dispatch_after实现原理深度探索

1   // 循环5次
2     for (int i =0; i < 5; i++ ) {
3         
4         NSLog(@"let‘s go %d",i);
5         // 设置2秒后执行block
6         dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
7             NSLog(@"This is my %d number!",i);
8         });
9     }

 1 2016-01-03 23:55:49.109 dispatchAfterDeep[1095:60579] let‘s go 0
 2 2016-01-03 23:55:49.110 dispatchAfterDeep[1095:60579] let‘s go 1
 3 2016-01-03 23:55:49.111 dispatchAfterDeep[1095:60579] let‘s go 2
 4 2016-01-03 23:55:49.111 dispatchAfterDeep[1095:60579] let‘s go 3
 5 2016-01-03 23:55:49.111 dispatchAfterDeep[1095:60579] let‘s go 4
 6 2016-01-03 23:55:51.111 dispatchAfterDeep[1095:60579] This is my 0 number!
 7 2016-01-03 23:55:51.300 dispatchAfterDeep[1095:60579] This is my 1 number!
 8 2016-01-03 23:55:51.301 dispatchAfterDeep[1095:60579] This is my 2 number!
 9 2016-01-03 23:55:51.301 dispatchAfterDeep[1095:60579] This is my 3 number!
10 2016-01-03 23:55:51.301 dispatchAfterDeep[1095:60579] This is my 4 number!

Declaration
void dispatch_after( dispatch_time_t when, dispatch_queue_t queue, dispatch_block_t block);

Parameters
when    
The temporal milestone returned by dispatch_time or dispatch_walltime.
queue    
The queue on which to submit the block. The queue is retained by the system until the block has run to completion. This parameter cannot be NULL.
block    
The block to submit. This function performs a Block_copy and Block_release on behalf of the caller. This parameter cannot be NULL.

Discussion
This function waits until the specified time and then asynchronously adds block to the specified queue.

1 delay (2){
2 
3         dispatch_async(DISPATCH_QUEUE_SERIAL, ^{
4 
5             (add block into main queue);
6 
7         });
8     }