/*!
* @function dispatch_get_main_queue
*
* @abstract
* Returns the default queue that is bound to the main thread.
*
* @discussion
* In order to invoke blocks submitted to the main queue, the application must
* call dispatch_main(), NSApplicationMain(), or use a CFRunLoop on the main
* thread.
*
* @result
* Returns the main queue. This queue is created automatically on behalf of
* the main thread before main() is called.
*/
__OSX_AVAILABLE_STARTING(__MAC_10_6,__IPHONE_4_0)
DISPATCH_EXPORT struct dispatch_queue_s _dispatch_main_q;
#define dispatch_get_main_queue() \
DISPATCH_GLOBAL_OBJECT(dispatch_queue_t, _dispatch_main_q)

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

 // 延迟2秒执行：
 double delayInSeconds = 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_t
 dispatch_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;
}