- (void)showOrHideNavPrompt{

BOOL show = YES;

double delayInSeconds =1.0;

dispatch_time_t popTime =dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));

dispatch_after(popTime,dispatch_get_main_queue(), ^(void){

? ? ? if(show) {? ? ? ? ? ?

? ? ? ? ? ? ?[self.navigationItemsetPrompt:@"Add photos with faces to Googlyify them!"];? ? ? ? }else {? ? ? ? ? ??

? ? ? ? ? ? ?[self.navigationItemsetPrompt:nil];? ?

?? ? }? ? });}

看 https://github.com/nixzhu/dev-blog/blob/master/2014-04-19-grand-central-dispatch-in-depth-part-1.md GCD深入理解時(shí)發(fā)現(xiàn)的。