dispatch_semaphore_wait 理解

dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

code

对semaphore减一，如果得值小于0（注意是小于0），则等待，如果不小于0，则执行code处代码

所以如果是执行多个任务的话，一般把wait放在dispatch_async的block的第一行，因为会有多个dispatch_async，把dispatch_semaphore_signal(semaphore);放在dispatch_async中耗时任务真正完成的时候

实现类似dispatch_group的功能

    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(5);  //0
    NSMutableArray *array = [NSMutableArray array];
    
    for (int index = 0; index < 5; index++) {
        
        
        NSLog(@"quene :%d", index);//1
        
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        dispatch_async(queue, ^(){
            
            NSLog(@"addd :%d", index);//2
            
            [array addObject:[NSNumber numberWithInt:index]];
            
            dispatch_semaphore_signal(semaphore);
            
        });
        
    }
        
    NSLog(@"执行完成");

异步改成同步

-(void)testSegiel
{
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);  //0
    NSMutableArray *array = [NSMutableArray array];
    
    for (int index = 0; index < 10; index++) {
        
        dispatch_async(queue, ^(){
            
            NSLog(@"quene :%d", index);//1
            
            dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
            
            NSLog(@"addd :%d", index);//2
            
            [array addObject:[NSNumber numberWithInt:index]];
            
            dispatch_semaphore_signal(semaphore);
            
        });
        
    }
}

运行结果
进行打印看结果，

2016-08-25 19:08:36.599 testNew[2880:2514251] async--->>>0

2016-08-25 19:08:36.599 testNew[2880:2513974] async--->>>1

2016-08-25 19:08:36.600 testNew[2880:2514252] async--->>>2

2016-08-25 19:08:36.600 testNew[2880:2514253] async--->>>3

2016-08-25 19:08:36.600 testNew[2880:2514254] async--->>>4

2016-08-25 19:08:36.600 testNew[2880:2514251] semaphore:0

2016-08-25 19:08:36.600 testNew[2880:2514255] async--->>>5

2016-08-25 19:08:36.600 testNew[2880:2514256] async--->>>6

2016-08-25 19:08:36.601 testNew[2880:2514251] async--->>>7

2016-08-25 19:08:36.601 testNew[2880:2514255] semaphore:5

2016-08-25 19:08:36.601 testNew[2880:2514257] async--->>>8

2016-08-25 19:08:36.601 testNew[2880:2514255] async--->>>9

2016-08-25 19:08:36.601 testNew[2880:2514252] semaphore:2

2016-08-25 19:08:36.601 testNew[2880:2514253] semaphore:3

2016-08-25 19:08:36.601 testNew[2880:2514254] semaphore:4

2016-08-25 19:08:36.601 testNew[2880:2513974] semaphore:1

2016-08-25 19:08:36.601 testNew[2880:2514256] semaphore:6

2016-08-25 19:08:36.602 testNew[2880:2514251] semaphore:7

2016-08-25 19:08:36.602 testNew[2880:2514257] semaphore:8

2016-08-25 19:08:36.602 testNew[2880:2514255] semaphore:9

发现全部执行

就能看到其中1部分全部基本是顺序执行，2部分无序执行。

如果把是dispatch_semaphore_create(1)改成dispatch_semaphore_create(0)的话

就能看到其中1部分全部基本是顺序执行，2部分一直没有执行。

因为dispatch_semaphore_wait是进行信号量减1操作，而dispatch_semaphore_signal是进行加1操作。如果dispatch_semaphore_wait减1前如果小于1，则一直等待。同时这两者之间是线程无顺序的抢占资源，但只能允许一个线程执行。所以猜测如果某一时刻进行了dispatch_semaphore_signal操作，则会继续执行

dispatch_semaphore_wait 理解

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