Dispatch Semaphore & NSCondition

dispatch_semaphore 使用 http://www.cnblogs.com/snailHL/p/3906112.html

两个作用

加锁/数据同步

dispatch_semaphore_create

dispatch_semaphore_signal

dispatch_semaphore_wait 

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- (void)methodWithABlock:(DHBLK)blk{
    blk(@"hahaha");
}

测试代码1 单个block

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dispatch_semaphore_t sem = dispatch_semaphore_create(0);      
[self methodWithABlock:^(id result){
    //写block中做的事情
    //结束等待
    dispatch_semaphore_signal(sem);
}];
//等待信号
dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);

测试代码2 多个 blcok

xxx.m

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dispatch_semaphore_t sem = dispatch_semaphore_create(0);
   [self methodWithABlock:^(id result){
       //写block中做的事情
       dispatch_semaphore_signal(sem);
       [self methodWithABlock:^(id result){
           //写block中做的事情
           dispatch_semaphore_signal(sem);
       }];
   }];
   dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
   dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);

测试代码3

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dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
__block int j = 0;
dispatch_async(queue, ^{
    j = 100;
    dispatch_semaphore_signal(semaphore);
});
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
NSLog(@"%d",j);
// j = 100

测试代码 4 加锁测试 ,反注释掉//

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for (int i = 0; i < 100; i++) {
      dispatch_async(queue, ^{
          // 相当于加锁
//            dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
          NSLog(@"i = %zd semaphore = %@", i, semaphore);
          // 相当于解锁
//            dispatch_semaphore_signal(semaphore);
      });

NSCondition 的使用

NSCondition 的对象实际上作为一个锁和一个线程检查器：锁主要为了当检测条件时保护数据源，执行条件引发的任务；线程检查器主要是根据条件决定是否继续运行线程，即线程是否被阻塞。
http://www.jianshu.com/p/5d20c15ae690

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NSCondition *condition = [[NSCondition alloc] init];
__block NSMutableArray *products = [[NSMutableArray alloc] initWithCapacity:10];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    [condition lock];
    while ([products count] == 0) {
        NSLog(@"wait for products ");
        [condition wait];
    }
    [products removeObjectAtIndex:0];
    NSLog(@"custome a product");
    [condition unlock];
});
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    [condition lock];
    products = [NSMutableArray array];
    [products addObject:@"oo"];
    NSLog(@"produce a obj");
    [condition signal];
    [condition unlock];
});

异步下载图片完成后同步完成数据库再通知UI刷新的机制

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- (void)receiveMessage:(MessageEntity *)message
{
    NSCondition *condition = [[NSCondition alloc]init];
    [condition lock];
    NSInteger imageCount ;
    void(^unlock)=^(){
        while(!imageCount)
            [condition signal];
    }
    imageCount++;
dowanloadImageFinished:^(UIImage *image){ //异步下载图片，下载完成的回调中
    imageCount--;
    unlock();
}
    while(imageCount){
        [condition wait];
    }
    [condition unLock];
}

以GPUImage 实现的延时摄影

用 semaphore 来筛选(关断)出时间间隔的图像 buffter.筛选出来的buffer 再筛入 WriterInputPixelBufferAdaptor.

https://github.com/yangfangxue/GPUImageRecord.git

1.dispatch timer 的用法 

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__weak typeof(self) ws =self;
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
_timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
dispatch_source_set_timer(_timer, dispatch_walltime(NULL, 0), 0.5 * NSEC_PER_SEC, 0);
dispatch_source_set_event_handler(_timer, ^{
    [ws defaultTimerHandel];
});
_seam = dispatch_semaphore_create(0);

//使用 timer 和销毁

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dispatch_resume(_timer);
dispatch_cancel(_timer);

GPUImageVideoCamera 中回调方法CMSampleBufferRef的 lock 

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[_camera addTarget:self.preview];
//获取buffer
     CVPixelBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
     CVPixelBufferLockBaseAddress(imageBuffer, 0);
     _imageBuffer =  CVPixelBufferRetain(imageBuffer);
     CVPixelBufferUnlockBaseAddress(imageBuffer, 0);

录制配置路径的时候需要检测是否存在,清除历史文件

完成录制时候加入condition 保护

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[writerInput markAsFinished];
if(videoWriter.status == AVAssetWriterStatusWriting){
    NSCondition *cond = [[NSCondition alloc] init];
    [cond lock];
    [videoWriter finishWritingWithCompletionHandler:^{
        [cond lock];
        [cond signal];
        [cond unlock];
    }];
    [cond wait];
    [cond unlock];
    [self savePhotoCmare:self.url];
}