objective c - How can I make every message an object receives thread-safe?

Question

I am developing an Objective-C application, and what I want to do, is something like the following:

+-----------------+              +---------------+
|   Some Object   | <----------  |  Synchronize  |
|(Not Thread Safe)|              |     Proxy     |
+-----------------+            / +---------------+
                              / 
                             /  Intercepts [someobject getCount]
                            /   @synchronize (someObject)
                           /               
    [someObject getCount] /
                +----------------------+
                |  Some Calling Object |
                +----------------------+

What I've asking is, how can I create an object in objective-c, that intercepts messages sent to another object, in order to perform code before the message is sent to that object.

Some things that I think will not work:

• Categories (I need this to only happen for certain instances of a class)
• Rewriting the object (I don't have access to the source of the object)
• Method swizzling (once again, this need to only happen for certain instances of a class)

Answer 1

You would implement an NSProxy that forwards messages to your non-thread-safe object.

Here is a nice writeup of message forwarding in Objective-C, and here is Apple's documentation.

To handle thread safety, it depends on what you need. If your non-thread-safe object must run on a specific thread then you can use a NSRunLoop on said thread to serialize messages to that object.

Here is an example of using NSInvocation in conjunction with NSRunLoop. In that example they're using performSelector:withObject:afterDelay: but to use it with performSelector:onThread:withObject:waitUntilDone: would be very similar.

Otherwise, just use a single NSRecursiveLock in your proxy.