NSConnection weirdness

Created 3/9/2006, updated 3/12/20202 minute read

Today I present an episode in the Great Mysteries of Cocoa series.

The mystery

Set up an NSConnection like this:

[NSConnection connectionWithReceivePort:[NSPort port] sendPort:[NSPort port]];

Pass that connection to another thread as remoteConnection. Now try to create a local connection, localConnection in the thread based on the passed connection:

[NSConnection connectionWithReceivePort:[remoteConnection sendPort]
                               sendPort:[remoteConnection receivePort]];

If you try to use that connection you’ll find that your code blocks forever as soon as you try to perform [localConnection rootProxy]. Pass the ports separately instead of passing the connection and the code works as expected without blocking. This is why the Apple example code gets you to pass not only the connection but the ports separately (in an array), even though it seems like an unnecessarily long-winded way of doing things.

Still, I can’t say that I really like the behaviour. Consider the following piece of code:

NSPort *port1 = [NSPort port];
NSPort *port2 = [NSPort port];
NSConnection *connection =
    [NSConnection connectionWithReceivePort:port1 sendPort:port2];
NSPort *port3 = [connection receivePort];   // does port3 == port1?
NSPort *port4 = [connection sendPort];      // does port4 == port2?

Is NSConnection returning the ports that were used to create it? If not, it definitely seems counter-intuitive given that the documentation says that receivePort, "Returns the NSPort on which the receiver receives incoming network messages." But the docs also go on to say:

You can inspect this object for debugging purposes or use it to create another NSConnection, but shouldn’t use it to send or receive messages explicitly.

So does port1 equal port3 or not? I whipped up a quick test project and found out that it is indeed equal (both according to a pointer comparison and also the isEqual: method). It’s equal if you perform the comparisons from the same thread, and also if you perform them from a secondary thread after passing the NSConnection to that thread. So the question is, why does the call to rootProxy block if you don’t pass the ports separately? I’m afraid it will have to remain one of the Great Mysteries of Cocoa…

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