NSOperation vs GCD | Techbirds
In addition to run loops, threads and operations, Snow Leopard adds Grand Central Dispatch (GCD), a very lightweight way to represent units of work and the style of concurrency they need, and have the system figure out how to schedule them.
It shouldn’t surprise you in the least to learn that NSOperation, on Snow Leopard, is built atop GCD. However, there are a number of differences between the two, and for that reason people have started to ask “How should I decide which to use when?”
you should generally be using NSOperation rather than using GCD directly. Not because of a difference in efficiency, but because NSOperation provides a higher-level abstraction atop the mechanisms of GCD.
For example, you can set up a dependency between two NSOperations such that the second will only be run after the first is complete — even if they’re run on different queues. You can use KVO to observe the completion (or cancellation) of different operations — and you can create operations that support being cancelled in the first place. You can set a completion block to run after an application has finished. And you can, of course, create operations from blocks using NSBlockOperation.
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