在 Objective-C 中同步异步任务

Question

我正在尝试使用 GCDAsyncSocket 向我的 iDevice 客户端发送一些图像文件（近 100MB）。

我想同步向客户端发送数据包。我的意思是在向第一个客户端发送 100MB 数据后，迭代到下一个客户端。但是由于 GCDAsyncSocket 的异步性质，我不知道如何序列化这些数据包发送。

我不能使用信号量，因为在发送图像之前，我会与每个客户端协商以了解我应该发送哪些图像，然后尝试发送这些图像。我找不到一种巧妙的方法来等待和向信号量发出信号。

- (void)sendImagesToTheClients:clients
{
    ...
    //negotiating with client to know which images should sent
    ...

    for(Client* client in clients)
    {
       packet = [packet addImages: images];
       [self sendPacket:packet toClient:client];
    }
}

- (void)sendPacket:packet toClient:client
{
 // Initialize Buffer
    NSMutableData *buffer = [[NSMutableData alloc] init];
    NSData *bufferData = [NSKeyedArchiver archivedDataWithRootObject:packet];

    uint64_t headerLength = [bufferData length];
    [buffer appendBytes:&headerLength length:sizeof(uint64_t)];
    [buffer appendBytes:[bufferData bytes] length:[bufferData length]];

    // Write Buffer
    [client.socket writeData:buffer withTimeout:-1.0 tag:0];
}

这是 AsyncSocket 写入数据的工作原理：

- (void)writeData:(NSData *)data withTimeout:(NSTimeInterval)timeout tag:(long)tag
{
    if ([data length] == 0) return;

    GCDAsyncWritePacket *packet = [[GCDAsyncWritePacket alloc] initWithData:data timeout:timeout tag:tag];

    dispatch_async(socketQueue, ^{ @autoreleasepool {

        LogTrace();

        if ((flags & kSocketStarted) && !(flags & kForbidReadsWrites))
        {
            [writeQueue addObject:packet];
            [self maybeDequeueWrite];
        }
    }});

    // Do not rely on the block being run in order to release the packet,
    // as the queue might get released without the block completing.
}

那么我怎样才能同步这个任务呢？

更新

对于套接字连接，我使用GCDAsyncSocket，它大量使用委托进行事件通知。（GCDAsyncSocket.h 和 GCDAsyncSocket.m）（没有带有 completionHandler 的方法）。

我编写了一个名为 TCPClient 的类，它处理套接字连接和数据包发送，并将其设置为初始化套接字的委托。写入数据包后，委托方法- (void)socket:(GCDAsyncSocket *)sock didWriteDataWithTag:(long)tag被调用。这只告诉我一些数据已经写入。在这里，我无法根据写入的数据来决定要调用dispatch_group_leave. 所以依赖委托方法是没有用的。

我在 GCDAsyncSocket.h 和 .m 文件中修改了 [client.socket writeData:buffer withTimeout:-1.0 tag:0] 以接受 completionBlock： [client.socket writeData:buffer withTimeout:-1.0 tag:0 completionBlock:completionBlock] 使用这种方法可以帮助我解决同步异步任务。

// perform your async task
dispatch_async(self.socketQueue, ^{
    [self sendPacket:packet toClient:client withCompletion:^(BOOL finished, NSError *error)
     {
         if (finished) {
             NSLog(@"images file sending finished");
             //leave the group when you're done
             dispatch_group_leave(group);
         }
         else if (!finished && error)
         {
             NSLog(@"images file sending FAILED");
         }
     }];

但问题是在更新 GCDAsyncsocket 后，我​​的代码可能会中断。在这里，我正在寻找一种巧妙的方法将完成处理程序添加到 GCDAsyncsocket 而无需直接修改它。比如围绕它创建一个包装器或使用 Objective-c 运行时的特性。你有什么主意吗？

Answer 1

您可以使用调度组完成此操作。对于带有完成块的异步任务：

//create & enter the group
dispatch_group_t group = dispatch_group_create();
dispatch_group_enter(group);

// perform your async task
dispatch_async(socketQueue, ^{

    //leave the group when you're done
    dispatch_group_leave(group);
});

// wait for the group to complete 
// (you can use DISPATCH_TIME_FOREVER to wait forever)
long status = dispatch_group_wait(group,
                    dispatch_time(DISPATCH_TIME_NOW,NSEC_PER_SEC * COMMAND_TIMEOUT));

// check to see if it timed out, or completed
if (status != 0) {
    // timed out
}

或者对于有委托的任务：

@property (nonatomic) dispatch_group_t group;

-(BOOL)doWorkSynchronously {

    self.group = dispatch_group_create();
    dispatch_group_enter(self.group);

    [object doAsyncWorkWithDelegate:self];

    long status = dispatch_group_wait(self.group,
                        dispatch_time(DISPATCH_TIME_NOW,NSEC_PER_SEC * COMMAND_TIMEOUT));

    // A
    if (status != 0) {
        // timed out
        return NO
    }

    return YES;
}

-(void)asyncWorkCompleted {}

    // after this call, control should jump back to point A in the doWorkSynchronously method
    dispatch_group_leave(self.group);
}