Where can I find a good introduction or documentation on Objective-C blocks?

I am try开发者_如何转开发ing to use blocks in Objective-C. I've worked through this tutorial, but I would like to know more about blocks.

Where should I look for better documentation or tutorials on Objective-C blocks and their usage?

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Collecting the answers from the comments (and turning this answer community wiki for fairness) so we have an answer to this question:

Apple's official documentation

http://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/Blocks/

Mike Ash

http://mikeash.com/pyblog/friday-qa-2009-08-14-practical-blocks.html

Cocoa With Love

http://cocoawithlove.com/2009/10/how-blocks-are-implemented-and.html