Comments on: Cocoa Tutorial: NSOperation and NSOperationQueue

By: Marcus Zarra

Marcus Zarra — Fri, 18 Mar 2011 16:53:37 +0000

I highly suggest posting questions like this on stackoverflow.com. You will get far more detailed answers.

By: abhisek

abhisek — Fri, 18 Mar 2011 12:17:23 +0000

If I’m creating a big application and I need to declare an NSOperationQueue which can be accessible everywhere by all the Operations(Threads), where shall I do it. In AppDelegate(should it be static?) ? or in main.m ?

By: Marcus Zarra

Marcus Zarra — Thu, 17 Mar 2011 20:10:13 +0000

static variables belong in the implementation file, not in the header. -init is always called before anything else on the instance of the class. It initializes the instance after its memory has been allocated via -alloc.

By: abhisek

abhisek — Thu, 17 Mar 2011 20:07:13 +0000

Marcus, great tutorial. one questn. Why can't we define variables as static in @interface, likewise you defined static in @implementation. Also as I am new to cocoa I'd like to knw if its sure that init in appdelegate will be called and will finish its work before applicationDidFinishLaunching is called??

Regards, Abhisek

By: Marcus Zarra

Marcus Zarra — Thu, 10 Mar 2011 17:25:03 +0000

Performance difference? No. Safety yes.

Yes an operation can be cancelled per the API.

By: chic

chic — Thu, 10 Mar 2011 14:32:39 +0000

thank you for this tutorial,

I have 2 questions after it: . is there big performance difference using nsoperation or nsthread . is it possible to cancel the operation, for example if user go back

By: karthik

karthik — Sat, 06 Nov 2010 16:57:03 +0000

nice tutorial.Thanks

By: Marcus Zarra

Marcus Zarra — Tue, 14 Sep 2010 15:18:36 +0000

It is set to nil for a couple of reasons:

If something after that call tries to access the ivar then it will be a no op because it is nil rather than an exception because it is a reference to a released object.
In GC land, -release is a no op and setting an object to nil is a further guarantee that the GC will behave and decrease the reference count.

In either case it is extra protection for your code and a good habit to get into.

By: octy

octy — Tue, 14 Sep 2010 14:03:58 +0000

Hi Marcus,

Great stuff, thanks for this post.

I'm wondering about one thing though... in the dealloc method (AppDelegate.m:41): why do you set the queue pointer to nil? Isn't the pointer going away when dealloc is called? I'm not nitpicking, just trying to understand a little bit more about object lifecycle in Objective-C.

Thanks!

By: Marcus Zarra

Marcus Zarra — Sat, 04 Sep 2010 17:42:21 +0000

The document is not leaked. It is released after the main thread has completed. Line 35 of PageLoadOperation.m to be specific.

By: Eaglelouk

Eaglelouk — Sat, 04 Sep 2010 17:38:08 +0000

Seems that the NSXMLDocument is leaked. maybe you should use [document autorelease] when using performSelectorOnMainThread.

Nice tuto anyway!

By: bguest

bguest — Thu, 20 May 2010 15:28:20 +0000

Very succinct tutorial, just what I needed, thank you.

By: Marcus Zarra

Marcus Zarra — Sun, 23 Nov 2008 23:27:57 +0000

The comma is used to chain commands together on one line. That is all that it does and it is a part of the C language. I use it when I do releases so that I do not forget to set the pointer to nil. This is a bit of protective coding to avoid accidently send a message to a released object.

By: CharlesAHunter

CharlesAHunter — Sun, 23 Nov 2008 22:44:30 +0000

I am curious about the comma in your dealloc routine between the release of targetURL and the assignment to nil. I have never seen comma syntax in this situation before and am wondering whether I’m missing something? (Kernighan & Ritchie, 1978, Appendix A.7.15) gives an example of comma syntax in an expression of “f(a,(t=3,t+2),c)” saying that f() is being pass three arguments: a, 5 and c. However, that is not what appears to be going on in this dealloc routine. Is it just a typo that happens to compile or is it more significant?

By: Marcus Zarra

Marcus Zarra — Thu, 27 Mar 2008 15:00:40 +0000

brg,

performSelectorOnMainThread "works" just like any other message call. It is a message that is passed to an object. In this example it is passed to the static reference to the AppDelegate. However it could easily have been passed to any other NSObject. When the receiver gets the message, it then makes a subsequent call to itself on the main thread using the selector that was passed in.

By: brg

brg — Thu, 27 Mar 2008 12:25:48 +0000

How does performSelectorOnMainThread work ?

Does it multi-cast the call to any object that implements the pageLoaded method (thats running in the main thread)?

Or does it have magic knowledge to send it to the AppDelegate?

By: jpdann

jpdann — Wed, 12 Mar 2008 18:51:44 +0000

Of course, how stupid of me! I should not post until I've had a coffee.

Thanks Marcus

By: Marcus Zarra

Marcus Zarra — Wed, 12 Mar 2008 16:04:54 +0000

NSURL, like NSString, is immutable therefore no copy is necessary since it is not possible for a user to alter an immutable object.

However, unlike NSString, NSURL does not have a mutable subclass so there is no concern. Therefore if you wanted to be crazy you could check to see if your incoming NSString is mutable and decide on a copy vs retain. But, I seriously down that the difference between a string copy vs. retain is going to save you a whole lot of memory.

By: jpdann

jpdann — Wed, 12 Mar 2008 15:59:37 +0000

Hi Marcus,

I noticed that your targetURL property is retained instead of copied. If you're going to be using this retained value in a new thread then why not copy it? Can the url change from under your feet, or does the NSOperation(Queue) system watch for that?

I ask this as I have a routine that matches regexes in a large string, retaining the string would save memory, but the user can potentially edit the string in an NSTextView while the matching is taken place.

Thanks,

Jon

By: jpdann

jpdann — Wed, 05 Mar 2008 23:22:07 +0000

Hi Marcus,

Thanks for the quick response. Glad to know my coding style isn't completely off.

Jon