锁的作用在于避免多线程同时访问某些公共资源的时候出错。

互斥锁（NSLock）：互斥锁可以满足大部分情况，可以保证多线程访问公共资源时的互斥性。

递归锁（NSRecursiveLock）：既然互斥锁能解决大部分问题，为什么还要有递归锁呢，问题在于互斥锁在同一线程多次上锁的时候会造成死锁，当同一线程中多次上锁的时候用递归锁。

条件锁（NSCondition）：有时候我们会有这样的需求再一个线程中不满足条件的时候等待，如果条件满足再通知该线程继续执行，适用于经典的生产者和消费者场景中。举个例子：异步下载5张图片，下载完成后存储到本地。

- (void)downloadTest{
    NSURLSession * session = [NSURLSession sharedSession];
    //下载第一张图片
    NSURLSessionDownloadTask * taskOne = [session downloadTaskWithURL:[NSURL URLWithString:@"http://b.hiphotos.baidu.com/image/pic/item/32fa828ba61ea8d3fcd2e9ce9e0a304e241f5803.jpg"] completionHandler:^(NSURL * _Nullable location, NSURLResponse * _Nullable response, NSError * _Nullable error) {
        [self.conditionLock lock];
        NSData * data = [NSData dataWithContentsOfURL:location];
        UIImage * image = [UIImage imageWithData:data];
        [self.dataArray addObject:data];
        [self.conditionLock signal];
        [self.conditionLock unlock];
        dispatch_async(dispatch_get_main_queue(), ^{
            [((UIImageView *)self.imageArray[0]) setImage:image];
        });
    }];
    [taskOne resume];
     //下载第二张图片
    NSURLSessionDownloadTask * taskTwo = [session downloadTaskWithURL:[NSURL URLWithString:@"http://b.hiphotos.baidu.com/image/pic/item/32fa828ba61ea8d3fcd2e9ce9e0a304e241f5803.jpg"] completionHandler:^(NSURL * _Nullable location, NSURLResponse * _Nullable response, NSError * _Nullable error) {
        [self.conditionLock lock];
        NSData * data = [NSData dataWithContentsOfURL:location];
        UIImage * image = [UIImage imageWithData:data];
        [self.dataArray addObject:data];
        [self.conditionLock signal];
        [self.conditionLock unlock];
        dispatch_async(dispatch_get_main_queue(), ^{
            [((UIImageView *)self.imageArray[1]) setImage:image];
        });
    }];
    [taskTwo resume];
     //下载第三张图片
    NSURLSessionDownloadTask * taskThree = [session downloadTaskWithURL:[NSURL URLWithString:@"http://b.hiphotos.baidu.com/image/pic/item/32fa828ba61ea8d3fcd2e9ce9e0a304e241f5803.jpg"] completionHandler:^(NSURL * _Nullable location, NSURLResponse * _Nullable response, NSError * _Nullable error) {
        [self.conditionLock lock];
        NSData * data = [NSData dataWithContentsOfURL:location];
        UIImage * image = [UIImage imageWithData:data];
        [self.dataArray addObject:data];
        [self.conditionLock signal];
        [self.conditionLock unlock];
        dispatch_async(dispatch_get_main_queue(), ^{
            [((UIImageView *)self.imageArray[2]) setImage:image];
        });
    }];
    [taskThree resume];
     //下载第四张图片
    NSURLSessionDownloadTask * taskFour = [session downloadTaskWithURL:[NSURL URLWithString:@"http://b.hiphotos.baidu.com/image/pic/item/32fa828ba61ea8d3fcd2e9ce9e0a304e241f5803.jpg"] completionHandler:^(NSURL * _Nullable location, NSURLResponse * _Nullable response, NSError * _Nullable error) {
        [self.conditionLock lock];
        NSData * data = [NSData dataWithContentsOfURL:location];
        UIImage * image = [UIImage imageWithData:data];
        [self.dataArray addObject:data];
        [self.conditionLock signal];
        [self.conditionLock unlock];
        dispatch_async(dispatch_get_main_queue(), ^{
            [((UIImageView *)self.imageArray[3]) setImage:image];
        });
    }];
    [taskFour resume];
     //下载第五张图片
    NSURLSessionDownloadTask * taskFive = [session downloadTaskWithURL:[NSURL URLWithString:@"http://b.hiphotos.baidu.com/image/pic/item/32fa828ba61ea8d3fcd2e9ce9e0a304e241f5803.jpg"] completionHandler:^(NSURL * _Nullable location, NSURLResponse * _Nullable response, NSError * _Nullable error) {
        [self.conditionLock lock];
        NSData * data = [NSData dataWithContentsOfURL:location];
        UIImage * image = [UIImage imageWithData:data];
        [self.dataArray addObject:data];
        [self.conditionLock signal];
        [self.conditionLock unlock];
        dispatch_async(dispatch_get_main_queue(), ^{
            [((UIImageView *)self.imageArray[4]) setImage:image];
        });
    }];
    [taskFive resume];
    [self storeData];
    
}
- (void) storeData{
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        [self.conditionLock lock];
        while (self.dataArray.count != 5) {
            [self.conditionLock wait];
        }
        dispatch_async(dispatch_get_main_queue(), ^{
            //存储到本地
        });
        [self.conditionLock unlock];
    });
}

锁保护了数组的写操作，且写过之后发送signal唤醒storeData部分wait部分继续判断当前下载状态。如果都下载完成执行写操作。

信号量：从另一个角度看信号量其实就是个有个特殊条件的条件锁（条件就是信号大于0）。更多用在控制多线程的最大并发量。