"NSMutableData data retain" on NSURLConnection - how calm this works?

On a lot of NSURLConnection examples I see the following lines:

NSURLConnection *theConnection = [[NSURLConnection alloc]initWithRequest:theRequest delegate:self];
if(theConnection)
{
    webData = [[NSMutableData data]retain];
}
else
    ...

I wonder - what is this supposed to do? and why does it work? I thought that data is an accessor method, and since your not calling it on an instanciated object, it will return nil, and by retaining it you actually do nothing.

This is the way I have seen to get data on connections like this:

NSURLConnection *connection = [[NSURLConnection alloc] initWithRequest:[NSURLRequest requestWithURL:url] delegate:self s开发者_如何学JAVAtartImmediately:YES];
if( connection )
{
    while (!finished) {
        [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
    }
}

finished is an ivar that gets set to YES on connectionDidFinishLoading:

Can anyone clear this up for me? Which should be used and what's the difference?

[NSMutableData data] is not an accessor but a so-called class method. You probably know [NSMutableData alloc], that too is a class method and means it is tied to the class but not an instance. The typical way to denote that a method is a class method is by prefixing it with a plus: +[NSMutableData data].

In this case, the method is inherited from the NSData class (but does return an NSMutableData instance, since you're calling it on that class).

An implementation might look like this:

@interface NSMutableData
+ (id)data
{
    return [[[self alloc] init] autorelease];
}
@end

Note that self in this case is the NSMutableData class.

In other languages like C++, C# and Java (AFAIK) these would be methods that you specify with the static keyword.