comparison GCD vs. performSelectorInBackground: dispatch_async not in background

Question

Grand Central Dispatch is great and reduces the amount of code but why I cannot run something on a background thread?
I have made a sample application to show what I mean (n

Answer 1

UIKit classes should be used only from an application’s main thread. (From iOS4, drawing to a graphics context is thread-safe.) You can't use UIKit stuff in a background thread.

Thus, you can only use dispatch_async(dispatch_get_main_queue(), block) in this situation.

dispatch_async(dispatch_get_main_queue(), ^(void) {

It will invoke the block on the main thread in the runloop of the main thread.

dispatch_async(dispatch_queue_create("myGCDqueue", NULL), ^(void) {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void) {

It will invoke the block in a background thread. You can't use it because you want to use UIKit in the block. And be careful dispatch_async(dispatch_queue_create(, it might cause memory leak, you have to release the serial queue that is created by dispatch_queue_create.

dispatch_sync(dispatch_queue_create("myGCDqueue", NULL), ^(void) {
dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void) {

dispatch_sync waits until the block is done.

dispatch_sync(dispatch_get_main_queue(), ^(void) {

It causes DEADLOCK.