//后台执行：dispatch_async(dispatch_get_global_queue(0,0), ^{//something});

//主线程执行：dispatch_async(dispatch_get_main_queue(), ^{//something});

//一次性执行：staticdispatch_once_t onceToken;

dispatch_once(&onceToken, ^{//code to be executed once});

//延迟2秒执行：doubledelayInSeconds =2.0;

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

dispatch_after(popTime, dispatch_get_main_queue(),^(void){//code to be executed on the main queue after delay});

//自定义dispatch_queue_tdispatch_queue_t urls_queue = dispatch_queue_create("blog.devtang.com", NULL);

dispatch_async(urls_queue,^{//your code}); dispatch_release(urls_queue);

//合并汇总结果dispatch_group_t group =dispatch_group_create();

dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{//并行执行的线程一});

dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{//并行执行的线程二});

dispatch_group_notify(group, dispatch_get_global_queue(0,0), ^{//汇总结果});

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0), ^{

NSURL* url = [NSURL URLWithString:@"http://www.baidu.com"];

NSError*error;

NSString* data = [NSString stringWithContentsOfURL:url encoding:NSUTF8StringEncoding error:&error];if(data !=nil) {

dispatch_async(dispatch_get_main_queue(),^{

NSLog(@"call back, the data is: %@", data);

});

}else{

NSLog(@"error when download:%@", error);

}

});

//AppDelegate.h文件@property (assign, nonatomic) UIBackgroundTaskIdentifier backgroundUpdateTask;//AppDelegate.m文件- (void)applicationDidEnterBackground:(UIApplication *)application

{

[self beingBackgroundUpdateTask];//在这里加上你需要长久运行的代码[self endBackgroundUpdateTask];

}- (void)beingBackgroundUpdateTask

{

self.backgroundUpdateTask= [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{

[self endBackgroundUpdateTask];

}];

}- (void)endBackgroundUpdateTask

{

[[UIApplication sharedApplication] endBackgroundTask: self.backgroundUpdateTask];

self.backgroundUpdateTask=UIBackgroundTaskInvalid;

}