How can I have an alert sent if the array changes? - php

Question

If NewMessage changes, I want an alert. I have tried live and change on and bind change, but nothing works.
Relevant PHP:
$message=6;
$myReturnData["Message"] = $message;
//JSON-encode and return
print json_encode($myReturnData);
Relevant jQuery:
setInterval(function(){
$.getJSON("foo.php", function(data){
var NewMessage=(data.Message);
if(NewMessage>0){
document.title= NewMessage + ' pm';}
$(NewMessage).live("change", function() {
alert(NewMessage);
});
});
}, 3000);

Answer 1

'change' is not doing what you think it should be. You should store the original message in a variable, then compare it to the new message you're getting. Like this:
var currentMessage = '';
setInterval(function(){
$.getJSON("foo.php", function(data){
var NewMessage=(data.Message);
if(NewMessage>0){
document.title= NewMessage + ' pm';
if (currentMessage !== NewMessage) {
alert(NewMessage);
currentMessage = NewMessage;
}
}
});
}, 3000);

Answer 2

Assuming you want function X to run when a variable controlled by the server changes. To that means you are using a polling mechanism.
So if you want your code to know when it receives a different message, you must store the previous message somewhere outside the scope of your callback function.
Most easy way:
var lastMessage = 'the server will never ever forever ever return this message';
setInterval(function(){
$.getJSON("foo.php", function(data){
// .. arbitrary code
if (lastMessage != data.Message)
{
alert('it changed');
lastMessage = data.Message;
}
// .. more arbitrary code
});
}, 3000);
i don't think the following code makes any sense or would even remotely work, because the NewMessage is not a DOM element, is it?
$(NewMessage).live("change", function() {
alert(NewMessage);
});