GCD系列:信号量(semaphore)

信号量有点像具备N个task并行能力的channel,当channel的并行能力未达到上限时可以任意往里边加task，
当达到channel上限时，需要task完成才可以加入新的task.

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基础函数

dispatch_semaphore_tdispatch_semaphore_create(long value);dispatch_semaphore_wait(dispatch_semaphore_t sema, dispatch_time_t timeout);dispatch_semaphore_signal(dispatch_semaphore_t dsema);

1.申明一个信号量:dispatch_semaphore_t dsema;
2.创建一个具有n个并行能力的semaphore:dsema = dispatch_semaphore_create(2); 这里创建一个具备2个并行能力的信号量dsema.
3.消耗1个并行能力,例子中当重复调用2次此函数，代码将造成阻塞,阻塞的时间是timeout，实际开发中，可以创建一dispatch_time_t实例，也可使用DISPATCH_TIME_FOREVER,将永远等待知道发送一个signal为止.因此,如果不能确定一定会发送signal，慎用DISPATCH_TIME_FOREVER.
4.发送1个signal.

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主要用途

1. 当前线程执行代码等待其它线程代码执行结束
2. 控制异步task并发数量

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• 实例1: 当前Thread同步执行等待其他Thread返回结果：

void normalizedSemaphore() {    NSLog(@"current thread begin...");    dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{        for(NSUInteger index = 0 ; index < 3 ; index ++) {            NSLog(@"other thread message:%lu",index);            if (index == 2) {                dispatch_semaphore_signal(semaphore);            }        }    });    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);    NSLog(@"current thread continue...");    dispatch_semaphore_signal(semaphore);}

代码中，创建了一个同时允许并发为1的semaphore,当执行到current thread continue…这条log时,执行被卡住了，等待异步代码块调用dispatch_semaphore_signal(semaphore);继续往下执行.
输出结果:
2017-02-27 00:26:11.392620 Semaphore[61812:11351508] current thread begin...
2017-02-27 00:26:11.392751 Semaphore[61812:11351539] other thread message:0
2017-02-27 00:26:11.392772 Semaphore[61812:11351539] other thread message:1
2017-02-27 00:26:11.392784 Semaphore[61812:11351539] other thread message:2
2017-02-27 00:26:11.392848 Semaphore[61812:11351508] current thread continue...

实例2：控制异步并发数量

- (void)managerAsynTaskDemo {    NSMutableArray *tasks = [NSMutableArray new];    for (NSUInteger index = 0 ; index < 10; index ++) {        [tasks addObject:@(index)];    }    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{        [tasks enumerateObjectsUsingBlock:^(id  _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {            [self logTaskIndex:[obj unsignedIntegerValue]];        }];    });}dispatch_semaphore_t _dsema;void (^logTaskIndexHandle)(NSUInteger) = ^(NSUInteger index) {    NSLog(@"task index:%lu",index);};- (void)logTaskIndex:(NSUInteger)index {    if (!_dsema) _dsema = dispatch_semaphore_create(5);    dispatch_semaphore_wait(_dsema, dispatch_time(DISPATCH_TIME_NOW, 1 * NSEC_PER_SEC));    logTaskIndexHandle(index);    dispatch_semaphore_wait(_dsema, dispatch_time(DISPATCH_TIME_NOW, 1 * NSEC_PER_SEC));    dispatch_semaphore_signal(_dsema);}

这个例子中，我们产生了10个异步的task,创建了一个并发量为5的信号量dsema,并且在很短的时间内都异步调用logTaskIndex方法,
2017-02-27 00:28:01.579778 Semaphore[61846:11355856] task index:0
2017-02-27 00:28:01.579822 Semaphore[61846:11355856] task index:1
2017-02-27 00:28:01.579838 Semaphore[61846:11355856] task index:2
2017-02-27 00:28:01.579850 Semaphore[61846:11355856] task index:3
2017-02-27 00:28:01.579862 Semaphore[61846:11355856] task index:4
2017-02-27 00:28:02.580413 Semaphore[61846:11355856] task index:5
2017-02-27 00:28:03.585241 Semaphore[61846:11355856] task index:6
2017-02-27 00:28:04.589975 Semaphore[61846:11355856] task index:7
2017-02-27 00:28:05.592348 Semaphore[61846:11355856] task index:8
2017-02-27 00:28:06.596368 Semaphore[61846:11355856] task index:9

通过日志看到,前5个任务完成时间间隔非常短，从index=5的task开始每隔1秒钟执行一个task。是因为，打印日志的时候，并没有调用dispatch_semaphore_signal(semaphore);函数，所以只能等待超时时间1秒.
如果这里的超时时间设置成了DISPATCH_TIME_FOREVER，请思考一下，最终的打印结果是什么呢？

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最后，推荐一个封装的semaphore函数;

void dispatch_semaphore_async_handle(dispatch_semaphore_t dsema,dispatch_time_t timeout, void (^block)(dispatch_semaphore_t dsema)) {            dsema = dsema ?: dispatch_semaphore_create(0);            timeout = timeout ?: DISPATCH_TIME_FOREVER;            dispatch_semaphore_wait(dsema, timeout);            block(dsema);            dispatch_semaphore_wait(dsema, timeout);            dispatch_semaphore_signal(dsema);        }

• 使用方法如下:

dispatch_semaphore_async_handle(nil, 0, ^(dispatch_semaphore_t dsema){//                some code...                dispatch_semaphore_signal(dsema);            });

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• 推荐一篇不错的GCD文章
  http://www.cocoachina.com/ios/20160225/15422.html