ios - 关于使这个线程安全但高效的建议？

Question

我一直在思考一个看起来很容易实现的问题，但一个高效且线程安全的解决方案却阻碍了我。我想要做的是创建某种工作对象。多个调用者可能会要求它在不同的线程中工作。一个要求是请求不能排队。换句话说，如果有人要求 worker 工作但看到它已经在工作，它应该早点返回。

一个简单的第一步是这样的:

@interface Worker : NSObject
@property (nonatomic, assign, getter = isWorking) BOOL working;
- (void)doWork;
@end

@implementation Worker
{
    dispatch_queue_t _workerQueue; //... a private serial queue
}

- (void)doWork
{
    if ( self.isWorking )
    {
        return;
    }
    self.working = YES;
    dispatch_async(_workerQueue, ^{
        // Do time consuming work here ... Done!
        self.working = NO;
    });
}
@end

问题在于 isWorking 属性不是线程安全的。将其标记为原子在这里无济于事，因为对它的访问需要在几个语句之间同步。

为了使其成为线程安全的，我需要使用锁来保护 isWorking:

@interface Worker : NSObject
@property (nonatomic, assign, getter = isWorking) BOOL working;
- (void)doWork;
@end

@implementation Worker
{
    dispatch_queue_t _workerQueue; //... a private serial queue
    NSLock *_lock; // assume this is created
}

- (void)doWork
{
    [_lock lock];
    if ( self.isWorking )
    {
        [_lock unlock];
        return;
    }
    self.working = YES;
    [_lock unlock];
    dispatch_async(_workerQueue, ^{
        // Do time consuming work here ... Done!
        [_lock lock];
        self.working = NO;
        [_lock unlock];
    });
}

@结束

虽然我确实相信这会是线程安全的，但我认为必须如此频繁地获取和放弃锁(一项昂贵的操作)是非常糟糕的。

那么，有没有更优雅的方案呢？

Answer 1

dispatch_semaphore is the idiomatic way to limit access to a finite resource ，如果您已经在使用 GCD。

// Add an ivar:
dispatch_semaphore_t _semaphore;

// To initialize:
_semaphore = dispatch_semaphore_create(1);

// To "do work" from any thread:
- (void)doWork
{
     if (dispatch_semaphore_wait(_semaphore, DISPATCH_TIME_NOW) == 0) {
         // We got the semaphore without waiting, so we're first in line.
         dispatch_async(_workerQueue, ^{
             // do time consuming work here, then when done:
             dispatch_semaphore_signal(_semaphore);
         });
     } else {
         // We would have had to wait for the semaphore, so somebody must have
         // been doing work already, and we should do nothing.
     }
}

Here's a blog post explaining in more detail.