Network push notification - iphone

Question

I am wondering if there is a simple way to push a notification, to all your users currently using your application.
So the next time they launch the application and they are connected to Wi-Fi, they receive a alert telling them that, i.e., "An update is available".
EDIT:
To explain in greater detail what I am looking for. I am developing an application that should only be used when it is the most recent version of the software, so I would like to be able to send out a message (notification) for when an update is available in the AppStore. This is seen in some games such as AngryBirds and Cut the Rope.
Maybe even change a BOOL in the code to TRUE, leaving a red flag if{} BOOL is TRUE. (Of course first I'd like an answer to the more basic version. This would be helpful though as well)
Hope this clears things up.
With regards,
SirKaydian

Answer 1

Local Notification Solution
Essentially what Oscar said previously, you could make this really simple. If you wanted to check after every launch of the application you could easily call some sort of local API (or similar) call from your application to a web service you might host. So for example you have a PHP file:
<?php
$currentVersion = 1.3;
echo $currentVersion;
?>
That simple PHP script can be updated by you whenever you release an update. The iPhone can recognize this by querying that PHP file on every startup (check NSURLConnection delegate methods to get the response from any particular URL). Now from previous checks by your application it can store those in an NSUserDefaults value by the following:
NSUserDefaults *defaults = [[NSUserDefaults alloc] init];
NSString *loadedVersion = [defaults valueForKey:#"MYAPP_CURR_VERSION"];
Now we need to compare the two from your NSURLConnection delegate method that receives the string back from the PHP file on your web service.
NSMutableURLRequest *urlRequest = [[NSMutableURLRequest alloc] initWithURL:[NSURL URLWithString:#"http://www.website.com/folder/version.php"]];
[urlRequest setHTTPMethod:#"GET"];
[urlRequest setHTTPBody:[postParams dataUsingEncoding:NSUTF8StringEncoding]];
NSURLConnection *connection = [[NSURLConnection alloc] initWithRequest:urlRequest delegate:self startImmediately:YES];
[connection start];
The above code will send the request to your server URL that you tell it to. Now when we get the response we'll do it like this:
-(void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
NSString *responseVersionFromServer = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
if(![loadedVersion isEqualToString:responseVersionFromServer]) {
//execute code here to show a local notification or UIAlertView
}
}
That will do all the comparison for you.
NOTE: You must add NSURLConnectionDelegate in your .h Header file!
Push Notifications Solution
There are services out there for allowing your application to have push notifications such as Urban Airship that you can log into and send a push notification to all of your users when an update becomes available. It's really simple to use, check out their website. (www.urbanairship.com)

Answer 2

You could consume a webservice and compare versions of your app (maybe save the version to NSUserDefaults), I'm sure there are other solutions but this one comes to mind. You could show an alert that links to the app store if the versions are different.