JS cross-domain XML--with AS URLLoader

<iframe id="hiddenIframe" style="width:0px; height:0px0; border:0px none; *width:1px; *height:0px; overflow:hidden;"></iframe>
<button value="Get data" onclick="getData()"></button>
<!-- Why not directly use display:none; or visibility: hidden;? [Explain later: tag] -->

document.domain = "xxx.com";
function getData(data) {
if (data == "" || data == undefined || data == null) {
sendData("a=1&b=2"); //The parameter format is arbitrary, equivalent to the parameters you send when making a get request
}
else {
alert("Acquired data: " + data)
} }
function sendData(param) {
var childWindow = document.getElementById('testIframe').contentWindow;
childWindow.sendData(param); // Driving function, driving the method in the hidden domain to call the API method in AS to obtain XML
}
function change() {
document.getElementById("testIframe").src = "xx.html"; //Hidden domain URL
}

document.domain = "xxx.com";
function sendData(paramFromParent) {
var serverUrl = 'xxx.com';
//Here, paramFromParent is processed to obtain the complete URL of xxx.xml
//Then call AS's URLLoader method to get the data under the URL
//After successful acquisition, call the following method to drive the getData method in the parent page
}
function getData(backData) {
window.parent.getData(backData);
}