Appendix A

Example of ARC, Blocks, and GCD

Let’s see an example using ARC, Blocks, and GCD. The following source code reads data from a URL and displays the result on the main thread. That is a very typical use case in iOS applications so you can paste this as a part of your application. This shows you how ARC, Block, and GCD can be used for an application such as a twitter or Tumblr client.

The source code is explained with its comments.

NSString *url = @"http://images.apple.com/"     "jp/iphone/features/includes/camera-gallery/03-20100607.jpg"  /*   * On the main thread, downloading data from the specified URL starts asynchronously.   */ [ASyncURLConnection request:url completeBlock:^(NSData *data) {     dispatch_queue_t queue =         dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, ...

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