call my HttpCommunicator class.HttpCommunicator.as (with error handling and stuff removed):

public class HttpCommunicator {
private var _externalHalder:function;
public function communicate(data:String, externalHandler:APIHandler):void {
// do encryption

[code]...

Will the second call trounce the first calls communicator variable? i.e. will it behave as if its static, as there is onlyone copy of the API object?If say there was a button on the GUI which had "update balance", and the user kept clicking on it, at the same time as say a URLLoader complete event hander being called which also cals the apis getBalance() function (i.e. flash being multithreaded).