"Finding" an object instance of a known class?

Question

My first post here (anywhere for that matter!), re. Cocoa/Obj-C (I'm NOT up to speed on either, please be patient!). I hope I haven't missed the answer already, I did try to find it.

I'm an old-school procedural dog (haven't done any programming since the mid 80's, so I probably just can't even learn new tricks), but OOP has my head spinning! My question is:

is there any means at all to "discover/find/identify" an instance of an object of a known class, given that some OTHER unknown process instantiated it?

eg. somthing that would accomplish this scenario:

(id) anObj = [someTarget getMostRecentInstanceOf:[aKnownClass class]];

for that matter, "getAnyInstance" or "getAllInstances" might do the trick too.

Background: I'm trying to write a plugin for a commercial application, so much of the heavy lifting is being done by the app, behind the scenes.

I have the SDK & header files, I know what class the object is, and what method I need to call (it has only instance methods), I just can't identify the object for targetting.

I've spent untold hours and days going over Apples documentation, tutorials and lots of example/sample code on the web (including here at Stack Overflow), and come up empty. Seems that everything requires a known target object to work, and I just don't have one.

Since I may not be expressing my problem as clearly as needed, I've put up a web page, with diagram & working sample pages to illustrate:

http://www.nulltime.com/svtest/index.html

Any help or guidance will be appreciated! Thanks.

Answer 1

This is not possible without having you register each instance in a dictionary as it is created. I.e., override some common factory method at a higher level which does this bookkeeping work. This will fall down when you use delegates that you may not control though, keep that in mind.

I do question the need to even do this at all, but I don't know your problem as well as I perhaps would need to, to recommend a different, more apt way of accomplishing the actual task at hand.

Just as a corollary to the above; I did look at the runtime to see if there was anything that I actually forgot about, but there is not. So my above statement with regards to you requiring to do that bookkeeping yourself, still holds I'm afraid.

Edit:

Based on your diagram (my apologies, just noticed the link after I posted this answer); I would suggest that if you control the classes that are being returned to you, just add a property to them. I.e., add a "name" property that you can set and keep unique. Then just pass the message to each instance, checking whether or not that object is the one you want. It's not particularly clever or anything like that, but it should work for your purposes.

Answer 2

I have the SDK & header files, I know what class the object is, and what method I need to call (it has only instance methods), I just can't identify the object for targetting.

If this is a publicly declared class with publicly declared instance methods (i.e., you have the header for the class and it has instance methods in it), there is probably a way in this application's API to get an instance of the class. Either you are meant to create one yourself, or the application has one (or more) and provides a way to get it (or them). Look at both the header for the class in question and the other headers.

I initially said “there must be a way…”, but I changed it, because there is an alternative reason why the header would have instance methods: The application developer does not intend those instance methods for plug-in use (and didn't mark them appropriately), or did not mean to include that header in the application/SDK (they included it by accident). You may want to ask the application developer for guidance.

If it is not a publicly declared class or its instance methods are not publicly declared, then the application does not support you working with instances of the class. Doing so is a breach of the API contract—not a legal contract, but the expectations that the application has of its plug-ins. If you breach the API contract, you will cause unexpected behavior, either now (not necessarily on your own machine/in your own tests) or in the future.

If the class's public declaration contains only class methods, then perhaps what you're after is not an instance at all—you're supposed to send those messages to the class itself.