another multithread uses

 

 

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.

//************************************************************************************

1 dispatch_async will not block and dispatch_sync will block

2 dispatch_get_main_queue(),will dispatch the task to main thread, so be careful not to cuase deadlock

3 example to call sth in multithread and update UI 

 

- (void)handleClickEvent { dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ [self backgroundProcessing]; dispatch_async(dispatch_get_main_queue(), ^{ [self updateUI]; }); }); }

http://blog.csdn.net/ericsuper/article/details/6998856

http://blog.devtang.com/blog/2012/02/22/use-gcd/

 

GCD的另一个用处是可以让程序在后台较长久的运行。在没有使用GCD时，当app被按home键退出后，app仅有最多5秒钟的时候做一些保存或清理资源的工作。但是在使用GCD后，app最多有10分钟的时间在后台长久运行。这个时间可以用来做清理本地缓存，发送统计数据等工作。