Objective C Unsure where method is being called? - objective-c

Question

Showing what a novice I am with Objective C here. The second of these two methods is getting called by the method above. Though I have absolutely no idea where? I want to be able to wrap the part that calls the second method in an if statement to determine if the file did exist based on the returned Boolean. Example code would be appreciated, if anyone could also explain how this second method gets called that would also be fantastic.
-(void) queryResponseForURL:(NSURL *)inURL {
NSMutableURLRequest * request = [NSMutableURLRequest requestWithURL:inURL];
[request setHTTPMethod:#"HEAD"];
NSURLConnection * connection = [NSURLConnection connectionWithRequest:request delegate:self];
// connection starts automatically
}
-(BOOL)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response{
if([(NSHTTPURLResponse *)response statusCode] == 200){
NSLog(#"file exists");
return YES;
}else return NO;
}

Answer 1

It is getting called by the NSURLConnection object, so you can't intervene in the code that calls it. But you shouldn't need to - the purpose of a delegate method is to enable you to hook in to that inaccessible code.
The method signature is this, by the way:
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
It doesn't have a return value and you can't change the method signature, as it's part of the NSURLConnection object's protocol. Remember, you don't call this method, the connection object does. But you can respond to the method. Instead of attempting to return a BOOL, you can deal with your 'if file..' code inside this method, or call another method from it. You should bear in mind that this is triggered (asynchronously) when a response is confirmed to have been initiated - so you can check HTTP header info such as mime type - but it doesn't guarantee a completed data transfer (there are other delegate methods for that).
See also the developer guide to NSURLConnection

Answer 2

The second method is a delegate or callback method. The callback method is called when your connection instance receives a response from the server your connecting with. NSURLConnection has a few informal protocols on NSObject. This means that methods defined in these protocols will automatically be called when any subclass of NSObject has implemented these methods. Your subclass of NSObject has implemented one of these callback methods, therefore this method is called. Most delegates are declared as formal protocols and need to be explicitly defined in your classes in order to make use of them. The informal protocols of NSURLConnection are exception to the rule, though this is about to change in future versions of iOS.
P.S.: a protocol in Objective-C is essentially the same as an interface in Java / C#.
More info here: http://developer.apple.com/library/ios/ipad/#documentation/Foundation/Reference/NSURLConnectionDataDelegate_protocol/Reference/Reference.html
I guess you want to cancel the connection when you receive the wrong response? What you should do is the following:
Store your URLConnection in an ivar or property (personally I'd prefer a property).
In your -connection:didReceiveResponse delegate method, if the response status code isn't an appropriate value, cancel your connection. You can use your ivar or property for this purpose.