iOS之GCD

Grand Central Dispatch (GCD)是Apple开发的一个多核编程的解决方法。
dispatch queue分成以下三种：

1. 运行在主线程的Main queue，通过dispatch_get_main_queue获取,dispatch_get_main_queue也是一种dispatch_queue_t。

/*!
 * @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.
 */
DISPATCH_INLINE DISPATCH_ALWAYS_INLINE DISPATCH_CONST DISPATCH_NOTHROW
dispatch_queue_t
dispatch_get_main_queue(void)
{
    return DISPATCH_GLOBAL_OBJECT(dispatch_queue_t, _dispatch_main_q);
}

2. 并行队列global dispatch queue，通过dispatch_get_global_queue获取，由系统创建三个不同优先级的dispatch queue。并行队列的执行顺序与其加入队列的顺序相同。

/*!
 * @function dispatch_get_global_queue
 *
 * @abstract
 * Returns a well-known global concurrent queue of a given quality of service
 * class.
 *
 * @discussion
 * The well-known global concurrent queues may not be modified. Calls to
 * dispatch_suspend(), dispatch_resume(), dispatch_set_context(), etc., will
 * have no effect when used with queues returned by this function.
 *
 * @param identifier
 * A quality of service class defined in qos_class_t or a priority defined in
 * dispatch_queue_priority_t.
 *
 * It is recommended to use quality of service class values to identify the
 * well-known global concurrent queues:
 *  - QOS_CLASS_USER_INTERACTIVE
 *  - QOS_CLASS_USER_INITIATED
 *  - QOS_CLASS_DEFAULT
 *  - QOS_CLASS_UTILITY
 *  - QOS_CLASS_BACKGROUND
 *
 * The global concurrent queues may still be identified by their priority,
 * which map to the following QOS classes:
 *  - DISPATCH_QUEUE_PRIORITY_HIGH:         QOS_CLASS_USER_INITIATED
 *  - DISPATCH_QUEUE_PRIORITY_DEFAULT:      QOS_CLASS_DEFAULT
 *  - DISPATCH_QUEUE_PRIORITY_LOW:          QOS_CLASS_UTILITY
 *  - DISPATCH_QUEUE_PRIORITY_BACKGROUND:   QOS_CLASS_BACKGROUND
 *
 * @param flags
 * Reserved for future use. Passing any value other than zero may result in
 * a NULL return value.
 *
 * @result
 * Returns the requested global queue or NULL if the requested global queue
 * does not exist.
 */
__OSX_AVAILABLE_STARTING(__MAC_10_6,__IPHONE_4_0)
DISPATCH_EXPORT DISPATCH_CONST DISPATCH_WARN_RESULT DISPATCH_NOTHROW
dispatch_queue_t
dispatch_get_global_queue(long identifier, unsigned long flags);

3. 串行队列serial queues一般用于按顺序同步访问，可创建任意数量的串行队列，各个串行队列之间是并发的。
   当想要任务按照某一个特定的顺序执行时，串行队列是很有用的。串行队列在同一个时间只执行一个任务。我们可以使用串行队列代替锁去保护共享的数据。和锁不同，一个串行队列可以保证任务在一个可预知的顺序下执行。
   serial queues通过dispatch_queue_create创建。

/*!
 * @function dispatch_queue_create
 *
 * @abstract
 * Creates a new dispatch queue to which blocks may be submitted.
 *
 * @discussion
 * Dispatch queues created with the DISPATCH_QUEUE_SERIAL or a NULL attribute
 * invoke blocks serially in FIFO order.
 *
 * Dispatch queues created with the DISPATCH_QUEUE_CONCURRENT attribute may
 * invoke blocks concurrently (similarly to the global concurrent queues, but
 * potentially with more overhead), and support barrier blocks submitted with
 * the dispatch barrier API, which e.g. enables the implementation of efficient
 * reader-writer schemes.
 *
 * When a dispatch queue is no longer needed, it should be released with
 * dispatch_release(). Note that any pending blocks submitted to a queue will
 * hold a reference to that queue. Therefore a queue will not be deallocated
 * until all pending blocks have finished.
 *
 * Passing the result of the dispatch_queue_attr_make_with_qos_class() function
 * to the attr parameter of this function allows a quality of service class and
 * relative priority to be specified for the newly created queue.
 * The quality of service class so specified takes precedence over the quality
 * of service class of the newly created dispatch queue's target queue (if any)
 * as long that does not result in a lower QOS class and relative priority.
 *
 * When no quality of service class is specified, the target queue of a newly
 * created dispatch queue is the default priority global concurrent queue.
 *
 * @param label
 * A string label to attach to the queue.
 * This parameter is optional and may be NULL.
 *
 * @param attr
 * A predefined attribute such as DISPATCH_QUEUE_SERIAL,
 * DISPATCH_QUEUE_CONCURRENT, or the result of a call to
 * a dispatch_queue_attr_make_with_* function.
 *
 * @result
 * The newly created dispatch queue.
 */
__OSX_AVAILABLE_STARTING(__MAC_10_6,__IPHONE_4_0)
DISPATCH_EXPORT DISPATCH_MALLOC DISPATCH_RETURNS_RETAINED DISPATCH_WARN_RESULT
DISPATCH_NOTHROW
dispatch_queue_t
dispatch_queue_create(const char *_Nullable label,
        dispatch_queue_attr_t _Nullable attr);

GCD的使用：

//  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 my_queue = dispatch_queue_create("com.hq.myQuene", NULL);
 dispatch_async(my_queue, ^{  
　 　// your code 
 });
 dispatch_release(my_queue); // 非ARC需要释放手动创建的队列

 // 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
 });

example

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);
        }
    });

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。

让程序在后台长久运行的示例代码如下：

// 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;
}