多线程编程之GCD(二)

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三、系统Dispatch_Queue

GCD提供了两个系统的Queue：1、Main Dispatch Queue；2、Global Dispatch Queue

Main Dispatch Queue：是在主线程中执行的Dispatch Queue，因为主线程只有一个，所以Main Dispatch Queue就是Serial Dispatch Queue；像一些界面刷新等必须在主线程下执行的操作，可以把任务追加到Main Dispatch Queue中，类似于[NSObject performSelectorOnMainThread:];

Global Dispatch Queue:是一个Concurrent Dispatch Queue；
Global Dispatch Queue 有四个执行优先级：DISPATCH_QUEUE_PRIORITY_HIGH、DISPATCH_QUEUE_PRIORITY_DEFAULT、DISPATCH_QUEUE_PRIORITY_LOW、DISPATCH_QUEUE_PRIORITY_BACKGROUND；

参照官方API
/*!
 *  @function dispatch_get_global_queue
 *  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);

获取Main Dispatch Queue
dispatch_queue_t mainQueue = dispatch_get_main_queue();
获取Global Dispatch Queue
dispatch_queue_t highQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);

注：通过 dispatch_queue_t queue = dispatch_queue_create("com.qmf", DISPATCH_QUEUE_CONCURRENT||DISPATCH_QUEUE_Serial);创建的queue的优先级和dispatch_queue_t defaultQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);相同；

四、dispatch_set_target_queue

通过 dispatch_queue_t queue = dispatch_queue_create("com.qmf", DISPATCH_QUEUE_CONCURRENT||DISPATCH_QUEUE_Serial);创建的queue的优先级和dispatch_queue_t defaultQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);相同。
当我们想改变自己创建的Dispatch Queue的优先级的时候可以使用dispatch_set_target_queue。

dispatch_set_target_queue有两个作用：
一、变更用户生成的Dispatch Queue的优先级

代码演示：

//设置优先级
dispatch_queue_t myqueue = dispatch_queue_create("com.qmf", NULL);
dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
// 第一个参数为要设置优先级的queue,第二个参数是参照物，既将第一个queue的优先级和第二个queue的优先级设置一样。Dispatch Queue
dispatch_set_target_queue(myqueue, globalQueue);

二、设置Dispatch Queue的执行层次

首先,我们需要阐述一下生成多个Serial DisPatch Queue时的注意事项Serial DisPatch Queue是一个串行队列,只能同时执行1个追加处理(即任务),当用Dispatch_queue_create函数生成多个Serial DisPatch Queue时,每个Serial DisPatch Queue均获得一个线程,即多个Serial DisPatch Queue可并发执行,同时处理添加到各个Serial DisPatch Queue中的任务,但要注意如果过多地使用多线程,就会消耗大量内存,引起大量的上下文切换,大幅度降低系统的响应性能,所以我们只在为了避免多个线程更新相同资源导致数据竞争时,使用Serial DisPatch Queue

第一种情况：使用dispatch_set_target_queue(queue1, queue2)实现队列的动态调度

- (void)testTargetQueue2 {
    //创建一个串行队列queue1
    dispatch_queue_t queue1 = dispatch_queue_create("test.1", DISPATCH_QUEUE_SERIAL);
    //创建一个串行队列queue2
    dispatch_queue_t queue2 = dispatch_queue_create("test.2", DISPATCH_QUEUE_SERIAL);
    
    //使用dispatch_set_target_queue()实现队列的动态调度管理
    dispatch_set_target_queue(queue1, queue2);
    
    
    /*
     
     <*>dispatch_set_target_queue(Dispatch Queue1, Dispatch Queue2);
     那么dispatchA上还未运行的block会在dispatchB上运行。这时如果暂停dispatchA运行：
     
     <*>dispatch_suspend(dispatchA);
     这时则只会暂停dispatchA上原来的block的执行，dispatchB的block则不受影响。而如果暂停dispatchB的运行，则会暂停dispatchA的运行。
     
     这里只简单举个例子，说明dispatch队列运行的灵活性，在实际应用中你会逐步发掘出它的潜力。
     
     dispatch队列不支持cancel（取消），没有实现dispatch_cancel()函数，不像NSOperationQueue，不得不说这是个小小的缺憾
     
     */
    
    dispatch_async(queue1, ^{
        for (NSInteger i = 0; i < 10; i++) {
            NSLog(@"queue1:%@, %ld", [NSThread currentThread], i);
            [NSThread sleepForTimeInterval:0.5];
            if (i == 5) {
                dispatch_suspend(queue1);
            }
        }
    });
    
    dispatch_async(queue1, ^{
        for (NSInteger i = 0; i < 100; i++) {
            NSLog(@"queue1:%@, %ld", [NSThread currentThread], i);
        }
        
    });
    
    dispatch_async(queue2, ^{
        for (NSInteger i = 0; i < 100; i++) {
            NSLog(@"queue2:%@, %ld", [NSThread currentThread], i);
        }
    });
    
}

第二种情况:使用dispatch_set_target_queue将多个串行的queue指定到了同一目标，那么着多个串行queue在目标queue上就是同步执行的，不再是并行执行。

- (void)testTargetQueue {
    //1.创建目标队列
    dispatch_queue_t targetQueue = dispatch_queue_create("test.target.queue", DISPATCH_QUEUE_SERIAL);
    
    //2.创建3个串行队列
    dispatch_queue_t queue1 = dispatch_queue_create("test.1", DISPATCH_QUEUE_SERIAL);
    dispatch_queue_t queue2 = dispatch_queue_create("test.2", DISPATCH_QUEUE_SERIAL);
    dispatch_queue_t queue3 = dispatch_queue_create("test.3", DISPATCH_QUEUE_SERIAL);
    
    //3.将3个串行队列分别添加到目标队列
    dispatch_set_target_queue(queue1, targetQueue);
    dispatch_set_target_queue(queue2, targetQueue);
    dispatch_set_target_queue(queue3, targetQueue);
    
    
    dispatch_async(queue1, ^{
        NSLog(@"1 in");
        [NSThread sleepForTimeInterval:3.f];
        NSLog(@"1 out");
    });
    
    dispatch_async(queue2, ^{
        NSLog(@"2 in");
        [NSThread sleepForTimeInterval:2.f];
        NSLog(@"2 out");
    });
    dispatch_async(queue3, ^{
        NSLog(@"3 in");
        [NSThread sleepForTimeInterval:1.f];
        NSLog(@"3 out");
    });
}

输出：

1 in
1 out
2 in
2 out
3 in
3 out

不懂就药问