几个函数的意义
信号创建，其中value是初始信号值
代码语言：javascript复制dispatch_semaphore_create(long value) 信号等待函数，dsema是信号，timeout是等待时间点，在等待时间点内，只有信号dsema的信号值大于等于1才放行，继续往下执行；放行之后信号值减1；
代码语言：javascript复制dispatch_semaphore_wait(dispatch_semaphore_t dsema, dispatch_time_t timeout); 增加信号值，每使用一次对应的dsema的信号值就加1
代码语言：javascript复制dispatch_semaphore_signal(dispatch_semaphore_t dsema);实际例子
1、普通，可以看出异步执行
代码语言：javascript复制- (void)normalTest{
dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
NSMutableArray *array = [NSMutableArray array];
for (int index = 0; index < 5; index ) {
dispatch_async(queue, ^(){
[array addObject:[NSNumber numberWithInt:index]];
int value = arc4random() % 100;
float sleep = value / 100.00;
[NSThread sleepForTimeInterval:sleep];
NSLog(@"add index :%d",index);
});
}
}
2、加了semaphore之后，变成同步执行
代码语言：javascript复制- (void)semaphoreTest{
dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);
NSMutableArray *array = [NSMutableArray array];
for (int index = 0; index < 5; index ) {
dispatch_async(queue, ^(){
dispatch_time_t waitTime = dispatch_time(DISPATCH_TIME_NOW,5 * NSEC_PER_SEC);//有效时间
dispatch_semaphore_wait(semaphore, waitTime);//这个函数本身就是一个判断函数，只有这个函数通过(有信号)，才会继续往下执行
[array addObject:[NSNumber numberWithInt:index]];
int value = arc4random() % 100;
float sleep = value / 100.00;
[NSThread sleepForTimeInterval:sleep];
NSLog(@"add index :%d",index);
dispatch_semaphore_signal(semaphore);
});
}
}
3、控制并发数
代码语言：javascript复制 dispatch_semaphore_t semaphore = dispatch_semaphore_create(2);
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
for (int i = 0; i < 10; i )
{
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
dispatch_async(queue, ^{
NSLog(@"%i",i);
sleep(2);
dispatch_semaphore_signal(semaphore);
});
}
可以看出，每次并发量被控制在2个
demo位置
以上代码都放在dispatch_semaphore demo
