General question regarding "parallelism" of iPhone execution

Question

I've got a general question and hope somebody can shed some light on this.
While I'm relatively new to iPhone development I've been in software programing for more than 15 years now and have a solid background in Java, C, C++, C# and other languages. I've created a couple of applications for the iPhone, easy and complex ones, some using UIKit, others using Open GL and Open AL. With that said I think it would be fair to say that I know pretty much what I'm talking about.
For those who tend to refer everybody back to the Apple Samples or PDF guides, please believe me, I've read them all.
Here's my "question":
I occasionally ran into a situation where I got the impression that code on the iPhone got executed in a "non sequential" or "parallel" manner even when I did not explicitly created threads. I'm not referring to code buried in a framework or so, I'm talking about code I've written. (English is not my mother tongue, so please excuse me if my explanation might be somewhat difficult to understand.)
I try to give an example. Assume a code snippet like this:
_gameEngine = [GameEngine alloc];
[_gameEngine initSomeStuff];
In my understanding the second statement (the initSomeStuff) gets executed once alloc returns and _gameEngine for sure is a valid pointer. Is that fair to say? I sometimes got the impression that the iPhone starts executing code without waiting that a method returned.
I had similar issues when I tried to show an ActivityIndicator while another method was doing a lengthy operation. Pseudocode like this:
[_activityIndicator show];
[self longLastingOperation];
[_activityIndicator hide];
In code like the one above: Can I be 100% sure, that the hide message is not sent until longLastingOperation returns? I ran into a situation where I had the impression that show and hide got called immediately without waiting until longLastingOperation returned.
Final example:
[self performSelectorOnMainThread:@selector(doSomething) withObject:nil waitUntilDone:YES];
[self nextStatement];
-(void)doSomething {
[self longLastingOperation];
1.) In the above code, I understand the idea is that doSomething will be executed on the MainThread and that nextStatement will in no way be executed before doSomething has returned. Is that true?
2.) doSomething is supposed to run on the MainThread but does this hold true for longLastingOperation, too? Or could it be that longLastingOperation WITHIN doSomething gets dispatched to another thread and doSomething returns PRIOR to longLastingOperation been finished?
This might all sound pretty wired but as I said from time to time I ran into a situation where I got totally confused as to how stuff executes in – or out of – sequence. I helped myself with using NSNotificationCenters etc. but I'd like to understand what's going on in detail and crystal clear here.
Your help is more than appreciated.
Thanks,
R.
Message was edited by: BeSharp

Answer 1

It is never safe to assume that any allocation was successful, and while it's incredibly unlikely that you're running into any such situation, it's entirely possible for a formal protocol to declare that a given message send should return immediately without waiting around:
http://developer.apple.com/documentation/Cocoa/Conceptual/ObjectiveC/Articles/ch apter13_section_8.html#//appleref/doc/uid/TP30001163-CH9-BAJIGHAF
An "impression" doesn't prove much unfortunately, so I'd recommend getting friendly with the debugger to see what's really going on.