信号量

• 信号量的使用：

//生成信号量
dispatch_semaphore_t semaphore = dispatch_semaphore_create(10);
//当信号量为0的时候会永久等待
dispatch_semaphore_wait(semaphore,  DISPATCH_TIME_FOREVER);
//信号量加一
dispatch_semaphore_signal(semaphore);

• 信号量使用的业务场景：

一、锁

YYKit加锁代码：

- (instancetype)init {
    self = [super init];
    _lock = dispatch_semaphore_create(1);
    return self;
}
- (NSURL *)imageURL {
    dispatch_semaphore_wait(_lock, DISPATCH_TIME_FOREVER);
    NSURL *imageURL = _imageURL;
    dispatch_semaphore_signal(_lock);
    return imageURL;
}

二、异步返回

- (NSArray *)tasksForKeyPath:(NSString *)keyPath {
    __block NSArray *tasks = nil;
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
    [self.session getTasksWithCompletionHandler:^(NSArray *dataTasks, NSArray *uploadTasks, NSArray *downloadTasks) {
        //task赋值，代码有点长，就不贴了
        dispatch_semaphore_signal(semaphore);
    }];
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
    return tasks;
}

三、控制并发量，降低性能。

dispatch_semaphore_t semaphore = dispatch_semaphore_create(5);
for (int i = 0; i < 1000; ++i) {
 dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        //多线程代码
        dispatch_semaphore_signal(semaphore);
    });
}