GCD vs performSelectorInBackground/performSelectorOnMainThread

Question

I am new in ios development. I have following questions:

1. When we use GCD(dispatch_group_async, dispatch_async(dispatch_get_main_queue()...) and when we use performSelectorInBackground/performSelectorOnMainThread?

2. What's the differences between those two.

   I know when we use performSelectorInBackground, we create a new NSThread. But isn't the same when we use dispatch_group_async? Because if we create more than one dispatch_group_async, it means we need to submit more than one blocks in queue. And those blocks might run on different queues. Therefore, when we create more than one dispatch_group_async, does it means we create a new thread? (because blocks may run on different queues) (I kind of confused about NSThread and block queue.....)

Thanks!!

Answer 1

When to use performSelectorInBackground:

Never. Do not use this method. It spawns an unbounded number of threads. Even before GCD was available, this was a horrible method.

When to use performSelectorOnMainThread:

Meh… Never, but just because it's inconvenient. There's nothing deeply wrong with this method. It's just not as useful as dispatch_async().

The difference between GCD and the old performSelector… methods (and NSThread in general) is that GCD manages a thread pool for you. In general, you should avoid manual threading in Cocoa. Instead, use NSOperationQueue or GCD (dispatch methods). They provide a more useful queue abstraction rather than forcing you to manually manage threads.

Be sure to read over Apple's Migrating Away from Threads for more info.

Answer 2

Actually after iOS 4.0 I can't find any single reason to use performSelectorInBackground/onMainThread. If you need to do something in background, use GCD( or, better, NSOperationQueue which is built on top of GCD since 4.0 and gives greater flexibility with little overhead), but be sure not to create retain cycles when using blocks.